// Copyright (c) 2017 Google Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Contains
//   - SPIR-V to MARK-V encoder
//   - MARK-V to SPIR-V decoder
//
// MARK-V is a compression format for SPIR-V binaries. It strips away
// non-essential information (such as result ids which can be regenerated) and
// uses various bit reduction techiniques to reduce the size of the binary.
//
// MarkvModel is a flatbuffers object containing a set of rules defining how
// compression/decompression is done (coding schemes, dictionaries).

#include <algorithm>
#include <cassert>
#include <cstring>
#include <functional>
#include <iostream>
#include <iterator>
#include <list>
#include <memory>
#include <numeric>
#include <string>
#include <vector>
#include <unordered_map>
#include <unordered_set>

#include "spirv/1.2/GLSL.std.450.h"
#include "spirv/1.2/OpenCL.std.h"
#include "spirv/1.2/spirv.h"

#include "binary.h"
#include "diagnostic.h"
#include "enum_string_mapping.h"
#include "extensions.h"
#include "ext_inst.h"
#include "id_descriptor.h"
#include "instruction.h"
#include "markv_autogen.h"
#include "opcode.h"
#include "operand.h"
#include "spirv-tools/libspirv.h"
#include "spirv-tools/markv.h"
#include "spirv_endian.h"
#include "spirv_validator_options.h"
#include "util/bit_stream.h"
#include "util/huffman_codec.h"
#include "util/move_to_front.h"
#include "util/parse_number.h"
#include "validate.h"
#include "val/instruction.h"
#include "val/validation_state.h"

using libspirv::IdDescriptorCollection;
using libspirv::Instruction;
using libspirv::ValidationState_t;
using libspirv::DiagnosticStream;
using spvtools::ValidateInstructionAndUpdateValidationState;
using spvutils::BitReaderWord64;
using spvutils::BitWriterWord64;
using spvutils::HuffmanCodec;
using MoveToFront = spvutils::MoveToFront<uint32_t>;
using MultiMoveToFront = spvutils::MultiMoveToFront<uint32_t>;

struct spv_markv_encoder_options_t {
  bool validate_spirv_binary = false;
};

struct spv_markv_decoder_options_t {
  bool validate_spirv_binary = false;
};

namespace {

const uint32_t kSpirvMagicNumber = SpvMagicNumber;
const uint32_t kMarkvMagicNumber = 0x07230303;

// Handles for move-to-front sequences. Enums which end with "Begin" define
// handle spaces which start at that value and span 16 or 32 bit wide.
enum : uint64_t {
  kMtfNone = 0,
  // All ids.
  kMtfAll,
  // All forward declared ids.
  kMtfForwardDeclared,
  // All type ids except for generated by OpTypeFunction.
  kMtfTypeNonFunction,
  // All labels.
  kMtfLabel,
  // All ids created by instructions which had type_id.
  kMtfObject,
  // All types generated by OpTypeFloat, OpTypeInt, OpTypeBool.
  kMtfTypeScalar,
  // All composite types.
  kMtfTypeComposite,
  // Boolean type or any vector type of it.
  kMtfTypeBoolScalarOrVector,
  // All float types or any vector floats type.
  kMtfTypeFloatScalarOrVector,
  // All int types or any vector int type.
  kMtfTypeIntScalarOrVector,
  // All types declared as return types in OpTypeFunction.
  kMtfTypeReturnedByFunction,
  // All object ids which are integer constants.
  kMtfConstInteger,
  // All composite objects.
  kMtfComposite,
  // All bool objects or vectors of bools.
  kMtfBoolScalarOrVector,
  // All float objects or vectors of float.
  kMtfFloatScalarOrVector,
  // All int objects or vectors of int.
  kMtfIntScalarOrVector,
  // All pointer types which point to composited.
  kMtfTypePointerToComposite,
  // Used by EncodeMtfRankHuffman.
  kMtfGenericNonZeroRank,
  // Handle space for ids of specific type.
  kMtfIdOfTypeBegin = 0x10000,
  // Handle space for ids generated by specific opcode.
  kMtfIdGeneratedByOpcode = 0x20000,
  // Handle space for ids of objects with type generated by specific opcode.
  kMtfIdWithTypeGeneratedByOpcodeBegin = 0x30000,
  // All vectors of specific component type.
  kMtfVectorOfComponentTypeBegin = 0x40000,
  // All vector types of specific size.
  kMtfTypeVectorOfSizeBegin = 0x50000,
  // All pointer types to specific type.
  kMtfPointerToTypeBegin = 0x60000,
  // All function types which return specific type.
  kMtfFunctionTypeWithReturnTypeBegin = 0x70000,
  // All function objects which return specific type.
  kMtfFunctionWithReturnTypeBegin = 0x80000,
  // All float vectors of specific size.
  kMtfFloatVectorOfSizeBegin = 0x90000,
  // Id descriptor space (32-bit).
  kMtfIdDescriptorSpaceBegin = 0x100000000,
};

// Used by "presumed index" technique which does special treatment of integer
// constants no greater than this value.
const uint32_t kMarkvMaxPresumedAccessIndex = 31;

// Signals that the value is not in the coding scheme and a fallback method
// needs to be used.
const uint64_t kMarkvNoneOfTheAbove = GetMarkvNonOfTheAbove();

// Mtf ranks smaller than this are encoded with Huffman coding.
const uint32_t kMtfSmallestRankEncodedByValue = 10;

// Signals that the mtf rank is too large to be encoded with Huffman.
const uint32_t kMtfRankEncodedByValueSignal =
    std::numeric_limits<uint32_t>::max();

const size_t kCommentNumWhitespaces = 2;

const size_t kByteBreakAfterInstIfLessThanUntilNextByte = 8;

// Returns a set of mtf rank codecs based on a plausible hand-coded
// distribution.
std::map<uint64_t, std::unique_ptr<HuffmanCodec<uint32_t>>>
GetMtfHuffmanCodecs() {
  std::map<uint64_t, std::unique_ptr<HuffmanCodec<uint32_t>>> codecs;

  std::unique_ptr<HuffmanCodec<uint32_t>> codec;

  codec.reset(new HuffmanCodec<uint32_t>(std::map<uint32_t, uint32_t>({
    { 0, 5 },
    { 1, 40 },
    { 2, 10 },
    { 3, 5 },
    { 4, 5 },
    { 5, 5 },
    { 6, 3 },
    { 7, 3 },
    { 8, 3 },
    { 9, 3 },
    { kMtfRankEncodedByValueSignal, 10 },
  })));
  codecs.emplace(kMtfAll, std::move(codec));

  codec.reset(new HuffmanCodec<uint32_t>(std::map<uint32_t, uint32_t>({
    { 1, 50 },
    { 2, 20 },
    { 3, 5 },
    { 4, 5 },
    { 5, 2 },
    { 6, 1 },
    { 7, 1 },
    { 8, 1 },
    { 9, 1 },
    { kMtfRankEncodedByValueSignal, 10 },
  })));
  codecs.emplace(kMtfGenericNonZeroRank, std::move(codec));

  return codecs;
}

// Encoding/decoding model containing various constants and codecs.
class MarkvModel {
 public:
  MarkvModel()
      : mtf_huffman_codecs_(GetMtfHuffmanCodecs()),
        opcode_and_num_operands_huffman_codec_(GetOpcodeAndNumOperandsHist()),
        opcode_and_num_operands_markov_huffman_codecs_(
            GetOpcodeAndNumOperandsMarkovHuffmanCodecs()),
        non_id_word_huffman_codecs_(GetNonIdWordHuffmanCodecs()),
        id_descriptor_huffman_codecs_(GetIdDescriptorHuffmanCodecs()),
        descriptors_with_coding_scheme_(GetDescriptorsWithCodingScheme()),
        literal_string_huffman_codecs_(GetLiteralStringHuffmanCodecs()) {}

  size_t opcode_chunk_length() const { return 7; }
  size_t num_operands_chunk_length() const { return 3; }
  size_t mtf_rank_chunk_length() const { return 5; }

  size_t u16_chunk_length() const { return 4; }
  size_t s16_chunk_length() const { return 4; }
  size_t s16_block_exponent() const { return 6; }

  size_t u32_chunk_length() const { return 8; }
  size_t s32_chunk_length() const { return 8; }
  size_t s32_block_exponent() const { return 10; }

  size_t u64_chunk_length() const { return 8; }
  size_t s64_chunk_length() const { return 8; }
  size_t s64_block_exponent() const { return 10; }

  // Returns Huffman codec for ranks of the mtf with given |handle|.
  // Different mtfs can use different rank distributions.
  // May return nullptr if the codec doesn't exist.
  const HuffmanCodec<uint32_t>* GetMtfHuffmanCodec(uint64_t handle) const {
    const auto it = mtf_huffman_codecs_.find(handle);
    if (it == mtf_huffman_codecs_.end())
      return nullptr;
    return it->second.get();
  }

  // Returns a codec for common opcode_and_num_operands words for the given
  // previous opcode. May return nullptr if the codec doesn't exist.
  const HuffmanCodec<uint64_t>* GetOpcodeAndNumOperandsMarkovHuffmanCodec(
      uint32_t prev_opcode) const {
    if (prev_opcode == SpvOpNop)
      return &opcode_and_num_operands_huffman_codec_;

    const auto it =
        opcode_and_num_operands_markov_huffman_codecs_.find(prev_opcode);
    if (it == opcode_and_num_operands_markov_huffman_codecs_.end())
      return nullptr;
    return it->second.get();
  }

  // Returns a codec for common non-id words used for given operand slot.
  // Operand slot is defined by the opcode and the operand index.
  // May return nullptr if the codec doesn't exist.
  const HuffmanCodec<uint64_t>* GetNonIdWordHuffmanCodec(
      uint32_t opcode, uint32_t operand_index) const {
    const auto it = non_id_word_huffman_codecs_.find(
        std::pair<uint32_t, uint32_t>(opcode, operand_index));
    if (it == non_id_word_huffman_codecs_.end())
      return nullptr;
    return it->second.get();
  }

  // Returns a codec for common id descriptos used for given operand slot.
  // Operand slot is defined by the opcode and the operand index.
  // May return nullptr if the codec doesn't exist.
  const HuffmanCodec<uint64_t>* GetIdDescriptorHuffmanCodec(
      uint32_t opcode, uint32_t operand_index) const {
    const auto it = id_descriptor_huffman_codecs_.find(
        std::pair<uint32_t, uint32_t>(opcode, operand_index));
    if (it == id_descriptor_huffman_codecs_.end())
      return nullptr;
    return it->second.get();
  }

  // Returns a codec for common strings used by the given opcode.
  // Operand slot is defined by the opcode and the operand index.
  // May return nullptr if the codec doesn't exist.
  const HuffmanCodec<std::string>* GetLiteralStringHuffmanCodec(
      uint32_t opcode) const {
    const auto it = literal_string_huffman_codecs_.find(opcode);
    if (it == literal_string_huffman_codecs_.end())
      return nullptr;
    return it->second.get();
  }

  bool DescriptorHasCodingScheme(uint32_t descriptor) const {
    return descriptors_with_coding_scheme_.count(descriptor);
  }

 private:
  // Huffman codecs for move-to-front ranks. The map key is mtf handle. Doesn't
  // need to contain a different codec for every handle as most use one and the
  // same.
  std::map<uint64_t, std::unique_ptr<HuffmanCodec<uint32_t>>>
      mtf_huffman_codecs_;

  // Huffman codec for base-rate of opcode_and_num_operands.
  HuffmanCodec<uint64_t> opcode_and_num_operands_huffman_codec_;

  // Huffman codecs for opcode_and_num_operands. The map key is previous opcode.
  std::map<uint32_t, std::unique_ptr<HuffmanCodec<uint64_t>>>
      opcode_and_num_operands_markov_huffman_codecs_;

  // Huffman codecs for non-id single-word operand values.
  // The map key is pair <opcode, operand_index>.
  std::map<std::pair<uint32_t, uint32_t>,
      std::unique_ptr<HuffmanCodec<uint64_t>>>
      non_id_word_huffman_codecs_;

  // Huffman codecs for id descriptors. The map key is pair
  // <opcode, operand_index>.
  std::map<std::pair<uint32_t, uint32_t>,
      std::unique_ptr<HuffmanCodec<uint64_t>>>
      id_descriptor_huffman_codecs_;

  std::unordered_set<uint32_t> descriptors_with_coding_scheme_;

  // Huffman codecs for literal strings. The map key is the opcode of the
  // current instruction. This assumes, that there is no more than one literal
  // string operand per instruction, but would still work even if this is not
  // the case. Names and debug information strings are not collected.
  std::map<uint32_t, std::unique_ptr<HuffmanCodec<std::string>>>
      literal_string_huffman_codecs_;
};

const MarkvModel* GetDefaultModel() {
  static MarkvModel model;
  return &model;
}

// Returns chunk length used for variable length encoding of spirv operand
// words. Returns zero if operand type corresponds to potentially multiple
// words or a word which is not expected to profit from variable width encoding.
// Chunk length is selected based on the size of expected value.
// Most of these values will later be encoded with probability-based coding,
// but variable width integer coding is a good quick solution.
// TODO(atgoo@github.com): Put this in MarkvModel flatbuffer.
size_t GetOperandVariableWidthChunkLength(spv_operand_type_t type) {
  switch (type) {
    case SPV_OPERAND_TYPE_TYPE_ID:
      return 4;
    case SPV_OPERAND_TYPE_RESULT_ID:
    case SPV_OPERAND_TYPE_ID:
    case SPV_OPERAND_TYPE_SCOPE_ID:
    case SPV_OPERAND_TYPE_MEMORY_SEMANTICS_ID:
      return 8;
    case SPV_OPERAND_TYPE_LITERAL_INTEGER:
    case SPV_OPERAND_TYPE_OPTIONAL_LITERAL_INTEGER:
      return 6;
    case SPV_OPERAND_TYPE_CAPABILITY:
      return 6;
    case SPV_OPERAND_TYPE_SOURCE_LANGUAGE:
    case SPV_OPERAND_TYPE_EXECUTION_MODEL:
      return 3;
    case SPV_OPERAND_TYPE_ADDRESSING_MODEL:
    case SPV_OPERAND_TYPE_MEMORY_MODEL:
      return 2;
    case SPV_OPERAND_TYPE_EXECUTION_MODE:
      return 6;
    case SPV_OPERAND_TYPE_STORAGE_CLASS:
      return 4;
    case SPV_OPERAND_TYPE_DIMENSIONALITY:
    case SPV_OPERAND_TYPE_SAMPLER_ADDRESSING_MODE:
      return 3;
    case SPV_OPERAND_TYPE_SAMPLER_FILTER_MODE:
      return 2;
    case SPV_OPERAND_TYPE_SAMPLER_IMAGE_FORMAT:
      return 6;
    case SPV_OPERAND_TYPE_FP_ROUNDING_MODE:
    case SPV_OPERAND_TYPE_LINKAGE_TYPE:
    case SPV_OPERAND_TYPE_ACCESS_QUALIFIER:
    case SPV_OPERAND_TYPE_OPTIONAL_ACCESS_QUALIFIER:
      return 2;
    case SPV_OPERAND_TYPE_FUNCTION_PARAMETER_ATTRIBUTE:
      return 3;
    case SPV_OPERAND_TYPE_DECORATION:
    case SPV_OPERAND_TYPE_BUILT_IN:
      return 6;
    case SPV_OPERAND_TYPE_GROUP_OPERATION:
    case SPV_OPERAND_TYPE_KERNEL_ENQ_FLAGS:
    case SPV_OPERAND_TYPE_KERNEL_PROFILING_INFO:
      return 2;
    case SPV_OPERAND_TYPE_FP_FAST_MATH_MODE:
    case SPV_OPERAND_TYPE_FUNCTION_CONTROL:
    case SPV_OPERAND_TYPE_LOOP_CONTROL:
    case SPV_OPERAND_TYPE_IMAGE:
    case SPV_OPERAND_TYPE_OPTIONAL_IMAGE:
    case SPV_OPERAND_TYPE_OPTIONAL_MEMORY_ACCESS:
    case SPV_OPERAND_TYPE_SELECTION_CONTROL:
      return 4;
    case SPV_OPERAND_TYPE_EXTENSION_INSTRUCTION_NUMBER:
    case SPV_OPERAND_TYPE_TYPED_LITERAL_NUMBER:
      return 6;
    default:
      return 0;
  }
  return 0;
}

// Returns true if the opcode has a fixed number of operands. May return a
// false negative.
bool OpcodeHasFixedNumberOfOperands(SpvOp opcode) {
  switch (opcode) {
    // TODO(atgoo@github.com) This is not a complete list.
    case SpvOpNop:
    case SpvOpName:
    case SpvOpUndef:
    case SpvOpSizeOf:
    case SpvOpLine:
    case SpvOpNoLine:
    case SpvOpDecorationGroup:
    case SpvOpExtension:
    case SpvOpExtInstImport:
    case SpvOpMemoryModel:
    case SpvOpCapability:
    case SpvOpTypeVoid:
    case SpvOpTypeBool:
    case SpvOpTypeInt:
    case SpvOpTypeFloat:
    case SpvOpTypeVector:
    case SpvOpTypeMatrix:
    case SpvOpTypeSampler:
    case SpvOpTypeSampledImage:
    case SpvOpTypeArray:
    case SpvOpTypePointer:
    case SpvOpConstantTrue:
    case SpvOpConstantFalse:
    case SpvOpLabel:
    case SpvOpBranch:
    case SpvOpFunction:
    case SpvOpFunctionParameter:
    case SpvOpFunctionEnd:
    case SpvOpBitcast:
    case SpvOpCopyObject:
    case SpvOpTranspose:
    case SpvOpSNegate:
    case SpvOpFNegate:
    case SpvOpIAdd:
    case SpvOpFAdd:
    case SpvOpISub:
    case SpvOpFSub:
    case SpvOpIMul:
    case SpvOpFMul:
    case SpvOpUDiv:
    case SpvOpSDiv:
    case SpvOpFDiv:
    case SpvOpUMod:
    case SpvOpSRem:
    case SpvOpSMod:
    case SpvOpFRem:
    case SpvOpFMod:
    case SpvOpVectorTimesScalar:
    case SpvOpMatrixTimesScalar:
    case SpvOpVectorTimesMatrix:
    case SpvOpMatrixTimesVector:
    case SpvOpMatrixTimesMatrix:
    case SpvOpOuterProduct:
    case SpvOpDot:
      return true;
    default:
      break;
  }
  return false;
}

size_t GetNumBitsToNextByte(size_t bit_pos) {
  return (8 - (bit_pos % 8)) % 8;
}

// Defines and returns current MARK-V version.
uint32_t GetMarkvVersion() {
  const uint32_t kVersionMajor = 1;
  const uint32_t kVersionMinor = 2;
  return kVersionMinor | (kVersionMajor << 16);
}

class CommentLogger {
 public:
  void AppendText(const std::string& str) {
    Append(str);
    use_delimiter_ = false;
  }

  void AppendTextNewLine(const std::string& str) {
    Append(str);
    Append("\n");
    use_delimiter_ = false;
  }

  void AppendBitSequence(const std::string& str) {
    if (use_delimiter_)
      Append("-");
    Append(str);
    use_delimiter_ = true;
  }

  void AppendWhitespaces(size_t num) {
    Append(std::string(num, ' '));
    use_delimiter_ = false;
  }

  void NewLine() {
    Append("\n");
    use_delimiter_ = false;
  }

  std::string GetText() const {
    return ss_.str();
  }

 private:
  void Append(const std::string& str) {
    ss_ << str;
    // std::cerr << str;
  }

  std::stringstream ss_;

  // If true a delimiter will be appended before the next bit sequence.
  // Used to generate outputs like: 1100-0 1110-1-1100-1-1111-0 110-0.
  bool use_delimiter_ = false;
};

// Creates spv_text object containing text from |str|.
// The returned value is owned by the caller and needs to be destroyed with
// spvTextDestroy.
spv_text CreateSpvText(const std::string& str) {
  spv_text out = new spv_text_t();
  assert(out);
  char* cstr = new char[str.length() + 1];
  assert(cstr);
  std::strncpy(cstr, str.c_str(), str.length());
  cstr[str.length()] = '\0';
  out->str = cstr;
  out->length = str.length();
  return out;
}

// Base class for MARK-V encoder and decoder. Contains common functionality
// such as:
// - Validator connection and validation state.
// - SPIR-V grammar and helper functions.
class MarkvCodecBase {
 public:
  virtual ~MarkvCodecBase() {
    spvValidatorOptionsDestroy(validator_options_);
  }

  MarkvCodecBase() = delete;

  void SetModel(const MarkvModel* model) {
    model_ = model;
  }

 protected:
  struct MarkvHeader {
    MarkvHeader() {
      magic_number = kMarkvMagicNumber;
      markv_version = GetMarkvVersion();
      markv_model = 0;
      markv_length_in_bits = 0;
      spirv_version = 0;
      spirv_generator = 0;
    }

    uint32_t magic_number;
    uint32_t markv_version;
    // Magic number to identify or verify MarkvModel used for encoding.
    uint32_t markv_model;
    uint32_t markv_length_in_bits;
    uint32_t spirv_version;
    uint32_t spirv_generator;
  };

  explicit MarkvCodecBase(spv_const_context context,
                          spv_validator_options validator_options)
      : validator_options_(validator_options), grammar_(context),
        model_(GetDefaultModel()), context_(context),
        vstate_(validator_options ?
                new ValidationState_t(context, validator_options_) : nullptr) {}

  // Validates a single instruction and updates validation state of the module.
  // Does nothing and returns SPV_SUCCESS if validator was not created.
  spv_result_t UpdateValidationState(const spv_parsed_instruction_t& inst) {
    if (!vstate_)
      return SPV_SUCCESS;

    return ValidateInstructionAndUpdateValidationState(vstate_.get(), &inst);
  }

  // Returns instruction which created |id| or nullptr if such instruction was
  // not registered.
  const Instruction* FindDef(uint32_t id) const {
    const auto it = id_to_def_instruction_.find(id);
    if (it == id_to_def_instruction_.end())
      return nullptr;
    return it->second;
  }

  // Returns type id of vector type component.
  uint32_t GetVectorComponentType(uint32_t vector_type_id) const {
    const Instruction* type_inst = FindDef(vector_type_id);
    assert(type_inst);
    assert(type_inst->opcode() == SpvOpTypeVector);

    const uint32_t component_type =
        type_inst->word(type_inst->operands()[1].offset);
    return component_type;
  }

  // Returns mtf handle for ids of given type.
  uint64_t GetMtfIdOfType(uint32_t type_id) const {
    return kMtfIdOfTypeBegin + type_id;
  }

  // Returns mtf handle for ids generated by given opcode.
  uint64_t GetMtfIdGeneratedByOpcode(SpvOp opcode) const {
    return kMtfIdGeneratedByOpcode + opcode;
  }

  // Returns mtf handle for ids of type generated by given opcode.
  uint64_t GetMtfIdWithTypeGeneratedByOpcode(SpvOp opcode) const {
    return kMtfIdWithTypeGeneratedByOpcodeBegin + opcode;
  }

  // Returns mtf handle for vectors of specific component type.
  uint64_t GetMtfVectorOfComponentType(uint32_t type_id) const {
    return kMtfVectorOfComponentTypeBegin + type_id;
  }

  // Returns mtf handle for float vectors of specific size.
  uint64_t GetMtfFloatVectorOfSize(uint32_t size) const {
    return kMtfFloatVectorOfSizeBegin + size;
  }

  // Returns mtf handle for vector type of specific size.
  uint64_t GetMtfTypeVectorOfSize(uint32_t size) const {
    return kMtfTypeVectorOfSizeBegin + size;
  }

  // Returns mtf handle for pointers to specific size.
  uint64_t GetMtfPointerToType(uint32_t type_id) const {
    return kMtfPointerToTypeBegin + type_id;
  }

  // Returns mtf handle for function types with given return type.
  uint64_t GetMtfFunctionTypeWithReturnType(uint32_t type_id) const {
    return kMtfFunctionTypeWithReturnTypeBegin + type_id;
  }

  // Returns mtf handle for functions with given return type.
  uint64_t GetMtfFunctionWithReturnType(uint32_t type_id) const {
    return kMtfFunctionWithReturnTypeBegin + type_id;
  }

  // Returns mtf handle for the given id descriptor.
  uint64_t GetMtfIdDescriptor(uint32_t descriptor) const {
    return kMtfIdDescriptorSpaceBegin + descriptor;
  }

  // Process data from the current instruction. This would update MTFs and
  // other data containers.
  void ProcessCurInstruction();

  // Returns move-to-front handle to be used for the current operand slot.
  // Mtf handle is chosen based on a set of rules defined by SPIR-V grammar.
  uint64_t GetRuleBasedMtf();

  // Returns words of the current instruction. Decoder has a different
  // implementation and the array is valid only until the previously decoded
  // word.
  virtual const uint32_t* GetInstWords() const {
    return inst_.words;
  }

  // Returns the opcode of the previous instruction.
  SpvOp GetPrevOpcode() const {
    if (instructions_.empty())
      return SpvOpNop;

    return instructions_.back()->opcode();
  }

  // Returns diagnostic stream, position index is set to instruction number.
  DiagnosticStream Diag(spv_result_t error_code) const {
    return DiagnosticStream({0, 0, instructions_.size()},
                            context_->consumer, error_code);
  }

  // Returns current id bound.
  uint32_t GetIdBound() const {
    return id_bound_;
  }

  // Sets current id bound, expected to be no lower than the previous one.
  void SetIdBound(uint32_t id_bound) {
    assert(id_bound >= id_bound_);
    id_bound_ = id_bound;
    if (vstate_)
      vstate_->setIdBound(id_bound);
  }

  spv_validator_options validator_options_ = nullptr;
  const libspirv::AssemblyGrammar grammar_;
  MarkvHeader header_;
  const MarkvModel* model_ = nullptr;

  // Current instruction, current operand and current operand index.
  spv_parsed_instruction_t inst_;
  spv_parsed_operand_t operand_;
  uint32_t operand_index_;

  // Maps a result ID to its type ID.  By convention:
  //  - a result ID that is a type definition maps to itself.
  //  - a result ID without a type maps to 0.  (E.g. for OpLabel)
  std::unordered_map<uint32_t, uint32_t> id_to_type_id_;

  // Container for all move-to-front sequences.
  MultiMoveToFront multi_mtf_;

  // Id of the current function or zero if outside of function.
  uint32_t cur_function_id_ = 0;

  // Return type of the current function.
  uint32_t cur_function_return_type_ = 0;

  // Remaining function parameter types. This container is filled on OpFunction,
  // and drained on OpFunctionParameter.
  std::list<uint32_t> remaining_function_parameter_types_;

  // List of ids local to the current function.
  std::vector<uint32_t> ids_local_to_cur_function_;

  // List of instructions in the order they are given in the module.
  std::vector<std::unique_ptr<const Instruction>> instructions_;

  // Maps used for the 'presumed id' techniques. Maps small constant integer
  // value to its id and back.
  std::map<uint32_t, uint32_t> presumed_index_to_id_;
  std::map<uint32_t, uint32_t> id_to_presumed_index_;

  // Container/computer for id descriptors.
  IdDescriptorCollection id_descriptors_;

 private:
  spv_const_context context_ = nullptr;

  std::unique_ptr<ValidationState_t> vstate_;

  // Maps result id to the instruction which defined it.
  std::unordered_map<uint32_t, const Instruction*> id_to_def_instruction_;

  uint32_t id_bound_ = 1;
};

// SPIR-V to MARK-V encoder. Exposes functions EncodeHeader and
// EncodeInstruction which can be used as callback by spvBinaryParse.
// Encoded binary is written to an internally maintained bitstream.
// After the last instruction is encoded, the resulting MARK-V binary can be
// acquired by calling GetMarkvBinary().
// The encoder uses SPIR-V validator to keep internal state, therefore
// SPIR-V binary needs to be able to pass validator checks.
// CreateCommentsLogger() can be used to enable the encoder to write comments
// on how encoding was done, which can later be accessed with GetComments().
class MarkvEncoder : public MarkvCodecBase {
 public:
  MarkvEncoder(spv_const_context context,
               spv_const_markv_encoder_options options)
      : MarkvCodecBase(context, GetValidatorOptions(options)),
        options_(options) {
    (void) options_;
  }

  // Writes data from SPIR-V header to MARK-V header.
  spv_result_t EncodeHeader(
      spv_endianness_t /* endian */, uint32_t /* magic */,
      uint32_t version, uint32_t generator, uint32_t id_bound,
      uint32_t /* schema */) {
    SetIdBound(id_bound);
    header_.spirv_version = version;
    header_.spirv_generator = generator;
    return SPV_SUCCESS;
  }

  // Encodes SPIR-V instruction to MARK-V and writes to bit stream.
  // Operation can fail if the instruction fails to pass the validator or if
  // the encoder stubmles on something unexpected.
  spv_result_t EncodeInstruction(const spv_parsed_instruction_t& inst);

  // Concatenates MARK-V header and the bit stream with encoded instructions
  // into a single buffer and returns it as spv_markv_binary. The returned
  // value is owned by the caller and needs to be destroyed with
  // spvMarkvBinaryDestroy().
  spv_markv_binary GetMarkvBinary() {
    header_.markv_length_in_bits =
        static_cast<uint32_t>(sizeof(header_) * 8 + writer_.GetNumBits());
    const size_t num_bytes = sizeof(header_) + writer_.GetDataSizeBytes();

    spv_markv_binary markv_binary = new spv_markv_binary_t();
    markv_binary->data = new uint8_t[num_bytes];
    markv_binary->length = num_bytes;
    assert(writer_.GetData());
    std::memcpy(markv_binary->data, &header_, sizeof(header_));
    std::memcpy(markv_binary->data + sizeof(header_),
                writer_.GetData(), writer_.GetDataSizeBytes());
    return markv_binary;
  }

  // Creates an internal logger which writes comments on the encoding process.
  // Output can later be accessed with GetComments().
  void CreateCommentsLogger() {
    logger_.reset(new CommentLogger());
    writer_.SetCallback([this](const std::string& str){
      logger_->AppendBitSequence(str);
    });
  }

  // Optionally adds disassembly to the comments.
  // Disassembly should contain all instructions in the module separated by
  // \n, and no header.
  void SetDisassembly(std::string&& disassembly) {
    disassembly_.reset(new std::stringstream(std::move(disassembly)));
  }

  // Extracts the next instruction line from the disassembly and logs it.
  void LogDisassemblyInstruction() {
    if (logger_ && disassembly_) {
      std::string line;
      std::getline(*disassembly_, line, '\n');
      logger_->AppendTextNewLine(line);
    }
  }

  // Extracts the text from the comment logger.
  std::string GetComments() const {
    if (!logger_)
      return "";
    return logger_->GetText();
  }

 private:
  // Creates and returns validator options. Returned value owned by the caller.
  static spv_validator_options GetValidatorOptions(
      spv_const_markv_encoder_options options) {
    return options->validate_spirv_binary ?
        spvValidatorOptionsCreate() : nullptr;
  }

  // Writes a single word to bit stream. operand_.type determines if the word is
  // encoded and how.
  spv_result_t EncodeNonIdWord(uint32_t word);

  // Writes both opcode and num_operands as a single code.
  // Returns SPV_UNSUPPORTED iff no suitable codec was found.
  spv_result_t EncodeOpcodeAndNumOperands(uint32_t opcode, uint32_t num_operands);

  // Writes mtf rank to bit stream. |mtf| is used to determine the codec
  // scheme. |fallback_method| is used if no codec defined for |mtf|.
  spv_result_t EncodeMtfRankHuffman(uint32_t rank, uint64_t mtf,
                                    uint64_t fallback_method);

  // Writes id using coding based on mtf associated with the id descriptor.
  // Returns SPV_UNSUPPORTED iff fallback method needs to be used.
  spv_result_t EncodeIdWithDescriptor(uint32_t id);

  // Writes id using coding based on the given |mtf|, which is expected to
  // contain the given |id|.
  spv_result_t EncodeExistingId(uint64_t mtf, uint32_t id);

  // Writes type id of the current instruction if can't be inferred.
  spv_result_t EncodeTypeId();

  // Writes result id of the current instruction if can't be inferred.
  spv_result_t EncodeResultId();

  // Writes ids which are neither type nor result ids.
  spv_result_t EncodeRefId(uint32_t id);

  // Writes bits to the stream until the beginning of the next byte if the
  // number of bits until the next byte is less than |byte_break_if_less_than|.
  void AddByteBreak(size_t byte_break_if_less_than);

  // Encodes a literal number operand and writes it to the bit stream.
  spv_result_t EncodeLiteralNumber(const spv_parsed_operand_t& operand);

  spv_const_markv_encoder_options options_;

  // Bit stream where encoded instructions are written.
  BitWriterWord64 writer_;

  // If not nullptr, encoder will write comments.
  std::unique_ptr<CommentLogger> logger_;

  // If not nullptr, disassembled instruction lines will be written to comments.
  // Format: \n separated instruction lines, no header.
  std::unique_ptr<std::stringstream> disassembly_;
};

// Decodes MARK-V buffers written by MarkvEncoder.
class MarkvDecoder : public MarkvCodecBase {
 public:
  MarkvDecoder(spv_const_context context,
               const uint8_t* markv_data,
               size_t markv_size_bytes,
               spv_const_markv_decoder_options options)
      : MarkvCodecBase(context, GetValidatorOptions(options)),
        options_(options), reader_(markv_data, markv_size_bytes) {
    (void) options_;
    SetIdBound(1);
    parsed_operands_.reserve(25);
    inst_words_.reserve(25);
  }

  // Decodes SPIR-V from MARK-V and stores the words in |spirv_binary|.
  // Can be called only once. Fails if data of wrong format or ends prematurely,
  // of if validation fails.
  spv_result_t DecodeModule(std::vector<uint32_t>* spirv_binary);

 private:
  // Describes the format of a typed literal number.
  struct NumberType {
    spv_number_kind_t type;
    uint32_t bit_width;
  };

  // Creates and returns validator options. Returned value owned by the caller.
  static spv_validator_options GetValidatorOptions(
      spv_const_markv_decoder_options options) {
    return options->validate_spirv_binary ?
        spvValidatorOptionsCreate() : nullptr;
  }

  // Reads a single bit from reader_. The read bit is stored in |bit|.
  // Returns false iff reader_ fails.
  bool ReadBit(bool* bit) {
    uint64_t bits = 0;
    const bool result = reader_.ReadBits(&bits, 1);
    if (result)
      *bit = bits ? true : false;
    return result;
  };

  // Returns ReadBit bound to the class object.
  std::function<bool(bool*)> GetReadBitCallback() {
    return std::bind(&MarkvDecoder::ReadBit, this, std::placeholders::_1);
  }

  // Reads a single non-id word from bit stream. operand_.type determines if
  // the word needs to be decoded and how.
  spv_result_t DecodeNonIdWord(uint32_t* word);

  // Reads and decodes both opcode and num_operands as a single code.
  // Returns SPV_UNSUPPORTED iff no suitable codec was found.
  spv_result_t DecodeOpcodeAndNumberOfOperands(uint32_t* opcode,
                                               uint32_t* num_operands);

  // Reads mtf rank from bit stream. |mtf| is used to determine the codec
  // scheme. |fallback_method| is used if no codec defined for |mtf|.
  spv_result_t DecodeMtfRankHuffman(uint64_t mtf, uint32_t fallback_method,
                                    uint32_t* rank);

  // Reads id using coding based on mtf associated with the id descriptor.
  // Returns SPV_UNSUPPORTED iff fallback method needs to be used.
  spv_result_t DecodeIdWithDescriptor(uint32_t* id);

  // Reads id using coding based on the given |mtf|, which is expected to
  // contain the needed |id|.
  spv_result_t DecodeExistingId(uint64_t mtf, uint32_t* id);

  // Reads type id of the current instruction if can't be inferred.
  spv_result_t DecodeTypeId();

  // Reads result id of the current instruction if can't be inferred.
  spv_result_t DecodeResultId();

  // Reads id which is neither type nor result id.
  spv_result_t DecodeRefId(uint32_t* id);

  // Reads and discards bits until the beginning of the next byte if the
  // number of bits until the next byte is less than |byte_break_if_less_than|.
  bool ReadToByteBreak(size_t byte_break_if_less_than);

  // Returns instruction words decoded up to this point.
  const uint32_t* GetInstWords() const override {
    return inst_words_.data();
  }

  // Reads a literal number as it is described in |operand| from the bit stream,
  // decodes and writes it to spirv_.
  spv_result_t DecodeLiteralNumber(const spv_parsed_operand_t& operand);

  // Reads instruction from bit stream, decodes and validates it.
  // Decoded instruction is valid until the next call of DecodeInstruction().
  spv_result_t DecodeInstruction();

  // Read operand from the stream decodes and validates it.
  spv_result_t DecodeOperand(size_t operand_offset,
                             const spv_operand_type_t type,
                             spv_operand_pattern_t* expected_operands);

  // Records the numeric type for an operand according to the type information
  // associated with the given non-zero type Id.  This can fail if the type Id
  // is not a type Id, or if the type Id does not reference a scalar numeric
  // type.  On success, return SPV_SUCCESS and populates the num_words,
  // number_kind, and number_bit_width fields of parsed_operand.
  spv_result_t SetNumericTypeInfoForType(spv_parsed_operand_t* parsed_operand,
                                         uint32_t type_id);

  // Records the number type for the current instruction, if it generates a
  // type. For types that aren't scalar numbers, record something with number
  // kind SPV_NUMBER_NONE.
  void RecordNumberType();

  spv_const_markv_decoder_options options_;

  // Temporary sink where decoded SPIR-V words are written. Once it contains the
  // entire module, the container is moved and returned.
  std::vector<uint32_t> spirv_;

  // Bit stream containing encoded data.
  BitReaderWord64 reader_;

  // Temporary storage for operands of the currently parsed instruction.
  // Valid until next DecodeInstruction call.
  std::vector<spv_parsed_operand_t> parsed_operands_;

  // Temporary storage for current instruction words.
  // Valid until next DecodeInstruction call.
  std::vector<uint32_t> inst_words_;

  // Maps a type ID to its number type description.
  std::unordered_map<uint32_t, NumberType> type_id_to_number_type_info_;

  // Maps an ExtInstImport id to the extended instruction type.
  std::unordered_map<uint32_t, spv_ext_inst_type_t> import_id_to_ext_inst_type_;
};

void MarkvCodecBase::ProcessCurInstruction() {
  instructions_.emplace_back(new Instruction(&inst_));

  const SpvOp opcode = SpvOp(inst_.opcode);

  if (inst_.result_id) {
    id_to_def_instruction_.emplace(inst_.result_id, instructions_.back().get());

    // Collect ids local to the current function.
    if (cur_function_id_){
      ids_local_to_cur_function_.push_back(inst_.result_id);
    }

    // Starting new function.
    if (opcode == SpvOpFunction) {
      cur_function_id_ = inst_.result_id;
      cur_function_return_type_ = inst_.type_id;
      multi_mtf_.Insert(GetMtfFunctionWithReturnType(inst_.type_id),
                        inst_.result_id);

      // Store function parameter types in a queue, so that we know which types
      // to expect in the following OpFunctionParameter instructions.
      const Instruction* def_inst = FindDef(inst_.words[4]);
      assert(def_inst);
      assert(def_inst->opcode() == SpvOpTypeFunction);
      for (uint32_t i = 3; i < def_inst->words().size(); ++i) {
        remaining_function_parameter_types_.push_back(def_inst->word(i));
      }
    }
  }

  // Remove local ids from MTFs if function end.
  if (opcode == SpvOpFunctionEnd) {
    cur_function_id_ = 0;
    for (uint32_t id : ids_local_to_cur_function_)
      multi_mtf_.RemoveFromAll(id);
    ids_local_to_cur_function_.clear();
    assert(remaining_function_parameter_types_.empty());
  }

  if (!inst_.result_id)
    return;

  {
    // Save the result ID to type ID mapping.
    // In the grammar, type ID always appears before result ID.
    // A regular value maps to its type. Some instructions (e.g. OpLabel)
    // have no type Id, and will map to 0. The result Id for a
    // type-generating instruction (e.g. OpTypeInt) maps to itself.
    auto insertion_result = id_to_type_id_.emplace(
        inst_.result_id,
        spvOpcodeGeneratesType(SpvOp(inst_.opcode)) ?
        inst_.result_id : inst_.type_id);
    (void)insertion_result;
    assert(insertion_result.second);
  }

  // Add result_id to MTFs.

  switch (opcode) {
    case SpvOpTypeFloat:
    case SpvOpTypeInt:
    case SpvOpTypeBool:
    case SpvOpTypeVector:
    case SpvOpTypePointer:
    case SpvOpExtInstImport:
    case SpvOpTypeSampledImage:
    case SpvOpTypeImage:
    case SpvOpTypeSampler:
      multi_mtf_.Insert(GetMtfIdGeneratedByOpcode(opcode), inst_.result_id);
      break;
    default:
      break;
  }

  if (spvOpcodeIsComposite(opcode)) {
    multi_mtf_.Insert(kMtfTypeComposite, inst_.result_id);
  }

  if (opcode == SpvOpLabel) {
    multi_mtf_.InsertOrPromote(kMtfLabel, inst_.result_id);
  }

  if (opcode == SpvOpTypeInt) {
    multi_mtf_.Insert(kMtfTypeScalar, inst_.result_id);
    multi_mtf_.Insert(kMtfTypeIntScalarOrVector, inst_.result_id);
  }

  if (opcode == SpvOpTypeFloat) {
    multi_mtf_.Insert(kMtfTypeScalar, inst_.result_id);
    multi_mtf_.Insert(kMtfTypeFloatScalarOrVector, inst_.result_id);
  }

  if (opcode == SpvOpTypeBool) {
    multi_mtf_.Insert(kMtfTypeScalar, inst_.result_id);
    multi_mtf_.Insert(kMtfTypeBoolScalarOrVector, inst_.result_id);
  }

  if (opcode == SpvOpTypeVector) {
    const uint32_t component_type_id = inst_.words[2];
    const uint32_t size = inst_.words[3];
    if (multi_mtf_.HasValue(GetMtfIdGeneratedByOpcode(SpvOpTypeFloat),
                            component_type_id)) {
      multi_mtf_.Insert(kMtfTypeFloatScalarOrVector, inst_.result_id);
    } else if (multi_mtf_.HasValue(GetMtfIdGeneratedByOpcode(SpvOpTypeInt),
                            component_type_id)) {
      multi_mtf_.Insert(kMtfTypeIntScalarOrVector, inst_.result_id);
    } else if (multi_mtf_.HasValue(GetMtfIdGeneratedByOpcode(SpvOpTypeBool),
                            component_type_id)) {
      multi_mtf_.Insert(kMtfTypeBoolScalarOrVector, inst_.result_id);
    }
    multi_mtf_.Insert(GetMtfTypeVectorOfSize(size), inst_.result_id);
  }

  if (inst_.opcode == SpvOpTypeFunction) {
    const uint32_t return_type = inst_.words[2];
    multi_mtf_.Insert(kMtfTypeReturnedByFunction, return_type);
    multi_mtf_.Insert(GetMtfFunctionTypeWithReturnType(return_type),
                      inst_.result_id);
  }

  if (inst_.type_id) {
    const Instruction* type_inst = FindDef(inst_.type_id);
    assert(type_inst);

    multi_mtf_.Insert(kMtfObject, inst_.result_id);

    multi_mtf_.Insert(GetMtfIdOfType(inst_.type_id), inst_.result_id);

    if (multi_mtf_.HasValue(kMtfTypeFloatScalarOrVector, inst_.type_id)) {
      multi_mtf_.Insert(kMtfFloatScalarOrVector, inst_.result_id);
    }

    if (multi_mtf_.HasValue(kMtfTypeIntScalarOrVector, inst_.type_id))
      multi_mtf_.Insert(kMtfIntScalarOrVector, inst_.result_id);

    if (multi_mtf_.HasValue(kMtfTypeBoolScalarOrVector, inst_.type_id))
      multi_mtf_.Insert(kMtfBoolScalarOrVector, inst_.result_id);

    if (multi_mtf_.HasValue(kMtfTypeComposite, inst_.type_id))
      multi_mtf_.Insert(kMtfComposite, inst_.result_id);

    if (inst_.opcode == SpvOpConstant) {
      if (multi_mtf_.HasValue(
          GetMtfIdGeneratedByOpcode(SpvOpTypeInt), inst_.type_id)) {
        multi_mtf_.Insert(kMtfConstInteger, inst_.result_id);
        const uint32_t presumed_index = inst_.words[3];
        if (presumed_index <= kMarkvMaxPresumedAccessIndex) {
          const auto result =
              presumed_index_to_id_.emplace(presumed_index, inst_.result_id);
          if (result.second) {
            id_to_presumed_index_.emplace(inst_.result_id, presumed_index);
          }
        }
      }
    }

    switch (type_inst->opcode()) {
      case SpvOpTypeInt:
      case SpvOpTypeBool:
      case SpvOpTypePointer:
      case SpvOpTypeVector:
      case SpvOpTypeImage:
      case SpvOpTypeSampledImage:
      case SpvOpTypeSampler:
        multi_mtf_.Insert(GetMtfIdWithTypeGeneratedByOpcode(
            type_inst->opcode()), inst_.result_id);
        break;
      default:
        break;
    }

    if (type_inst->opcode() == SpvOpTypeVector) {
      const uint32_t component_type = type_inst->word(2);
      multi_mtf_.Insert(GetMtfVectorOfComponentType(component_type),
                        inst_.result_id);
    }

    if (type_inst->opcode() == SpvOpTypePointer) {
      assert(type_inst->operands().size() > 2);
      assert(type_inst->words().size() > type_inst->operands()[2].offset);
      const uint32_t data_type =
          type_inst->word(type_inst->operands()[2].offset);
      multi_mtf_.Insert(GetMtfPointerToType(data_type), inst_.result_id);

      if (multi_mtf_.HasValue(kMtfTypeComposite, data_type))
        multi_mtf_.Insert(kMtfTypePointerToComposite, inst_.result_id);
    }
  }

  if (spvOpcodeGeneratesType(opcode)) {
    if (opcode != SpvOpTypeFunction) {
      multi_mtf_.Insert(kMtfTypeNonFunction, inst_.result_id);
    }
  }

  const uint32_t descriptor = id_descriptors_.ProcessInstruction(inst_);
  if (model_->DescriptorHasCodingScheme(descriptor))
    multi_mtf_.Insert(GetMtfIdDescriptor(descriptor), inst_.result_id);
}

uint64_t MarkvCodecBase::GetRuleBasedMtf() {
  // This function is only called for id operands (but not result ids).
  assert(spvIsIdType(operand_.type) ||
         operand_.type == SPV_OPERAND_TYPE_OPTIONAL_ID);
  assert(operand_.type != SPV_OPERAND_TYPE_RESULT_ID);

  const SpvOp opcode = static_cast<SpvOp>(inst_.opcode);

  // All operand slots which expect label id.
  if ((inst_.opcode == SpvOpLoopMerge && operand_index_ <= 1) ||
      (inst_.opcode == SpvOpSelectionMerge && operand_index_ == 0) ||
      (inst_.opcode == SpvOpBranch && operand_index_ == 0) ||
      (inst_.opcode == SpvOpBranchConditional &&
       (operand_index_ == 1 || operand_index_ == 2 )) ||
      (inst_.opcode == SpvOpPhi && operand_index_ >= 3 &&
       operand_index_ % 2 == 1) ||
      (inst_.opcode == SpvOpSwitch && operand_index_ > 0)) {
    return kMtfLabel;
  }

  switch (opcode) {
    case SpvOpFAdd:
    case SpvOpFSub:
    case SpvOpFMul:
    case SpvOpFDiv:
    case SpvOpFRem:
    case SpvOpFMod:
    case SpvOpFNegate: {
      if (operand_index_ == 0)
        return kMtfTypeFloatScalarOrVector;

      return GetMtfIdOfType(inst_.type_id);
    }

    case SpvOpISub:
    case SpvOpIAdd:
    case SpvOpIMul:
    case SpvOpSDiv:
    case SpvOpUDiv:
    case SpvOpSMod:
    case SpvOpUMod:
    case SpvOpSRem:
    case SpvOpSNegate: {
      if (operand_index_ == 0)
        return kMtfTypeIntScalarOrVector;

      return kMtfIntScalarOrVector;
    }

    // TODO(atgoo@github.com) Add OpConvertFToU and other opcodes.

    case SpvOpFOrdEqual:
    case SpvOpFUnordEqual:
    case SpvOpFOrdNotEqual:
    case SpvOpFUnordNotEqual:
    case SpvOpFOrdLessThan:
    case SpvOpFUnordLessThan:
    case SpvOpFOrdGreaterThan:
    case SpvOpFUnordGreaterThan:
    case SpvOpFOrdLessThanEqual:
    case SpvOpFUnordLessThanEqual:
    case SpvOpFOrdGreaterThanEqual:
    case SpvOpFUnordGreaterThanEqual: {
      if (operand_index_ == 0)
        return kMtfTypeBoolScalarOrVector;
      if (operand_index_ == 2)
        return kMtfFloatScalarOrVector;
      if (operand_index_ == 3) {
        const uint32_t first_operand_id = GetInstWords()[3];
        const uint32_t first_operand_type =
            id_to_type_id_.at(first_operand_id);
        return GetMtfIdOfType(first_operand_type);
      }
      break;
    }

    case SpvOpVectorShuffle: {
      if (operand_index_ == 0) {
        assert(inst_.num_operands > 4);
        return GetMtfTypeVectorOfSize(inst_.num_operands - 4);
      }

      assert(inst_.type_id);
      if (operand_index_ == 2 || operand_index_ == 3)
        return GetMtfVectorOfComponentType(
            GetVectorComponentType(inst_.type_id));
      break;
    }

    case SpvOpVectorTimesScalar: {
      if (operand_index_ == 0) {
        // TODO(atgoo@github.com) Could be narrowed to vector of floats.
        return GetMtfIdGeneratedByOpcode(SpvOpTypeVector);
      }

      assert(inst_.type_id);
      if (operand_index_ == 2)
        return GetMtfIdOfType(inst_.type_id);
      if (operand_index_ == 3)
        return GetMtfIdOfType(GetVectorComponentType(inst_.type_id));
      break;
    }

    case SpvOpDot: {
      if (operand_index_ == 0)
        return GetMtfIdGeneratedByOpcode(SpvOpTypeFloat);

      assert(inst_.type_id);
      if (operand_index_ == 2)
        return GetMtfVectorOfComponentType(inst_.type_id);
      if (operand_index_ == 3) {
        const uint32_t vector_id = GetInstWords()[3];
        const uint32_t vector_type = id_to_type_id_.at(vector_id);
        return GetMtfIdOfType(vector_type);
      }
      break;
    }

    case SpvOpTypeVector: {
      if (operand_index_ == 1) {
        return kMtfTypeScalar;
      }
      break;
    }

    case SpvOpTypeMatrix: {
      if (operand_index_ == 1) {
        return GetMtfIdGeneratedByOpcode(SpvOpTypeVector);
      }
      break;
    }

    case SpvOpTypePointer: {
      if (operand_index_ == 2) {
        return kMtfTypeNonFunction;
      }
      break;
    }

    case SpvOpTypeStruct: {
      if (operand_index_ >= 1) {
        return kMtfTypeNonFunction;
      }
      break;
    }

    case SpvOpTypeFunction: {
      if (operand_index_ == 1) {
        return kMtfTypeNonFunction;
      }

      if (operand_index_ >= 2) {
        return kMtfTypeNonFunction;
      }
      break;
    }

    case SpvOpLoad: {
      if (operand_index_ == 0)
        return kMtfTypeNonFunction;

      if (operand_index_ == 2) {
        assert(inst_.type_id);
        return GetMtfPointerToType(inst_.type_id);
      }
      break;
    }

    case SpvOpStore: {
      if (operand_index_ == 0)
        return GetMtfIdWithTypeGeneratedByOpcode(SpvOpTypePointer);
      if (operand_index_ == 1) {
        const uint32_t pointer_id = GetInstWords()[1];
        const uint32_t pointer_type = id_to_type_id_.at(pointer_id);
        const Instruction* pointer_inst = FindDef(pointer_type);
        assert(pointer_inst);
        assert(pointer_inst->opcode() == SpvOpTypePointer);
        const uint32_t data_type =
            pointer_inst->word(pointer_inst->operands()[2].offset);
        return GetMtfIdOfType(data_type);
      }
      break;
    }

    case SpvOpVariable: {
      if (operand_index_ == 0)
        return GetMtfIdGeneratedByOpcode(SpvOpTypePointer);
      break;
    }

    case SpvOpAccessChain: {
      if (operand_index_ == 0)
        return GetMtfIdGeneratedByOpcode(SpvOpTypePointer);
      if (operand_index_ == 2)
        return kMtfTypePointerToComposite;
      if (operand_index_ >= 3)
        return GetMtfIdWithTypeGeneratedByOpcode(SpvOpTypeInt);
      break;
    }

    case SpvOpCompositeConstruct: {
      if (operand_index_ == 0)
        return kMtfTypeComposite;
      if (operand_index_ >= 2) {
        const uint32_t composite_type = GetInstWords()[1];
        if (multi_mtf_.HasValue(kMtfTypeFloatScalarOrVector, composite_type))
          return kMtfFloatScalarOrVector;
        if (multi_mtf_.HasValue(kMtfTypeIntScalarOrVector, composite_type))
          return kMtfIntScalarOrVector;
        if (multi_mtf_.HasValue(kMtfTypeBoolScalarOrVector, composite_type))
          return kMtfBoolScalarOrVector;
      }
      break;
    }

    case SpvOpCompositeExtract: {
      if (operand_index_ == 2)
        return kMtfComposite;
      break;
    }

    case SpvOpConstantComposite: {
      if (operand_index_ == 0)
        return kMtfTypeComposite;
      if (operand_index_ >= 2) {
        const Instruction* composite_type_inst = FindDef(inst_.type_id);
        assert(composite_type_inst);
        if (composite_type_inst->opcode() == SpvOpTypeVector) {
          return GetMtfIdOfType(composite_type_inst->word(2));
        }
      }
      break;
    }

    case SpvOpExtInst: {
      if (operand_index_ == 2)
        return GetMtfIdGeneratedByOpcode(SpvOpExtInstImport);
      if (operand_index_ >= 4) {
        const uint32_t return_type = GetInstWords()[1];
        const uint32_t ext_inst_type = inst_.ext_inst_type;
        const uint32_t ext_inst_index = GetInstWords()[4];
        // TODO(atgoo@github.com) The list of extended instructions is
        // incomplete. Only common instructions and low-hanging fruits listed.
        if (ext_inst_type == SPV_EXT_INST_TYPE_GLSL_STD_450) {
          switch (ext_inst_index) {
            case GLSLstd450FAbs:
            case GLSLstd450FClamp:
            case GLSLstd450FMax:
            case GLSLstd450FMin:
            case GLSLstd450FMix:
            case GLSLstd450Step:
            case GLSLstd450SmoothStep:
            case GLSLstd450Fma:
            case GLSLstd450Pow:
            case GLSLstd450Exp:
            case GLSLstd450Exp2:
            case GLSLstd450Log:
            case GLSLstd450Log2:
            case GLSLstd450Sqrt:
            case GLSLstd450InverseSqrt:
            case GLSLstd450Fract:
            case GLSLstd450Floor:
            case GLSLstd450Ceil:
            case GLSLstd450Radians:
            case GLSLstd450Degrees:
            case GLSLstd450Sin:
            case GLSLstd450Cos:
            case GLSLstd450Tan:
            case GLSLstd450Sinh:
            case GLSLstd450Cosh:
            case GLSLstd450Tanh:
            case GLSLstd450Asin:
            case GLSLstd450Acos:
            case GLSLstd450Atan:
            case GLSLstd450Atan2:
            case GLSLstd450Asinh:
            case GLSLstd450Acosh:
            case GLSLstd450Atanh:
            case GLSLstd450MatrixInverse:
            case GLSLstd450Cross:
            case GLSLstd450Normalize:
            case GLSLstd450Reflect:
            case GLSLstd450FaceForward:
              return GetMtfIdOfType(return_type);
            case GLSLstd450Length:
            case GLSLstd450Distance:
            case GLSLstd450Refract:
              return kMtfFloatScalarOrVector;
            default:
              break;
          }
        } else if (ext_inst_type == SPV_EXT_INST_TYPE_OPENCL_STD) {
          switch (ext_inst_index) {
            case OpenCLLIB::Fabs:
            case OpenCLLIB::FClamp:
            case OpenCLLIB::Fmax:
            case OpenCLLIB::Fmin:
            case OpenCLLIB::Step:
            case OpenCLLIB::Smoothstep:
            case OpenCLLIB::Fma:
            case OpenCLLIB::Pow:
            case OpenCLLIB::Exp:
            case OpenCLLIB::Exp2:
            case OpenCLLIB::Log:
            case OpenCLLIB::Log2:
            case OpenCLLIB::Sqrt:
            case OpenCLLIB::Rsqrt:
            case OpenCLLIB::Fract:
            case OpenCLLIB::Floor:
            case OpenCLLIB::Ceil:
            case OpenCLLIB::Radians:
            case OpenCLLIB::Degrees:
            case OpenCLLIB::Sin:
            case OpenCLLIB::Cos:
            case OpenCLLIB::Tan:
            case OpenCLLIB::Sinh:
            case OpenCLLIB::Cosh:
            case OpenCLLIB::Tanh:
            case OpenCLLIB::Asin:
            case OpenCLLIB::Acos:
            case OpenCLLIB::Atan:
            case OpenCLLIB::Atan2:
            case OpenCLLIB::Asinh:
            case OpenCLLIB::Acosh:
            case OpenCLLIB::Atanh:
            case OpenCLLIB::Cross:
            case OpenCLLIB::Normalize:
              return GetMtfIdOfType(return_type);
            case OpenCLLIB::Length:
            case OpenCLLIB::Distance:
              return kMtfFloatScalarOrVector;
            default:
              break;
          }
        }
      }
      break;
    }

    case SpvOpFunction: {
      if (operand_index_ == 0)
        return kMtfTypeReturnedByFunction;

      if (operand_index_ == 3) {
        const uint32_t return_type = GetInstWords()[1];
        return GetMtfFunctionTypeWithReturnType(return_type);
      }
      break;
    }

    case SpvOpFunctionCall: {
      if (operand_index_ == 0)
        return kMtfTypeReturnedByFunction;

      if (operand_index_ == 2) {
        const uint32_t return_type = GetInstWords()[1];
        return GetMtfFunctionWithReturnType(return_type);
      }

      if (operand_index_ >= 3) {
        const uint32_t function_id = GetInstWords()[3];
        const Instruction* function_inst = FindDef(function_id);
        if (!function_inst)
          return kMtfObject;

        assert(function_inst->opcode() == SpvOpFunction);

        const uint32_t function_type_id = function_inst->word(4);
        const Instruction* function_type_inst = FindDef(function_type_id);
        assert(function_type_inst);
        assert(function_type_inst->opcode() == SpvOpTypeFunction);

        const uint32_t argument_type =
            function_type_inst->word(operand_index_);
        return GetMtfIdOfType(argument_type);
      }
      break;
    }

    case SpvOpReturnValue: {
      if (operand_index_ == 0)
        return GetMtfIdOfType(cur_function_return_type_);
      break;
    }

    case SpvOpBranchConditional: {
      if (operand_index_ == 0)
        return GetMtfIdWithTypeGeneratedByOpcode(SpvOpTypeBool);
      break;
    }

    case SpvOpSampledImage: {
      if (operand_index_ == 0)
        return GetMtfIdGeneratedByOpcode(SpvOpTypeSampledImage);
      if (operand_index_ == 2)
        return GetMtfIdWithTypeGeneratedByOpcode(SpvOpTypeImage);
      if (operand_index_ == 3)
        return GetMtfIdWithTypeGeneratedByOpcode(SpvOpTypeSampler);
      break;
    }

    case SpvOpImageSampleImplicitLod: {
      if (operand_index_ == 0)
        return GetMtfIdGeneratedByOpcode(SpvOpTypeVector);
      if (operand_index_ == 2)
        return GetMtfIdWithTypeGeneratedByOpcode(SpvOpTypeSampledImage);
      if (operand_index_ == 3)
        return GetMtfIdWithTypeGeneratedByOpcode(SpvOpTypeVector);
      break;
    }

    default:
      break;
  }

  return kMtfNone;
}

spv_result_t MarkvEncoder::EncodeNonIdWord(uint32_t word) {
  auto* codec = model_->GetNonIdWordHuffmanCodec(inst_.opcode, operand_index_);

  if (codec) {
    uint64_t bits = 0;
    size_t num_bits = 0;
    if (codec->Encode(word, &bits, &num_bits)) {
      // Encoding successful.
      writer_.WriteBits(bits, num_bits);
      return SPV_SUCCESS;
    } else {
      // Encoding failed, write kMarkvNoneOfTheAbove flag.
      if (!codec->Encode(kMarkvNoneOfTheAbove, &bits, &num_bits))
        return Diag(SPV_ERROR_INTERNAL)
            << "Non-id word Huffman table for "
            << spvOpcodeString(SpvOp(inst_.opcode))
            << " operand index " << operand_index_
            << " is missing kMarkvNoneOfTheAbove";
      writer_.WriteBits(bits, num_bits);
    }
  }

  // Fallback encoding.
  const size_t chunk_length = GetOperandVariableWidthChunkLength(operand_.type);
  if (chunk_length) {
    writer_.WriteVariableWidthU32(word, chunk_length);
  } else {
    writer_.WriteUnencoded(word);
  }
  return SPV_SUCCESS;
}

spv_result_t MarkvDecoder::DecodeNonIdWord(uint32_t* word) {
  auto* codec = model_->GetNonIdWordHuffmanCodec(inst_.opcode, operand_index_);

  if (codec) {
    uint64_t decoded_value = 0;
    if (!codec->DecodeFromStream(GetReadBitCallback(), &decoded_value))
      return Diag(SPV_ERROR_INVALID_BINARY)
          << "Failed to decode non-id word with Huffman";

    if (decoded_value != kMarkvNoneOfTheAbove) {
      // The word decoded successfully.
      *word = uint32_t(decoded_value);
      assert(*word == decoded_value);
      return SPV_SUCCESS;
    }

    // Received kMarkvNoneOfTheAbove signal, use fallback decoding.
  }

  const size_t chunk_length = GetOperandVariableWidthChunkLength(operand_.type);
  if (chunk_length) {
    if (!reader_.ReadVariableWidthU32(word, chunk_length))
      return Diag(SPV_ERROR_INVALID_BINARY)
          << "Failed to decode non-id word with varint";
  } else {
    if (!reader_.ReadUnencoded(word))
      return Diag(SPV_ERROR_INVALID_BINARY)
          << "Failed to read unencoded non-id word";
  }
  return SPV_SUCCESS;
}

spv_result_t MarkvEncoder::EncodeOpcodeAndNumOperands(
    uint32_t opcode, uint32_t num_operands) {
  uint64_t bits = 0;
  size_t num_bits = 0;

  const uint32_t word = opcode | (num_operands << 16);

  // First try to use the Markov chain codec.
  auto* codec = model_->GetOpcodeAndNumOperandsMarkovHuffmanCodec(GetPrevOpcode());
  if (codec) {
    if (codec->Encode(word, &bits, &num_bits)) {
      // The word was successfully encoded into bits/num_bits.
      writer_.WriteBits(bits, num_bits);
      return SPV_SUCCESS;
    } else {
      // The word is not in the Huffman table. Write kMarkvNoneOfTheAbove
      // and use fallback encoding.
      if (!codec->Encode(kMarkvNoneOfTheAbove, &bits, &num_bits))
        return Diag(SPV_ERROR_INTERNAL)
            << "opcode_and_num_operands Huffman table for "
            << spvOpcodeString(GetPrevOpcode())
            << "is missing kMarkvNoneOfTheAbove";
      writer_.WriteBits(bits, num_bits);
    }
  }

  // Fallback to base-rate codec.
  codec = model_->GetOpcodeAndNumOperandsMarkovHuffmanCodec(SpvOpNop);
  assert(codec);
  if (codec->Encode(word, &bits, &num_bits)) {
    // The word was successfully encoded into bits/num_bits.
    writer_.WriteBits(bits, num_bits);
    return SPV_SUCCESS;
  } else {
    // The word is not in the Huffman table. Write kMarkvNoneOfTheAbove
    // and return false.
    if (!codec->Encode(kMarkvNoneOfTheAbove, &bits, &num_bits))
      return Diag(SPV_ERROR_INTERNAL)
          << "Global opcode_and_num_operands Huffman table is missing "
          << "kMarkvNoneOfTheAbove";
    writer_.WriteBits(bits, num_bits);
    return SPV_UNSUPPORTED;
  }
}

spv_result_t MarkvDecoder::DecodeOpcodeAndNumberOfOperands(
    uint32_t* opcode, uint32_t* num_operands) {
  // First try to use the Markov chain codec.
  auto* codec = model_->GetOpcodeAndNumOperandsMarkovHuffmanCodec(GetPrevOpcode());
  if (codec) {
    uint64_t decoded_value = 0;
    if (!codec->DecodeFromStream(GetReadBitCallback(), &decoded_value))
      return Diag(SPV_ERROR_INTERNAL)
          << "Failed to decode opcode_and_num_operands, previous opcode is "
          << spvOpcodeString(GetPrevOpcode());

    if (decoded_value != kMarkvNoneOfTheAbove) {
      // The word was successfully decoded.
      *opcode = uint32_t(decoded_value & 0xFFFF);
      *num_operands = uint32_t(decoded_value >> 16);
      return SPV_SUCCESS;
    }

    // Received kMarkvNoneOfTheAbove signal, use fallback decoding.
  }

  // Fallback to base-rate codec.
  codec = model_->GetOpcodeAndNumOperandsMarkovHuffmanCodec(SpvOpNop);
  assert(codec);
  uint64_t decoded_value = 0;
  if (!codec->DecodeFromStream(GetReadBitCallback(), &decoded_value))
    return Diag(SPV_ERROR_INTERNAL)
        << "Failed to decode opcode_and_num_operands with global codec";

  if (decoded_value == kMarkvNoneOfTheAbove) {
    // Received kMarkvNoneOfTheAbove signal, fallback further.
    return SPV_UNSUPPORTED;
  }

  *opcode = uint32_t(decoded_value & 0xFFFF);
  *num_operands = uint32_t(decoded_value >> 16);
  return SPV_SUCCESS;
}

spv_result_t MarkvEncoder::EncodeMtfRankHuffman(uint32_t rank, uint64_t mtf,
                                                uint64_t fallback_method) {
  const auto* codec = model_->GetMtfHuffmanCodec(mtf);
  if (!codec) {
    assert(fallback_method != kMtfNone);
    codec = model_->GetMtfHuffmanCodec(fallback_method);
  }

  if (!codec)
    return Diag(SPV_ERROR_INTERNAL) << "No codec to encode MTF rank";

  uint64_t bits = 0;
  size_t num_bits = 0;
  if (rank < kMtfSmallestRankEncodedByValue) {
    // Encode using Huffman coding.
    if (!codec->Encode(rank, &bits, &num_bits))
      return Diag(SPV_ERROR_INTERNAL)
          << "Failed to encode MTF rank with Huffman";

    writer_.WriteBits(bits, num_bits);
  } else {
    // Encode by value.
    if (!codec->Encode(kMtfRankEncodedByValueSignal, &bits, &num_bits))
      return Diag(SPV_ERROR_INTERNAL)
          << "Failed to encode kMtfRankEncodedByValueSignal";

    writer_.WriteBits(bits, num_bits);
    writer_.WriteVariableWidthU32(rank - kMtfSmallestRankEncodedByValue,
                                  model_->mtf_rank_chunk_length());
  }
  return SPV_SUCCESS;
}

spv_result_t MarkvDecoder::DecodeMtfRankHuffman(
    uint64_t mtf, uint32_t fallback_method, uint32_t* rank) {
  const auto* codec = model_->GetMtfHuffmanCodec(mtf);
  if (!codec) {
    assert(fallback_method != kMtfNone);
    codec = model_->GetMtfHuffmanCodec(fallback_method);
  }

  if (!codec)
    return Diag(SPV_ERROR_INTERNAL) << "No codec to decode MTF rank";

  uint32_t decoded_value = 0;
  if (!codec->DecodeFromStream(GetReadBitCallback(), &decoded_value))
    return Diag(SPV_ERROR_INTERNAL)
        << "Failed to decode MTF rank with Huffman";

  if (decoded_value == kMtfRankEncodedByValueSignal) {
    // Decode by value.
    if (!reader_.ReadVariableWidthU32(rank, model_->mtf_rank_chunk_length()))
      return Diag(SPV_ERROR_INTERNAL)
          << "Failed to decode MTF rank with varint";
    *rank += kMtfSmallestRankEncodedByValue;
  } else {
    // Decode using Huffman coding.
    assert(decoded_value < kMtfSmallestRankEncodedByValue);
    *rank = decoded_value;
  }
  return SPV_SUCCESS;
}

spv_result_t MarkvEncoder::EncodeIdWithDescriptor(uint32_t id) {
  auto* codec = model_->GetIdDescriptorHuffmanCodec(inst_.opcode,
                                                    operand_index_);
  if (!codec)
    return SPV_UNSUPPORTED;

  uint64_t bits = 0;
  size_t num_bits = 0;

  // Get the descriptor for id.
  const uint32_t descriptor = id_descriptors_.GetDescriptor(id);

  if (descriptor && codec->Encode(descriptor, &bits, &num_bits)) {
    // If the descriptor exists and is in the table, write the descriptor and
    // proceed to encoding the rank.
    writer_.WriteBits(bits, num_bits);
  } else {
    // The descriptor doesn't exist or we have no coding for it. Write
    // kMarkvNoneOfTheAbove and go to fallback method.
    if (!codec->Encode(kMarkvNoneOfTheAbove, &bits, &num_bits))
      return Diag(SPV_ERROR_INTERNAL)
          << "Descriptor Huffman table for "
          << spvOpcodeString(SpvOp(inst_.opcode))
          << " operand index " << operand_index_
          << " is missing kMarkvNoneOfTheAbove";

    writer_.WriteBits(bits, num_bits);
    return SPV_UNSUPPORTED;
  }

  // Descriptor has been encoded. Now encode the rank of the id in the
  // associated mtf sequence.
  const uint64_t mtf = GetMtfIdDescriptor(descriptor);
  return EncodeExistingId(mtf, id);
}

spv_result_t MarkvDecoder::DecodeIdWithDescriptor(uint32_t* id) {
  auto* codec = model_->GetIdDescriptorHuffmanCodec(inst_.opcode,
                                                    operand_index_);
  if (!codec)
    return SPV_UNSUPPORTED;

  uint64_t decoded_value = 0;
  if (!codec->DecodeFromStream(GetReadBitCallback(), &decoded_value))
    return Diag(SPV_ERROR_INTERNAL)
        << "Failed to decode descriptor with Huffman";

  if (decoded_value == kMarkvNoneOfTheAbove)
    return SPV_UNSUPPORTED;

  // If descriptor exists then the id was encoded through descriptor mtf.
  const uint32_t descriptor = uint32_t(decoded_value);
  assert(descriptor == decoded_value);
  assert(descriptor);

  const uint64_t mtf = GetMtfIdDescriptor(descriptor);
  return DecodeExistingId(mtf, id);
}

spv_result_t MarkvEncoder::EncodeExistingId(uint64_t mtf, uint32_t id) {
  assert(multi_mtf_.GetSize(mtf) > 0);
  if (multi_mtf_.GetSize(mtf) == 1) {
    // If the sequence has only one element no need to write rank, the decoder
    // would make the same decision.
    return SPV_SUCCESS;
  }

  uint32_t rank = 0;
  if (!multi_mtf_.RankFromValue(mtf, id, &rank))
    return Diag(SPV_ERROR_INTERNAL)
        << "Id is not in the MTF sequence";

  return EncodeMtfRankHuffman(rank, mtf, kMtfGenericNonZeroRank);
}

spv_result_t MarkvDecoder::DecodeExistingId(uint64_t mtf, uint32_t* id) {
  assert(multi_mtf_.GetSize(mtf) > 0);
  *id = 0;

  uint32_t rank = 0;

  if (multi_mtf_.GetSize(mtf) == 1) {
    rank = 1;
  } else {
    const spv_result_t result =
        DecodeMtfRankHuffman(mtf, kMtfGenericNonZeroRank, &rank);
    if (result != SPV_SUCCESS)
      return result;
  }

  assert(rank);
  if (!multi_mtf_.ValueFromRank(mtf, rank, id))
    return Diag(SPV_ERROR_INTERNAL)
        << "MTF rank is out of bounds";

  return SPV_SUCCESS;
}

spv_result_t MarkvEncoder::EncodeRefId(uint32_t id) {
  // TODO(atgoo@github.com) This might not be needed as EncodeIdWithDescriptor
  // can handle SpvOpAccessChain indices if enough statistics is collected.
  if (inst_.opcode == SpvOpAccessChain && operand_index_ >= 3) {
    const auto it = id_to_presumed_index_.find(id);
    if (it != id_to_presumed_index_.end()) {
      writer_.WriteBits(1, 1);
      writer_.WriteFixedWidth(it->second, kMarkvMaxPresumedAccessIndex);
      return SPV_SUCCESS;
    }

    writer_.WriteBits(0, 1);
  }

  {
    // Try to encode using id descriptor mtfs.
    const spv_result_t result = EncodeIdWithDescriptor(id);
    if (result != SPV_UNSUPPORTED)
      return result;
    // If can't be done continue with other methods.
  }

  // Encode using rule-based mtf.
  uint64_t mtf = GetRuleBasedMtf();
  const bool can_forward_declare =
      spvOperandCanBeForwardDeclaredFunction(
          SpvOp(inst_.opcode))(operand_index_);

  if (mtf != kMtfNone && !can_forward_declare) {
    assert(multi_mtf_.HasValue(kMtfAll, id));
    return EncodeExistingId(mtf, id);
  }

  if (mtf == kMtfNone)
    mtf = kMtfAll;

  uint32_t rank = 0;

  if (!multi_mtf_.RankFromValue(mtf, id, &rank)) {
    // This is the first occurrence of a forward declared id.
    multi_mtf_.Insert(kMtfAll, id);
    multi_mtf_.Insert(kMtfForwardDeclared, id);
    if (mtf != kMtfAll)
      multi_mtf_.Insert(mtf, id);
    rank = 0;
  }

  return EncodeMtfRankHuffman(rank, mtf, kMtfAll);
}

spv_result_t MarkvDecoder::DecodeRefId(uint32_t* id) {
  if (inst_.opcode == SpvOpAccessChain && operand_index_ >= 3) {
    uint64_t use_presumed_index_technique = 0;
    if (!reader_.ReadBits(&use_presumed_index_technique, 1))
      return Diag(SPV_ERROR_INVALID_BINARY)
          << "Failed to read use_presumed_index_technique flag";

    if (use_presumed_index_technique) {
      uint64_t value = 0;
      if (!reader_.ReadFixedWidth(&value, kMarkvMaxPresumedAccessIndex))
        return Diag(SPV_ERROR_INVALID_BINARY)
            << "Failed to read presumed_index";

      const uint32_t presumed_index = static_cast<uint32_t>(value);

      const auto it = presumed_index_to_id_.find(presumed_index);
      if (it == presumed_index_to_id_.end()) {
        assert(0);
        return Diag(SPV_ERROR_INTERNAL)
            << "Presumed index id not found";
      }

      *id = it->second;
      return SPV_SUCCESS;
    }
  }

  {
    const spv_result_t result = DecodeIdWithDescriptor(id);
    if (result != SPV_UNSUPPORTED)
      return result;
  }

  uint64_t mtf = GetRuleBasedMtf();
  const bool can_forward_declare =
      spvOperandCanBeForwardDeclaredFunction(
          SpvOp(inst_.opcode))(operand_index_);

  if (mtf != kMtfNone && !can_forward_declare) {
    return DecodeExistingId(mtf, id);
  }

  if (mtf == kMtfNone)
    mtf = kMtfAll;

  *id = 0;

  uint32_t rank = 0;

  {
    const spv_result_t result = DecodeMtfRankHuffman(mtf, kMtfAll, &rank);
    if (result != SPV_SUCCESS)
      return result;
  }

  if (rank == 0) {
    // This is the first occurrence of a forward declared id.
    *id = GetIdBound();
    SetIdBound(*id + 1);
    multi_mtf_.Insert(kMtfAll, *id);
    multi_mtf_.Insert(kMtfForwardDeclared, *id);
    if (mtf != kMtfAll)
      multi_mtf_.Insert(mtf, *id);
  } else {
    if (!multi_mtf_.ValueFromRank(mtf, rank, id))
      return Diag(SPV_ERROR_INTERNAL) << "MTF rank out of bounds";
  }

  assert(*id);
  return SPV_SUCCESS;
}

spv_result_t MarkvEncoder::EncodeTypeId() {
  if (inst_.opcode == SpvOpFunctionParameter) {
    assert(!remaining_function_parameter_types_.empty());
    assert(inst_.type_id == remaining_function_parameter_types_.front());
    remaining_function_parameter_types_.pop_front();
    return SPV_SUCCESS;
  }

  {
    // Try to encode using id descriptor mtfs.
    const spv_result_t result = EncodeIdWithDescriptor(inst_.type_id);
    if (result != SPV_UNSUPPORTED)
      return result;
    // If can't be done continue with other methods.
  }

  uint64_t mtf = GetRuleBasedMtf();
  assert(!spvOperandCanBeForwardDeclaredFunction(
      SpvOp(inst_.opcode))(operand_index_));

  if (mtf == kMtfNone) {
    mtf = kMtfTypeNonFunction;
    // Function types should have been handled by GetRuleBasedMtf.
    assert(inst_.opcode != SpvOpFunction);
  }

  return EncodeExistingId(mtf, inst_.type_id);
}

spv_result_t MarkvDecoder::DecodeTypeId() {
  if (inst_.opcode == SpvOpFunctionParameter) {
    assert(!remaining_function_parameter_types_.empty());
    inst_.type_id = remaining_function_parameter_types_.front();
    remaining_function_parameter_types_.pop_front();
    return SPV_SUCCESS;
  }

  {
    const spv_result_t result = DecodeIdWithDescriptor(&inst_.type_id);
    if (result != SPV_UNSUPPORTED)
      return result;
  }

  uint64_t mtf = GetRuleBasedMtf();
  assert(!spvOperandCanBeForwardDeclaredFunction(
      SpvOp(inst_.opcode))(operand_index_));

  if (mtf == kMtfNone) {
    mtf = kMtfTypeNonFunction;
    // Function types should have been handled by GetRuleBasedMtf.
    assert(inst_.opcode != SpvOpFunction);
  }

  return DecodeExistingId(mtf, &inst_.type_id);
}

spv_result_t MarkvEncoder::EncodeResultId() {
  uint32_t rank = 0;

  const uint64_t num_still_forward_declared =
      multi_mtf_.GetSize(kMtfForwardDeclared);

  if (num_still_forward_declared) {
    // We write the rank only if kMtfForwardDeclared is not empty. If it is
    // empty the decoder knows that there are no forward declared ids to expect.
    if (multi_mtf_.RankFromValue(kMtfForwardDeclared,
                                 inst_.result_id, &rank)) {
      // This is a definition of a forward declared id. We can remove the id
      // from kMtfForwardDeclared.
      if (!multi_mtf_.Remove(kMtfForwardDeclared, inst_.result_id))
        return Diag(SPV_ERROR_INTERNAL)
            << "Failed to remove id from kMtfForwardDeclared";
      writer_.WriteBits(1, 1);
      writer_.WriteVariableWidthU32(
          rank, model_->mtf_rank_chunk_length());
    } else {
      rank = 0;
      writer_.WriteBits(0, 1);
    }
  }

  if (!rank) {
    multi_mtf_.Insert(kMtfAll, inst_.result_id);
  }

  return SPV_SUCCESS;
}

spv_result_t MarkvDecoder::DecodeResultId() {
  uint32_t rank = 0;

  const uint64_t num_still_forward_declared =
      multi_mtf_.GetSize(kMtfForwardDeclared);

  if (num_still_forward_declared) {
    // Some ids were forward declared. Check if this id is one of them.
    uint64_t id_was_forward_declared;
    if (!reader_.ReadBits(&id_was_forward_declared, 1))
      return Diag(SPV_ERROR_INVALID_BINARY)
          << "Failed to read id_was_forward_declared flag";

    if (id_was_forward_declared) {
      if (!reader_.ReadVariableWidthU32(
          &rank, model_->mtf_rank_chunk_length()))
        return Diag(SPV_ERROR_INVALID_BINARY)
            << "Failed to read MTF rank of forward declared id";

      if (rank) {
        // The id was forward declared, recover it from kMtfForwardDeclared.
        if (!multi_mtf_.ValueFromRank(kMtfForwardDeclared,
                                     rank, &inst_.result_id))
          return Diag(SPV_ERROR_INTERNAL)
              << "Forward declared MTF rank is out of bounds";

        // We can now remove the id from kMtfForwardDeclared.
        if (!multi_mtf_.Remove(kMtfForwardDeclared, inst_.result_id))
          return Diag(SPV_ERROR_INTERNAL)
              << "Failed to remove id from kMtfForwardDeclared";
      }
    }
  }

  if (inst_.result_id == 0) {
    // The id was not forward declared, issue a new id.
    inst_.result_id = GetIdBound();
    SetIdBound(inst_.result_id + 1);
  }

  if (!rank) {
    multi_mtf_.Insert(kMtfAll, inst_.result_id);
  }

  return SPV_SUCCESS;
}

spv_result_t MarkvEncoder::EncodeLiteralNumber(
    const spv_parsed_operand_t& operand) {
  if (operand.number_bit_width <= 32) {
    const uint32_t word = inst_.words[operand.offset];
    return EncodeNonIdWord(word);
  } else {
    assert(operand.number_bit_width <= 64);
    const uint64_t word =
        uint64_t(inst_.words[operand.offset]) |
        (uint64_t(inst_.words[operand.offset + 1]) << 32);
    if (operand.number_kind == SPV_NUMBER_UNSIGNED_INT) {
      writer_.WriteVariableWidthU64(word, model_->u64_chunk_length());
    } else if (operand.number_kind == SPV_NUMBER_SIGNED_INT) {
      int64_t val = 0;
      std::memcpy(&val, &word, 8);
      writer_.WriteVariableWidthS64(val, model_->s64_chunk_length(),
                                    model_->s64_block_exponent());
    } else if (operand.number_kind == SPV_NUMBER_FLOATING) {
      writer_.WriteUnencoded(word);
    } else {
      return Diag(SPV_ERROR_INTERNAL) << "Unsupported bit length";
    }
  }
  return SPV_SUCCESS;
}

spv_result_t MarkvDecoder::DecodeLiteralNumber(
    const spv_parsed_operand_t& operand) {
  if (operand.number_bit_width <= 32) {
    uint32_t word = 0;
    const spv_result_t result = DecodeNonIdWord(&word);
    if (result != SPV_SUCCESS)
      return result;
    inst_words_.push_back(word);
  } else {
    assert(operand.number_bit_width <= 64);
    uint64_t word = 0;
    if (operand.number_kind == SPV_NUMBER_UNSIGNED_INT) {
      if (!reader_.ReadVariableWidthU64(&word, model_->u64_chunk_length()))
        return Diag(SPV_ERROR_INVALID_BINARY)
            << "Failed to read literal U64";
    } else if (operand.number_kind == SPV_NUMBER_SIGNED_INT) {
      int64_t val = 0;
      if (!reader_.ReadVariableWidthS64(&val, model_->s64_chunk_length(),
                                        model_->s64_block_exponent()))
        return Diag(SPV_ERROR_INVALID_BINARY)
            << "Failed to read literal S64";
      std::memcpy(&word, &val, 8);
    } else if (operand.number_kind == SPV_NUMBER_FLOATING) {
      if (!reader_.ReadUnencoded(&word))
        return Diag(SPV_ERROR_INVALID_BINARY)
            << "Failed to read literal F64";
    } else {
      return Diag(SPV_ERROR_INTERNAL) << "Unsupported bit length";
    }
    inst_words_.push_back(static_cast<uint32_t>(word));
    inst_words_.push_back(static_cast<uint32_t>(word >> 32));
  }
  return SPV_SUCCESS;
}

void MarkvEncoder::AddByteBreak(size_t byte_break_if_less_than) {
  const size_t num_bits_to_next_byte =
      GetNumBitsToNextByte(writer_.GetNumBits());
  if (num_bits_to_next_byte == 0 ||
      num_bits_to_next_byte > byte_break_if_less_than)
    return;

  if (logger_) {
    logger_->AppendWhitespaces(kCommentNumWhitespaces);
    logger_->AppendText("<byte break>");
  }

  writer_.WriteBits(0, num_bits_to_next_byte);
}

bool MarkvDecoder::ReadToByteBreak(size_t byte_break_if_less_than) {
  const size_t num_bits_to_next_byte =
      GetNumBitsToNextByte(reader_.GetNumReadBits());
  if (num_bits_to_next_byte == 0 ||
      num_bits_to_next_byte > byte_break_if_less_than)
    return true;


  uint64_t bits = 0;
  if (!reader_.ReadBits(&bits, num_bits_to_next_byte))
    return false;

  assert(bits == 0);
  if (bits != 0)
    return false;

  return true;
}

spv_result_t MarkvEncoder::EncodeInstruction(
    const spv_parsed_instruction_t& inst) {
  SpvOp opcode = SpvOp(inst.opcode);
  inst_ = inst;

  const spv_result_t validation_result = UpdateValidationState(inst);
  if (validation_result != SPV_SUCCESS)
    return validation_result;

  LogDisassemblyInstruction();

  const spv_result_t opcode_encodig_result =
      EncodeOpcodeAndNumOperands(opcode, inst.num_operands);
  if (opcode_encodig_result < 0)
    return opcode_encodig_result;

  if (opcode_encodig_result != SPV_SUCCESS) {
    // Fallback encoding for opcode and num_operands.
    writer_.WriteVariableWidthU32(opcode, model_->opcode_chunk_length());

    if (!OpcodeHasFixedNumberOfOperands(opcode)) {
      // If the opcode has a variable number of operands, encode the number of
      // operands with the instruction.

      if (logger_)
        logger_->AppendWhitespaces(kCommentNumWhitespaces);

      writer_.WriteVariableWidthU16(inst.num_operands,
                                    model_->num_operands_chunk_length());
    }
  }

  // Write operands.
  const uint32_t num_operands = inst_.num_operands;
  for (operand_index_ = 0; operand_index_ < num_operands; ++operand_index_) {
    operand_ = inst_.operands[operand_index_];

    if (logger_) {
      logger_->AppendWhitespaces(kCommentNumWhitespaces);
      logger_->AppendText("<");
      logger_->AppendText(spvOperandTypeStr(operand_.type));
      logger_->AppendText(">");
    }

    switch (operand_.type) {
      case SPV_OPERAND_TYPE_RESULT_ID:
      case SPV_OPERAND_TYPE_TYPE_ID:
      case SPV_OPERAND_TYPE_ID:
      case SPV_OPERAND_TYPE_OPTIONAL_ID:
      case SPV_OPERAND_TYPE_SCOPE_ID:
      case SPV_OPERAND_TYPE_MEMORY_SEMANTICS_ID: {
        const uint32_t id = inst_.words[operand_.offset];
        if (operand_.type == SPV_OPERAND_TYPE_TYPE_ID) {
          const spv_result_t result = EncodeTypeId();
          if (result != SPV_SUCCESS)
            return result;
        } else if (operand_.type == SPV_OPERAND_TYPE_RESULT_ID) {
          const spv_result_t result = EncodeResultId();
          if (result != SPV_SUCCESS)
            return result;
        } else {
          const spv_result_t result = EncodeRefId(id);
          if (result != SPV_SUCCESS)
            return result;
        }

        multi_mtf_.Promote(id);
        break;
      }

      case SPV_OPERAND_TYPE_LITERAL_INTEGER: {
        const spv_result_t result =
            EncodeNonIdWord(inst_.words[operand_.offset]);
        if (result != SPV_SUCCESS)
          return result;
        break;
      }

      case SPV_OPERAND_TYPE_TYPED_LITERAL_NUMBER: {
        const spv_result_t result = EncodeLiteralNumber(operand_);
        if (result != SPV_SUCCESS)
          return result;
        break;
      }

      case SPV_OPERAND_TYPE_LITERAL_STRING: {
        const char* src = reinterpret_cast<const char*>(
            &inst_.words[operand_.offset]);

        auto* codec = model_->GetLiteralStringHuffmanCodec(opcode);
        if (codec) {
          uint64_t bits = 0;
          size_t num_bits = 0;
          const std::string str = src;
          if (codec->Encode(str, &bits, &num_bits)) {
            writer_.WriteBits(bits, num_bits);
            break;
          } else {
            bool result = codec->Encode("kMarkvNoneOfTheAbove",
                                        &bits, &num_bits);
            (void)result;
            assert(result);
            writer_.WriteBits(bits, num_bits);
          }
        }

        const size_t length = spv_strnlen_s(src, operand_.num_words * 4);
        if (length == operand_.num_words * 4)
          return Diag(SPV_ERROR_INVALID_BINARY)
              << "Failed to find terminal character of literal string";
        for (size_t i = 0; i < length + 1; ++i)
          writer_.WriteUnencoded(src[i]);
        break;
      }

      default: {
        for (int i = 0; i < operand_.num_words; ++i) {
          const uint32_t word = inst_.words[operand_.offset + i];
          const spv_result_t result = EncodeNonIdWord(word);
          if (result != SPV_SUCCESS)
            return result;
        }
        break;
      }
    }
  }

  AddByteBreak(kByteBreakAfterInstIfLessThanUntilNextByte);

  if (logger_) {
    logger_->NewLine();
    logger_->NewLine();
  }

  ProcessCurInstruction();

  return SPV_SUCCESS;
}

spv_result_t MarkvDecoder::DecodeModule(std::vector<uint32_t>* spirv_binary) {
  const bool header_read_success =
      reader_.ReadUnencoded(&header_.magic_number) &&
      reader_.ReadUnencoded(&header_.markv_version) &&
      reader_.ReadUnencoded(&header_.markv_model) &&
      reader_.ReadUnencoded(&header_.markv_length_in_bits) &&
      reader_.ReadUnencoded(&header_.spirv_version) &&
      reader_.ReadUnencoded(&header_.spirv_generator);

  if (!header_read_success)
    return Diag(SPV_ERROR_INVALID_BINARY)
        << "Unable to read MARK-V header";

  if (header_.markv_length_in_bits == 0)
    return Diag(SPV_ERROR_INVALID_BINARY)
        << "Header markv_length_in_bits field is zero";

  if (header_.magic_number != kMarkvMagicNumber)
    return Diag(SPV_ERROR_INVALID_BINARY)
        << "MARK-V binary has incorrect magic number";

  // TODO(atgoo@github.com): Print version strings.
  if (header_.markv_version != GetMarkvVersion())
    return Diag(SPV_ERROR_INVALID_BINARY)
        << "MARK-V binary and the codec have different versions";

  spirv_.reserve(header_.markv_length_in_bits / 2); // Heuristic.
  spirv_.resize(5, 0);
  spirv_[0] = kSpirvMagicNumber;
  spirv_[1] = header_.spirv_version;
  spirv_[2] = header_.spirv_generator;

  while (reader_.GetNumReadBits() < header_.markv_length_in_bits) {
    inst_ = {};
    const spv_result_t decode_result = DecodeInstruction();
    if (decode_result != SPV_SUCCESS)
      return decode_result;

    const spv_result_t validation_result = UpdateValidationState(inst_);
    if (validation_result != SPV_SUCCESS)
      return validation_result;
  }


  if (reader_.GetNumReadBits() != header_.markv_length_in_bits ||
      !reader_.OnlyZeroesLeft()) {
    return Diag(SPV_ERROR_INVALID_BINARY)
        << "MARK-V binary has wrong stated bit length "
        << reader_.GetNumReadBits() << " " << header_.markv_length_in_bits;
  }

  // Decoding of the module is finished, validation state should have correct
  // id bound.
  spirv_[3] = GetIdBound();

  *spirv_binary = std::move(spirv_);
  return SPV_SUCCESS;
}

// TODO(atgoo@github.com): The implementation borrows heavily from
// Parser::parseOperand.
// Consider coupling them together in some way once MARK-V codec is more mature.
// For now it's better to keep the code independent for experimentation
// purposes.
spv_result_t MarkvDecoder::DecodeOperand(
    size_t operand_offset,
    const spv_operand_type_t type,
    spv_operand_pattern_t* expected_operands) {
  const SpvOp opcode = static_cast<SpvOp>(inst_.opcode);

  memset(&operand_, 0, sizeof(operand_));

  assert((operand_offset >> 16) == 0);
  operand_.offset = static_cast<uint16_t>(operand_offset);
  operand_.type = type;

  // Set default values, may be updated later.
  operand_.number_kind = SPV_NUMBER_NONE;
  operand_.number_bit_width = 0;

  const size_t first_word_index = inst_words_.size();

  switch (type) {
    case SPV_OPERAND_TYPE_RESULT_ID: {
      const spv_result_t result = DecodeResultId();
      if (result != SPV_SUCCESS)
        return result;

      inst_words_.push_back(inst_.result_id);
      SetIdBound(std::max(GetIdBound(), inst_.result_id + 1));
      multi_mtf_.Promote(inst_.result_id);
      break;
    }

    case SPV_OPERAND_TYPE_TYPE_ID: {
      const spv_result_t result = DecodeTypeId();
      if (result != SPV_SUCCESS)
        return result;

      inst_words_.push_back(inst_.type_id);
      SetIdBound(std::max(GetIdBound(), inst_.type_id + 1));
      multi_mtf_.Promote(inst_.type_id);
      break;
    }

    case SPV_OPERAND_TYPE_ID:
    case SPV_OPERAND_TYPE_OPTIONAL_ID:
    case SPV_OPERAND_TYPE_SCOPE_ID:
    case SPV_OPERAND_TYPE_MEMORY_SEMANTICS_ID: {
      uint32_t id = 0;
      const spv_result_t result = DecodeRefId(&id);
      if (result != SPV_SUCCESS)
        return result;

      if (id == 0)
        return Diag(SPV_ERROR_INVALID_BINARY) << "Decoded id is 0";

      if (type == SPV_OPERAND_TYPE_ID ||
          type == SPV_OPERAND_TYPE_OPTIONAL_ID) {

        operand_.type = SPV_OPERAND_TYPE_ID;

        if (opcode == SpvOpExtInst && operand_.offset == 3) {
          // The current word is the extended instruction set id.
          // Set the extended instruction set type for the current
          // instruction.
          auto ext_inst_type_iter = import_id_to_ext_inst_type_.find(id);
          if (ext_inst_type_iter == import_id_to_ext_inst_type_.end()) {
            return Diag(SPV_ERROR_INVALID_ID)
                << "OpExtInst set id " << id
                << " does not reference an OpExtInstImport result Id";
          }
          inst_.ext_inst_type = ext_inst_type_iter->second;
        }
      }

      inst_words_.push_back(id);
      SetIdBound(std::max(GetIdBound(), id + 1));
      multi_mtf_.Promote(id);
      break;
    }

    case SPV_OPERAND_TYPE_EXTENSION_INSTRUCTION_NUMBER: {
      uint32_t word = 0;
      const spv_result_t result = DecodeNonIdWord(&word);
      if (result != SPV_SUCCESS)
        return result;

      inst_words_.push_back(word);

      assert(SpvOpExtInst == opcode);
      assert(inst_.ext_inst_type != SPV_EXT_INST_TYPE_NONE);
      spv_ext_inst_desc ext_inst;
      if (grammar_.lookupExtInst(inst_.ext_inst_type, word, &ext_inst))
        return Diag(SPV_ERROR_INVALID_BINARY)
            << "Invalid extended instruction number: " << word;
      spvPushOperandTypes(ext_inst->operandTypes, expected_operands);
      break;
    }

    case SPV_OPERAND_TYPE_LITERAL_INTEGER:
    case SPV_OPERAND_TYPE_OPTIONAL_LITERAL_INTEGER: {
      // These are regular single-word literal integer operands.
      // Post-parsing validation should check the range of the parsed value.
      operand_.type = SPV_OPERAND_TYPE_LITERAL_INTEGER;
      // It turns out they are always unsigned integers!
      operand_.number_kind = SPV_NUMBER_UNSIGNED_INT;
      operand_.number_bit_width = 32;

      uint32_t word = 0;
      const spv_result_t result = DecodeNonIdWord(&word);
      if (result != SPV_SUCCESS)
        return result;

      inst_words_.push_back(word);
      break;
    }

    case SPV_OPERAND_TYPE_TYPED_LITERAL_NUMBER:
    case SPV_OPERAND_TYPE_OPTIONAL_TYPED_LITERAL_INTEGER: {
      operand_.type = SPV_OPERAND_TYPE_TYPED_LITERAL_NUMBER;
      if (opcode == SpvOpSwitch) {
        // The literal operands have the same type as the value
        // referenced by the selector Id.
        const uint32_t selector_id = inst_words_.at(1);
        const auto type_id_iter = id_to_type_id_.find(selector_id);
        if (type_id_iter == id_to_type_id_.end() ||
            type_id_iter->second == 0) {
          return Diag(SPV_ERROR_INVALID_BINARY)
              << "Invalid OpSwitch: selector id " << selector_id
              << " has no type";
        }
        uint32_t type_id = type_id_iter->second;

        if (selector_id == type_id) {
          // Recall that by convention, a result ID that is a type definition
          // maps to itself.
          return Diag(SPV_ERROR_INVALID_BINARY)
              << "Invalid OpSwitch: selector id " << selector_id
              << " is a type, not a value";
        }
        if (auto error = SetNumericTypeInfoForType(&operand_, type_id))
          return error;
        if (operand_.number_kind != SPV_NUMBER_UNSIGNED_INT &&
            operand_.number_kind != SPV_NUMBER_SIGNED_INT) {
          return Diag(SPV_ERROR_INVALID_BINARY)
              << "Invalid OpSwitch: selector id " << selector_id
              << " is not a scalar integer";
        }
      } else {
        assert(opcode == SpvOpConstant || opcode == SpvOpSpecConstant);
        // The literal number type is determined by the type Id for the
        // constant.
        assert(inst_.type_id);
        if (auto error = SetNumericTypeInfoForType(&operand_, inst_.type_id))
          return error;
      }

      if (auto error = DecodeLiteralNumber(operand_))
        return error;

      break;
    }

    case SPV_OPERAND_TYPE_LITERAL_STRING:
    case SPV_OPERAND_TYPE_OPTIONAL_LITERAL_STRING: {
      operand_.type = SPV_OPERAND_TYPE_LITERAL_STRING;
      std::vector<char> str;
      auto* codec = model_->GetLiteralStringHuffmanCodec(inst_.opcode);

      if (codec) {
        std::string decoded_string;
        const bool huffman_result =
            codec->DecodeFromStream(GetReadBitCallback(), &decoded_string);
        assert(huffman_result);
        if (!huffman_result)
          return Diag(SPV_ERROR_INVALID_BINARY)
              << "Failed to read literal string";

        if (decoded_string != "kMarkvNoneOfTheAbove") {
          std::copy(decoded_string.begin(), decoded_string.end(),
                    std::back_inserter(str));
          str.push_back('\0');
        }
      }

      // The loop is expected to terminate once we encounter '\0' or exhaust
      // the bit stream.
      if (str.empty()) {
        while (true) {
          char ch = 0;
          if (!reader_.ReadUnencoded(&ch))
            return Diag(SPV_ERROR_INVALID_BINARY)
                << "Failed to read literal string";

          str.push_back(ch);

          if (ch == '\0')
            break;
        }
      }

      while (str.size() % 4 != 0)
        str.push_back('\0');

      inst_words_.resize(inst_words_.size() + str.size() / 4);
      std::memcpy(&inst_words_[first_word_index], str.data(), str.size());

      if (SpvOpExtInstImport == opcode) {
        // Record the extended instruction type for the ID for this import.
        // There is only one string literal argument to OpExtInstImport,
        // so it's sufficient to guard this just on the opcode.
        const spv_ext_inst_type_t ext_inst_type =
            spvExtInstImportTypeGet(str.data());
        if (SPV_EXT_INST_TYPE_NONE == ext_inst_type) {
          return Diag(SPV_ERROR_INVALID_BINARY)
              << "Invalid extended instruction import '" << str.data() << "'";
        }
        // We must have parsed a valid result ID.  It's a condition
        // of the grammar, and we only accept non-zero result Ids.
        assert(inst_.result_id);
        const bool inserted = import_id_to_ext_inst_type_.emplace(
            inst_.result_id, ext_inst_type).second;
        (void)inserted;
        assert(inserted);
      }
      break;
    }

    case SPV_OPERAND_TYPE_CAPABILITY:
    case SPV_OPERAND_TYPE_SOURCE_LANGUAGE:
    case SPV_OPERAND_TYPE_EXECUTION_MODEL:
    case SPV_OPERAND_TYPE_ADDRESSING_MODEL:
    case SPV_OPERAND_TYPE_MEMORY_MODEL:
    case SPV_OPERAND_TYPE_EXECUTION_MODE:
    case SPV_OPERAND_TYPE_STORAGE_CLASS:
    case SPV_OPERAND_TYPE_DIMENSIONALITY:
    case SPV_OPERAND_TYPE_SAMPLER_ADDRESSING_MODE:
    case SPV_OPERAND_TYPE_SAMPLER_FILTER_MODE:
    case SPV_OPERAND_TYPE_SAMPLER_IMAGE_FORMAT:
    case SPV_OPERAND_TYPE_FP_ROUNDING_MODE:
    case SPV_OPERAND_TYPE_LINKAGE_TYPE:
    case SPV_OPERAND_TYPE_ACCESS_QUALIFIER:
    case SPV_OPERAND_TYPE_OPTIONAL_ACCESS_QUALIFIER:
    case SPV_OPERAND_TYPE_FUNCTION_PARAMETER_ATTRIBUTE:
    case SPV_OPERAND_TYPE_DECORATION:
    case SPV_OPERAND_TYPE_BUILT_IN:
    case SPV_OPERAND_TYPE_GROUP_OPERATION:
    case SPV_OPERAND_TYPE_KERNEL_ENQ_FLAGS:
    case SPV_OPERAND_TYPE_KERNEL_PROFILING_INFO: {
      // A single word that is a plain enum value.
      uint32_t word = 0;
      const spv_result_t result = DecodeNonIdWord(&word);
      if (result != SPV_SUCCESS)
        return result;

      inst_words_.push_back(word);

      // Map an optional operand type to its corresponding concrete type.
      if (type == SPV_OPERAND_TYPE_OPTIONAL_ACCESS_QUALIFIER)
        operand_.type = SPV_OPERAND_TYPE_ACCESS_QUALIFIER;

      spv_operand_desc entry;
      if (grammar_.lookupOperand(type, word, &entry)) {
        return Diag(SPV_ERROR_INVALID_BINARY)
            << "Invalid "
            << spvOperandTypeStr(operand_.type)
            << " operand: " << word;
      }

      // Prepare to accept operands to this operand, if needed.
      spvPushOperandTypes(entry->operandTypes, expected_operands);
      break;
    }

    case SPV_OPERAND_TYPE_FP_FAST_MATH_MODE:
    case SPV_OPERAND_TYPE_FUNCTION_CONTROL:
    case SPV_OPERAND_TYPE_LOOP_CONTROL:
    case SPV_OPERAND_TYPE_IMAGE:
    case SPV_OPERAND_TYPE_OPTIONAL_IMAGE:
    case SPV_OPERAND_TYPE_OPTIONAL_MEMORY_ACCESS:
    case SPV_OPERAND_TYPE_SELECTION_CONTROL: {
      // This operand is a mask.
      uint32_t word = 0;
      const spv_result_t result = DecodeNonIdWord(&word);
      if (result != SPV_SUCCESS)
        return result;

      inst_words_.push_back(word);

      // Map an optional operand type to its corresponding concrete type.
      if (type == SPV_OPERAND_TYPE_OPTIONAL_IMAGE)
        operand_.type = SPV_OPERAND_TYPE_IMAGE;
      else if (type == SPV_OPERAND_TYPE_OPTIONAL_MEMORY_ACCESS)
        operand_.type = SPV_OPERAND_TYPE_MEMORY_ACCESS;

      // Check validity of set mask bits. Also prepare for operands for those
      // masks if they have any.  To get operand order correct, scan from
      // MSB to LSB since we can only prepend operands to a pattern.
      // The only case in the grammar where you have more than one mask bit
      // having an operand is for image operands.  See SPIR-V 3.14 Image
      // Operands.
      uint32_t remaining_word = word;
      for (uint32_t mask = (1u << 31); remaining_word; mask >>= 1) {
        if (remaining_word & mask) {
          spv_operand_desc entry;
          if (grammar_.lookupOperand(type, mask, &entry)) {
            return Diag(SPV_ERROR_INVALID_BINARY)
                   << "Invalid " << spvOperandTypeStr(operand_.type)
                   << " operand: " << word << " has invalid mask component "
                   << mask;
          }
          remaining_word ^= mask;
          spvPushOperandTypes(entry->operandTypes, expected_operands);
        }
      }
      if (word == 0) {
        // An all-zeroes mask *might* also be valid.
        spv_operand_desc entry;
        if (SPV_SUCCESS == grammar_.lookupOperand(type, 0, &entry)) {
          // Prepare for its operands, if any.
          spvPushOperandTypes(entry->operandTypes, expected_operands);
        }
      }
      break;
    }
    default:
      return Diag(SPV_ERROR_INVALID_BINARY)
          << "Internal error: Unhandled operand type: " << type;
  }

  operand_.num_words = uint16_t(inst_words_.size() - first_word_index);

  assert(int(SPV_OPERAND_TYPE_FIRST_CONCRETE_TYPE) <= int(operand_.type));
  assert(int(SPV_OPERAND_TYPE_LAST_CONCRETE_TYPE) >= int(operand_.type));

  parsed_operands_.push_back(operand_);

  return SPV_SUCCESS;
}

spv_result_t MarkvDecoder::DecodeInstruction() {
  parsed_operands_.clear();
  inst_words_.clear();

  // Opcode/num_words placeholder, the word will be filled in later.
  inst_words_.push_back(0);

  bool num_operands_still_unknown = true;
  {
    uint32_t opcode = 0;
    uint32_t num_operands = 0;

    const spv_result_t opcode_decoding_result =
        DecodeOpcodeAndNumberOfOperands(&opcode, &num_operands);
    if (opcode_decoding_result < 0)
      return opcode_decoding_result;

    if (opcode_decoding_result == SPV_SUCCESS) {
      inst_.num_operands = static_cast<uint16_t>(num_operands);
      num_operands_still_unknown = false;
    } else {
      if (!reader_.ReadVariableWidthU32(
          &opcode, model_->opcode_chunk_length())) {
        return Diag(SPV_ERROR_INVALID_BINARY)
            << "Failed to read opcode of instruction";
      }
    }

    inst_.opcode = static_cast<uint16_t>(opcode);
  }

  const SpvOp opcode = static_cast<SpvOp>(inst_.opcode);

  spv_opcode_desc opcode_desc;
  if (grammar_.lookupOpcode(opcode, &opcode_desc) != SPV_SUCCESS) {
    return Diag(SPV_ERROR_INVALID_BINARY) << "Invalid opcode";
  }

  spv_operand_pattern_t expected_operands;
  expected_operands.reserve(opcode_desc->numTypes);
  for (auto i = 0; i < opcode_desc->numTypes; i++) {
    expected_operands.push_back(
        opcode_desc->operandTypes[opcode_desc->numTypes - i - 1]);
  }

  if (num_operands_still_unknown) {
    if (!OpcodeHasFixedNumberOfOperands(opcode)) {
      if (!reader_.ReadVariableWidthU16(&inst_.num_operands,
                                        model_->num_operands_chunk_length()))
        return Diag(SPV_ERROR_INVALID_BINARY)
            << "Failed to read num_operands of instruction";
    } else {
      inst_.num_operands = static_cast<uint16_t>(expected_operands.size());
    }
  }

  for (operand_index_ = 0;
       operand_index_ < static_cast<size_t>(inst_.num_operands);
       ++operand_index_) {
    assert(!expected_operands.empty());
    const spv_operand_type_t type =
        spvTakeFirstMatchableOperand(&expected_operands);

    const size_t operand_offset = inst_words_.size();

    const spv_result_t decode_result = DecodeOperand(
        operand_offset, type, &expected_operands);

    if (decode_result != SPV_SUCCESS)
      return decode_result;
  }


  assert(inst_.num_operands == parsed_operands_.size());

  // Only valid while inst_words_ and parsed_operands_ remain unchanged (until
  // next DecodeInstruction call).
  inst_.words = inst_words_.data();
  inst_.operands = parsed_operands_.empty() ? nullptr : parsed_operands_.data();
  inst_.num_words = static_cast<uint16_t>(inst_words_.size());
  inst_words_[0] = spvOpcodeMake(inst_.num_words, SpvOp(inst_.opcode));

  std::copy(inst_words_.begin(), inst_words_.end(), std::back_inserter(spirv_));

  assert(inst_.num_words == std::accumulate(
      parsed_operands_.begin(), parsed_operands_.end(), 1,
      [](int num_words, const spv_parsed_operand_t& operand) {
        return num_words += operand.num_words;
  }) && "num_words in instruction doesn't correspond to the sum of num_words"
        "in the operands");

  RecordNumberType();
  ProcessCurInstruction();

  if (!ReadToByteBreak(kByteBreakAfterInstIfLessThanUntilNextByte))
    return Diag(SPV_ERROR_INVALID_BINARY)
        << "Failed to read to byte break";

  return SPV_SUCCESS;
}

spv_result_t MarkvDecoder::SetNumericTypeInfoForType(
    spv_parsed_operand_t* parsed_operand, uint32_t type_id) {
  assert(type_id != 0);
  auto type_info_iter = type_id_to_number_type_info_.find(type_id);
  if (type_info_iter == type_id_to_number_type_info_.end()) {
    return Diag(SPV_ERROR_INVALID_BINARY)
        << "Type Id " << type_id << " is not a type";
  }

  const NumberType& info = type_info_iter->second;
  if (info.type == SPV_NUMBER_NONE) {
    // This is a valid type, but for something other than a scalar number.
    return Diag(SPV_ERROR_INVALID_BINARY)
        << "Type Id " << type_id << " is not a scalar numeric type";
  }

  parsed_operand->number_kind = info.type;
  parsed_operand->number_bit_width = info.bit_width;
  // Round up the word count.
  parsed_operand->num_words = static_cast<uint16_t>((info.bit_width + 31) / 32);
  return SPV_SUCCESS;
}

void MarkvDecoder::RecordNumberType() {
  const SpvOp opcode = static_cast<SpvOp>(inst_.opcode);
  if (spvOpcodeGeneratesType(opcode)) {
    NumberType info = {SPV_NUMBER_NONE, 0};
    if (SpvOpTypeInt == opcode) {
      info.bit_width = inst_.words[inst_.operands[1].offset];
      info.type = inst_.words[inst_.operands[2].offset] ?
          SPV_NUMBER_SIGNED_INT : SPV_NUMBER_UNSIGNED_INT;
    } else if (SpvOpTypeFloat == opcode) {
      info.bit_width = inst_.words[inst_.operands[1].offset];
      info.type = SPV_NUMBER_FLOATING;
    }
    // The *result* Id of a type generating instruction is the type Id.
    type_id_to_number_type_info_[inst_.result_id] = info;
  }
}

spv_result_t EncodeHeader(
    void* user_data, spv_endianness_t endian, uint32_t magic,
    uint32_t version, uint32_t generator, uint32_t id_bound,
    uint32_t schema) {
  MarkvEncoder* encoder = reinterpret_cast<MarkvEncoder*>(user_data);
  return encoder->EncodeHeader(
      endian, magic, version, generator, id_bound, schema);
}

spv_result_t EncodeInstruction(
    void* user_data, const spv_parsed_instruction_t* inst) {
  MarkvEncoder* encoder = reinterpret_cast<MarkvEncoder*>(user_data);
  return encoder->EncodeInstruction(*inst);
}

}  // namespace

spv_result_t spvSpirvToMarkv(spv_const_context context,
                             const uint32_t* spirv_words,
                             const size_t spirv_num_words,
                             spv_const_markv_encoder_options options,
                             spv_markv_binary* markv_binary,
                             spv_text* comments, spv_diagnostic* diagnostic) {
  spv_context_t hijack_context = *context;
  if (diagnostic) {
    *diagnostic = nullptr;
    libspirv::UseDiagnosticAsMessageConsumer(&hijack_context, diagnostic);
  }

  spv_const_binary_t spirv_binary = {spirv_words, spirv_num_words};

  spv_endianness_t endian;
  spv_position_t position = {};
  if (spvBinaryEndianness(&spirv_binary, &endian)) {
    return DiagnosticStream(position, hijack_context.consumer,
                                      SPV_ERROR_INVALID_BINARY)
        << "Invalid SPIR-V magic number.";
  }

  spv_header_t header;
  if (spvBinaryHeaderGet(&spirv_binary, endian, &header)) {
    return DiagnosticStream(position, hijack_context.consumer,
                            SPV_ERROR_INVALID_BINARY)
        << "Invalid SPIR-V header.";
  }

  MarkvEncoder encoder(&hijack_context, options);

  if (comments) {
    encoder.CreateCommentsLogger();

    spv_text text = nullptr;
    if (spvBinaryToText(&hijack_context, spirv_words, spirv_num_words,
                        SPV_BINARY_TO_TEXT_OPTION_NO_HEADER, &text, nullptr)
        != SPV_SUCCESS) {
      return DiagnosticStream(position, hijack_context.consumer,
                              SPV_ERROR_INVALID_BINARY)
          << "Failed to disassemble SPIR-V binary.";
    }
    assert(text);
    encoder.SetDisassembly(std::string(text->str, text->length));
    spvTextDestroy(text);
  }

  if (spvBinaryParse(
      &hijack_context, &encoder, spirv_words, spirv_num_words, EncodeHeader,
      EncodeInstruction, diagnostic) != SPV_SUCCESS) {
    return DiagnosticStream(position, hijack_context.consumer,
                            SPV_ERROR_INVALID_BINARY)
        << "Unable to encode to MARK-V.";
  }

  if (comments)
    *comments = CreateSpvText(encoder.GetComments());

  *markv_binary = encoder.GetMarkvBinary();
  return SPV_SUCCESS;
}

spv_result_t spvMarkvToSpirv(spv_const_context context,
                             const uint8_t* markv_data,
                             size_t markv_size_bytes,
                             spv_const_markv_decoder_options options,
                             spv_binary* spirv_binary,
                             spv_text* /* comments */,
                             spv_diagnostic* diagnostic) {
  spv_position_t position = {};
  spv_context_t hijack_context = *context;
  if (diagnostic) {
    *diagnostic = nullptr;
    libspirv::UseDiagnosticAsMessageConsumer(&hijack_context, diagnostic);
  }

  MarkvDecoder decoder(&hijack_context, markv_data, markv_size_bytes, options);

  std::vector<uint32_t> words;

  if (decoder.DecodeModule(&words) != SPV_SUCCESS) {
    return DiagnosticStream(position, hijack_context.consumer,
                            SPV_ERROR_INVALID_BINARY)
        << "Unable to decode MARK-V.";
  }

  assert(!words.empty());

  *spirv_binary = new spv_binary_t();
  (*spirv_binary)->code = new uint32_t[words.size()];
  (*spirv_binary)->wordCount = words.size();
  std::memcpy((*spirv_binary)->code, words.data(), 4 * words.size());

  return SPV_SUCCESS;
}

void spvMarkvBinaryDestroy(spv_markv_binary binary) {
  if (!binary) return;
  delete[] binary->data;
  delete binary;
}

spv_markv_encoder_options spvMarkvEncoderOptionsCreate() {
  return new spv_markv_encoder_options_t;
}

void spvMarkvEncoderOptionsDestroy(spv_markv_encoder_options options) {
  delete options;
}

spv_markv_decoder_options spvMarkvDecoderOptionsCreate() {
  return new spv_markv_decoder_options_t;
}

void spvMarkvDecoderOptionsDestroy(spv_markv_decoder_options options) {
  delete options;
}