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/* Copyright 2010 Google Inc. All Rights Reserved.
Distributed under MIT license.
See file LICENSE for detail or copy at
// Entropy encoding (Huffman) utilities.
#include <string.h>
#include "./histogram.h"
#include "./prefix.h"
#include "./types.h"
namespace brotli {
// A node of a Huffman tree.
struct HuffmanTree {
HuffmanTree() {}
HuffmanTree(uint32_t count, int16_t left, int16_t right)
: total_count_(count),
index_right_or_value_(right) {
uint32_t total_count_;
int16_t index_left_;
int16_t index_right_or_value_;
void SetDepth(const HuffmanTree &p, HuffmanTree *pool,
uint8_t *depth, uint8_t level);
// This function will create a Huffman tree.
// The (data,length) contains the population counts.
// The tree_limit is the maximum bit depth of the Huffman codes.
// The depth contains the tree, i.e., how many bits are used for
// the symbol.
// The actual Huffman tree is constructed in the tree[] array, which has to
// be at least 2 * length + 1 long.
// See
void CreateHuffmanTree(const uint32_t *data,
const size_t length,
const int tree_limit,
HuffmanTree* tree,
uint8_t *depth);
// Change the population counts in a way that the consequent
// Huffman tree compression, especially its rle-part will be more
// likely to compress this data more efficiently.
// length contains the size of the histogram.
// counts contains the population counts.
// good_for_rle is a buffer of at least length size
void OptimizeHuffmanCountsForRle(size_t length, uint32_t* counts,
uint8_t* good_for_rle);
// Write a Huffman tree from bit depths into the bitstream representation
// of a Huffman tree. The generated Huffman tree is to be compressed once
// more using a Huffman tree
void WriteHuffmanTree(const uint8_t* depth,
size_t num,
size_t* tree_size,
uint8_t* tree,
uint8_t* extra_bits_data);
// Get the actual bit values for a tree of bit depths.
void ConvertBitDepthsToSymbols(const uint8_t *depth,
size_t len,
uint16_t *bits);
template<int kSize>
struct EntropyCode {
// How many bits for symbol.
uint8_t depth_[kSize];
// Actual bits used to represent the symbol.
uint16_t bits_[kSize];
// How many non-zero depth.
int count_;
// First four symbols with non-zero depth.
int symbols_[4];
static const int kCodeLengthCodes = 18;
// Literal entropy code.
typedef EntropyCode<256> EntropyCodeLiteral;
// Prefix entropy codes.
typedef EntropyCode<kNumCommandPrefixes> EntropyCodeCommand;
typedef EntropyCode<kNumDistancePrefixes> EntropyCodeDistance;
typedef EntropyCode<kNumBlockLenPrefixes> EntropyCodeBlockLength;
// Context map entropy code, 256 Huffman tree indexes + 16 run length codes.
typedef EntropyCode<272> EntropyCodeContextMap;
// Block type entropy code, 256 block types + 2 special symbols.
typedef EntropyCode<258> EntropyCodeBlockType;
} // namespace brotli