src/codec/SkJpegSourceMgr.cpp - skia - Git at Google

 /*
  * Copyright 2023 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */

 #include "src/codec/SkJpegSourceMgr.h"

 #include "include/core/SkTypes.h"

 #ifdef SK_CODEC_DECODES_JPEG
 #include "include/core/SkData.h"
 #include "include/core/SkRefCnt.h"
 #include "include/core/SkStream.h"
 #include "src/codec/SkCodecPriv.h"

 #ifdef SK_CODEC_DECODES_JPEG_GAINMAPS
 #include "src/codec/SkJpegSegmentScan.h"
 #endif  // SK_CODEC_DECODES_JPEG_GAINMAPS

 ////////////////////////////////////////////////////////////////////////////////////////////////////
 // SkStream helpers.

 /*
  * Class that will will rewind an SkStream, and then restore it to its original position when it
  * goes out of scope. If the SkStream is not seekable, then the stream will not be altered at all,
  * and will return false from canRestore.
  */

 class ScopedSkStreamRestorer {
 public:
     ScopedSkStreamRestorer(SkStream* stream) : fStream(stream), fPosition(stream->getPosition()) {
         if (!fStream->rewind()) {
             SkCodecPrintf("Failed to rewind decoder stream.\n");
         }
     }
     ~ScopedSkStreamRestorer() {
         if (!fStream->seek(fPosition)) {
             SkCodecPrintf("Failed to restore decoder stream.\n");
         }
     }

 private:
     SkStream* const fStream;
     const size_t fPosition;
 };

 ////////////////////////////////////////////////////////////////////////////////////////////////////
 // SkJpegMemorySourceMgr

 class SkJpegMemorySourceMgr : public SkJpegSourceMgr {
 public:
     SkJpegMemorySourceMgr(SkStream* stream) : SkJpegSourceMgr(stream) {}
     ~SkJpegMemorySourceMgr() override {}

     void initSource(const uint8_t*& nextInputByte, size_t& bytesInBuffer) override {
         nextInputByte = reinterpret_cast<const uint8_t*>(fStream->getMemoryBase());
         bytesInBuffer = static_cast<size_t>(fStream->getLength());
     }
     bool fillInputBuffer(const uint8_t*& nextInputByte, size_t& bytesInBuffer) override {
         // The whole JPEG data is expected to reside in the supplied memory buffer, so any request
         // for more data beyond the given buffer size is treated as an error.
         SkCodecPrintf("Asked to re-fill a memory-mapped stream.\n");
         return false;
     }
     bool skipInputBytes(size_t bytesToSkip,
                         const uint8_t*& nextInputByte,
                         size_t& bytesInBuffer) override {
         if (bytesToSkip > bytesInBuffer) {
             SkCodecPrintf("Asked to read past end of a memory-mapped stream.\n");
             return false;
         }
         nextInputByte += bytesToSkip;
         bytesInBuffer -= bytesToSkip;
         return true;
     }
 #ifdef SK_CODEC_DECODES_JPEG_GAINMAPS
     const std::vector<SkJpegSegment>& getAllSegments() override {
         if (fScanner) {
             return fScanner->getSegments();
         }
         fScanner = std::make_unique<SkJpegSegmentScanner>(SkJpegSegmentScanner::kMarkerEndOfImage);
         fScanner->onBytes(fStream->getMemoryBase(), fStream->getLength());
         return fScanner->getSegments();
     }
     std::unique_ptr<SkStream> getSubsetStream(size_t offset, size_t size) override {
         if (offset > fStream->getLength() || size > fStream->getLength() - offset) {
             return nullptr;
         }
         return SkMemoryStream::MakeCopy(
                 reinterpret_cast<const uint8_t*>(fStream->getMemoryBase()) + offset, size);
     }
 #endif  // SK_CODEC_DECODES_JPEG_GAINMAPS
 };

 ////////////////////////////////////////////////////////////////////////////////////////////////////
 // SkJpegBufferedSourceMgr

 class SkJpegBufferedSourceMgr : public SkJpegSourceMgr {
 public:
     SkJpegBufferedSourceMgr(SkStream* stream, size_t bufferSize) : SkJpegSourceMgr(stream) {
         fBuffer = SkData::MakeUninitialized(bufferSize);
     }
     ~SkJpegBufferedSourceMgr() override {}

     void initSource(const uint8_t*& nextInputByte, size_t& bytesInBuffer) override {
         nextInputByte = fBuffer->bytes();
         bytesInBuffer = 0;
     }
     bool fillInputBuffer(const uint8_t*& nextInputByte, size_t& bytesInBuffer) override {
         size_t bytesRead = fStream->read(fBuffer->writable_data(), fBuffer->size());
         if (bytesRead == 0) {
             // Fail if we read zero bytes (libjpeg will accept any non-zero number of bytes).
             SkCodecPrintf("Hit end of file reading a buffered stream.\n");
             return false;
         }
         nextInputByte = fBuffer->bytes();
         bytesInBuffer = bytesRead;
         return true;
     }
     bool skipInputBytes(size_t bytesToSkip,
                         const uint8_t*& nextInputByte,
                         size_t& bytesInBuffer) override {
         // Skip through the already-read (or already in memory) buffer.
         if (bytesToSkip <= bytesInBuffer) {
             nextInputByte += bytesToSkip;
             bytesInBuffer -= bytesToSkip;
             return true;
         }
         bytesToSkip -= bytesInBuffer;

         // Fail if we skip past the end of the stream.
         if (fStream->skip(bytesToSkip) != bytesToSkip) {
             SkCodecPrintf("Failed to skip through buffered stream.\n");
             return false;
         }

         bytesInBuffer = 0;
         nextInputByte = fBuffer->bytes();
         return true;
     }
 #ifdef SK_CODEC_DECODES_JPEG_GAINMAPS
     const std::vector<SkJpegSegment>& getAllSegments() override {
         if (fScanner) {
             return fScanner->getSegments();
         }
         ScopedSkStreamRestorer streamRestorer(fStream);
         fScanner = std::make_unique<SkJpegSegmentScanner>(SkJpegSegmentScanner::kMarkerEndOfImage);
         while (!fScanner->isDone() && !fScanner->hadError()) {
             constexpr size_t kBufferSize = 1024;
             uint8_t buffer[kBufferSize];
             size_t bytesRead = fStream->read(buffer, kBufferSize);
             if (bytesRead == 0) {
                 SkCodecPrintf("Unexpected EOF.\n");
                 break;
             }
             fScanner->onBytes(buffer, bytesRead);
         }
         return fScanner->getSegments();
     }
     std::unique_ptr<SkStream> getSubsetStream(size_t offset, size_t size) override {
         ScopedSkStreamRestorer streamRestorer(fStream);
         if (!fStream->seek(offset)) {
             SkCodecPrintf("Failed to seek to subset stream position.\n");
             return nullptr;
         }
         sk_sp<SkData> data = SkData::MakeUninitialized(size);
         if (fStream->read(data->writable_data(), size) != size) {
             SkCodecPrintf("Failed to read subset stream data.\n");
             return nullptr;
         }
         return SkMemoryStream::Make(data);
     }
 #endif  // SK_CODEC_DECODES_JPEG_GAINMAPS

 private:
     sk_sp<SkData> fBuffer;
 };

 ////////////////////////////////////////////////////////////////////////////////////////////////////
 // SkJpegUnseekableSourceMgr

 #ifdef SK_CODEC_DECODES_JPEG_GAINMAPS
 /*
  * This class implements SkJpegSourceMgr for a stream that cannot seek or rewind. It scans the data
  * as it is presented to the decoder. This allows it to track the position of segments, so that it
  * can extract subsets at a specific offset (e.g, relative to the EndOfImage segment for JpegR or
  * relative to an MPF segment for MPF).
  */
 class SkJpegUnseekableSourceMgr : public SkJpegSourceMgr {
 public:
     SkJpegUnseekableSourceMgr(SkStream* stream, size_t bufferSize) : SkJpegSourceMgr(stream) {
         fBuffer = SkData::MakeUninitialized(bufferSize);
         fScanner = std::make_unique<SkJpegSegmentScanner>(SkJpegSegmentScanner::kMarkerEndOfImage);
     }
     ~SkJpegUnseekableSourceMgr() override {}

     void initSource(const uint8_t*& nextInputByte, size_t& bytesInBuffer) override {
         nextInputByte = fBuffer->bytes();
         bytesInBuffer = 0;
     }
     bool fillInputBuffer(const uint8_t*& nextInputByte, size_t& bytesInBuffer) override {
         if (!readToBufferAndScan(fBuffer->size())) {
             SkCodecPrintf("Failure filling unseekable input buffer.\n");
             return false;
         }
         nextInputByte = fBuffer->bytes();
         bytesInBuffer = fLastReadSize;
         return true;
     }
     bool skipInputBytes(size_t bytesToSkip,
                         const uint8_t*& nextInputByte,
                         size_t& bytesInBuffer) override {
         // Skip through the already-read (or already in memory) buffer.
         if (bytesToSkip <= bytesInBuffer) {
             nextInputByte += bytesToSkip;
             bytesInBuffer -= bytesToSkip;
             return true;
         }
         bytesToSkip -= bytesInBuffer;

         // Read the remaining bytes to skip into fBuffer and feed them into fScanner.
         while (bytesToSkip > 0) {
             if (!readToBufferAndScan(std::min(bytesToSkip, fBuffer->size()))) {
                 SkCodecPrintf("Failure filling unseekable input buffer.\n");
                 return false;
             }
             bytesToSkip -= fLastReadSize;
         }

         // Indicate to libjpeg that it it needs to call fillInputBuffer.
         bytesInBuffer = 0;
         nextInputByte = fBuffer->bytes();
         return true;
     }
     const std::vector<SkJpegSegment>& getAllSegments() override {
         while (!fScanner->isDone() && !fScanner->hadError()) {
             if (!readToBufferAndScan(fBuffer->size())) {
                 SkCodecPrintf("Failure finishing unseekable input buffer.\n");
                 break;
             }
         }
         return fScanner->getSegments();
     }
     std::unique_ptr<SkStream> getSubsetStream(size_t offset, size_t size) override {
         // This function should not be called before the scan has completed.
         SkASSERT(fScanner->isDone() || fScanner->hadError());

         // If we have read past offset, we can never get that data back again.
         if (offset < fLastReadOffset) {
             SkCodecPrintf("Requested subset stream for data that is gone.\n");
             return nullptr;
         }

         // Allocate the memory to return.
         sk_sp<SkData> subsetData = SkData::MakeUninitialized(size);
         uint8_t* subsetDataCurrent = reinterpret_cast<uint8_t*>(subsetData->writable_data());

         // Determine the relationship between the offset we're reading from and |fBuffer|.
         size_t offsetIntoBuffer = offset - fLastReadOffset;
         if (offsetIntoBuffer >= fLastReadSize) {
             // We have to skip past |fBuffer| to get to |offset|.
             fLastReadOffset += fLastReadSize;
             fLastReadSize = 0;

             // Skip any additional bytes needed to get to |offset|.
             size_t bytesToSkip = offset - fLastReadOffset;
             while (bytesToSkip > 0) {
                 size_t bytesSkipped = fStream->skip(bytesToSkip);
                 if (bytesSkipped == 0) {
                     SkCodecPrintf("Failed to skip bytes before subset.\n");
                     return nullptr;
                 }
                 bytesToSkip -= bytesSkipped;
                 fLastReadOffset += bytesSkipped;
             }
         } else {
             // This assert is to emphatically document the side of the branch we're on.
             SkASSERT(offsetIntoBuffer < fLastReadSize);

             // Some of the data we want to copy has already been read into |fBuffer|. Copy that data
             // to |subsetData|
             size_t bytesToReadFromBuffer = std::min(fLastReadSize - offsetIntoBuffer, size);
             memcpy(subsetDataCurrent, fBuffer->bytes() + offsetIntoBuffer, bytesToReadFromBuffer);
             size -= bytesToReadFromBuffer;
             subsetDataCurrent += bytesToReadFromBuffer;

             // If all of the data that we needed was in |fBuffer|, then return early.
             if (size == 0) {
                 return SkMemoryStream::Make(subsetData);
             }
             // We will now have to read beyond |fBuffer|, so reset it.
             fLastReadOffset += fLastReadSize;
             fLastReadSize = 0;
         }

         // Read the remaining data from |fStream|.
         while (size > 0) {
             size_t bytesRead = fStream->read(subsetDataCurrent, size);
             if (bytesRead == 0) {
                 SkCodecPrintf("Failed to read subset stream data.\n");
                 return nullptr;
             }
             size -= bytesRead;
             subsetDataCurrent += bytesRead;
             fLastReadOffset += bytesRead;
         }

         return SkMemoryStream::Make(subsetData);
     }

 private:
     // Read the specified number of bytes into fBuffer and feed them to fScanner. The number of
     // bytes must not be larger than fBuffer's size.
     bool readToBufferAndScan(size_t bytesToRead) {
         SkASSERT(bytesToRead <= fBuffer->size());
         fLastReadOffset += fLastReadSize;
         fLastReadSize = fStream->read(fBuffer->writable_data(), bytesToRead);
         if (fLastReadSize == 0) {
             SkCodecPrintf("Hit end of file reading an unseekable stream.\n");
             return false;
         }
         fScanner->onBytes(fBuffer->bytes(), fLastReadSize);
         return true;
     }

     sk_sp<SkData> fBuffer;

     // The number of bytes that were most recently read into fBuffer (this can be less than the size
     // of fBuffer).
     size_t fLastReadSize = 0;

     // The offset into the stream (total number of bytes read) at the time of our most recent read
     // into fBuffer.
     size_t fLastReadOffset = 0;
 };

 sk_sp<SkData> SkJpegSourceMgr::copyParameters(const SkJpegSegment& segment,
                                               const void* signature,
                                               const size_t signatureLength) {
     // This functionality is only available for seekable streams.
     if (!fStream->hasPosition()) {
         return nullptr;
     }

     constexpr size_t kParameterLengthSize = SkJpegSegmentScanner::kParameterLengthSize;
     constexpr size_t kMarkerCodeSize = SkJpegSegmentScanner::kMarkerCodeSize;
     // If the segment's parameter length isn't long enough for the signature and the length,
     // early-out.
     if (segment.parameterLength < signatureLength + kParameterLengthSize) {
         return nullptr;
     }
     size_t sizeToRead = segment.parameterLength - signatureLength - kParameterLengthSize;

     // Seek to the start of the segment.
     if (!fStream->seek(segment.offset)) {
         SkCodecPrintf("Failed to seek to segment\n");
         return nullptr;
     }

     // Read the marker and verify it matches `segment`.
     uint8_t markerCode[kMarkerCodeSize] = {0};
     if (fStream->read(markerCode, kMarkerCodeSize) != kMarkerCodeSize) {
         SkCodecPrintf("Failed to read segment marker code\n");
         return nullptr;
     }
     SkASSERT(markerCode[0] == 0xFF);
     SkASSERT(markerCode[1] == segment.marker);

     // Read the parameter length and verify it matches `segment`.
     uint8_t parameterLength[kParameterLengthSize] = {0};
     if (fStream->read(parameterLength, kParameterLengthSize) != kParameterLengthSize) {
         SkCodecPrintf("Failed to read parameter length\n");
         return nullptr;
     }
     SkASSERT(256 * parameterLength[0] + parameterLength[1] == segment.parameterLength);

     // Check the next bytes against `signature`.
     auto segmentSignature = SkData::MakeUninitialized(signatureLength);
     if (fStream->read(segmentSignature->writable_data(), segmentSignature->size()) !=
         segmentSignature->size()) {
         SkCodecPrintf("Failed to read parameters\n");
         return nullptr;
     }
     if (memcmp(segmentSignature->data(), signature, signatureLength) != 0) {
         return nullptr;
     }

     // Finally, read the remainder of the segment.
     auto result = SkData::MakeUninitialized(sizeToRead);
     if (fStream->read(result->writable_data(), sizeToRead) != sizeToRead) {
         return nullptr;
     }

     return result;
 }
 #endif  // SK_CODEC_DECODES_JPEG_GAINMAPS

 // static
 std::unique_ptr<SkJpegSourceMgr> SkJpegSourceMgr::Make(SkStream* stream, size_t bufferSize) {
 #ifdef SK_CODEC_DECODES_JPEG_GAINMAPS
     if (!stream->hasPosition()) {
         return std::make_unique<SkJpegUnseekableSourceMgr>(stream, bufferSize);
     }
 #endif
     if (stream->hasLength() && stream->getMemoryBase()) {
         return std::make_unique<SkJpegMemorySourceMgr>(stream);
     }
     return std::make_unique<SkJpegBufferedSourceMgr>(stream, bufferSize);
 }

 SkJpegSourceMgr::SkJpegSourceMgr(SkStream* stream) : fStream(stream) {}

 SkJpegSourceMgr::~SkJpegSourceMgr() = default;

 #else // SK_CODEC_DECODES_JPEG

 std::unique_ptr<SkJpegSourceMgr> SkJpegSourceMgr::Make(SkStream* stream, size_t bufferSize) {
     return nullptr;
 }

 SkJpegSourceMgr::SkJpegSourceMgr(SkStream* stream): fStream(nullptr) {}

 SkJpegSourceMgr::~SkJpegSourceMgr() = default;

 #endif
	/*
	* Copyright 2023 Google Inc.
	*
	* Use of this source code is governed by a BSD-style license that can be
	* found in the LICENSE file.
	*/

	#include "src/codec/SkJpegSourceMgr.h"

	#include "include/core/SkTypes.h"

	#ifdef SK_CODEC_DECODES_JPEG
	#include "include/core/SkData.h"
	#include "include/core/SkRefCnt.h"
	#include "include/core/SkStream.h"
	#include "src/codec/SkCodecPriv.h"

	#ifdef SK_CODEC_DECODES_JPEG_GAINMAPS
	#include "src/codec/SkJpegSegmentScan.h"
	#endif // SK_CODEC_DECODES_JPEG_GAINMAPS

	////////////////////////////////////////////////////////////////////////////////////////////////////
	// SkStream helpers.

	/*
	* Class that will will rewind an SkStream, and then restore it to its original position when it
	* goes out of scope. If the SkStream is not seekable, then the stream will not be altered at all,
	* and will return false from canRestore.
	*/

	class ScopedSkStreamRestorer {
	public:
	ScopedSkStreamRestorer(SkStream* stream) : fStream(stream), fPosition(stream->getPosition()) {
	if (!fStream->rewind()) {
	SkCodecPrintf("Failed to rewind decoder stream.\n");
	}
	}
	~ScopedSkStreamRestorer() {
	if (!fStream->seek(fPosition)) {
	SkCodecPrintf("Failed to restore decoder stream.\n");
	}
	}

	private:
	SkStream* const fStream;
	const size_t fPosition;
	};

	////////////////////////////////////////////////////////////////////////////////////////////////////
	// SkJpegMemorySourceMgr

	class SkJpegMemorySourceMgr : public SkJpegSourceMgr {
	public:
	SkJpegMemorySourceMgr(SkStream* stream) : SkJpegSourceMgr(stream) {}
	~SkJpegMemorySourceMgr() override {}

	void initSource(const uint8_t*& nextInputByte, size_t& bytesInBuffer) override {
	nextInputByte = reinterpret_cast<const uint8_t*>(fStream->getMemoryBase());
	bytesInBuffer = static_cast<size_t>(fStream->getLength());
	}
	bool fillInputBuffer(const uint8_t*& nextInputByte, size_t& bytesInBuffer) override {
	// The whole JPEG data is expected to reside in the supplied memory buffer, so any request
	// for more data beyond the given buffer size is treated as an error.
	SkCodecPrintf("Asked to re-fill a memory-mapped stream.\n");
	return false;
	}
	bool skipInputBytes(size_t bytesToSkip,
	const uint8_t*& nextInputByte,
	size_t& bytesInBuffer) override {
	if (bytesToSkip > bytesInBuffer) {
	SkCodecPrintf("Asked to read past end of a memory-mapped stream.\n");
	return false;
	}
	nextInputByte += bytesToSkip;
	bytesInBuffer -= bytesToSkip;
	return true;
	}
	#ifdef SK_CODEC_DECODES_JPEG_GAINMAPS
	const std::vector<SkJpegSegment>& getAllSegments() override {
	if (fScanner) {
	return fScanner->getSegments();
	}
	fScanner = std::make_unique<SkJpegSegmentScanner>(SkJpegSegmentScanner::kMarkerEndOfImage);
	fScanner->onBytes(fStream->getMemoryBase(), fStream->getLength());
	return fScanner->getSegments();
	}
	std::unique_ptr<SkStream> getSubsetStream(size_t offset, size_t size) override {
	if (offset > fStream->getLength() \|\| size > fStream->getLength() - offset) {
	return nullptr;
	}
	return SkMemoryStream::MakeCopy(
	reinterpret_cast<const uint8_t*>(fStream->getMemoryBase()) + offset, size);
	}
	#endif // SK_CODEC_DECODES_JPEG_GAINMAPS
	};

	////////////////////////////////////////////////////////////////////////////////////////////////////
	// SkJpegBufferedSourceMgr

	class SkJpegBufferedSourceMgr : public SkJpegSourceMgr {
	public:
	SkJpegBufferedSourceMgr(SkStream* stream, size_t bufferSize) : SkJpegSourceMgr(stream) {
	fBuffer = SkData::MakeUninitialized(bufferSize);
	}
	~SkJpegBufferedSourceMgr() override {}

	void initSource(const uint8_t*& nextInputByte, size_t& bytesInBuffer) override {
	nextInputByte = fBuffer->bytes();
	bytesInBuffer = 0;
	}
	bool fillInputBuffer(const uint8_t*& nextInputByte, size_t& bytesInBuffer) override {
	size_t bytesRead = fStream->read(fBuffer->writable_data(), fBuffer->size());
	if (bytesRead == 0) {
	// Fail if we read zero bytes (libjpeg will accept any non-zero number of bytes).
	SkCodecPrintf("Hit end of file reading a buffered stream.\n");
	return false;
	}
	nextInputByte = fBuffer->bytes();
	bytesInBuffer = bytesRead;
	return true;
	}
	bool skipInputBytes(size_t bytesToSkip,
	const uint8_t*& nextInputByte,
	size_t& bytesInBuffer) override {
	// Skip through the already-read (or already in memory) buffer.
	if (bytesToSkip <= bytesInBuffer) {
	nextInputByte += bytesToSkip;
	bytesInBuffer -= bytesToSkip;
	return true;
	}
	bytesToSkip -= bytesInBuffer;

	// Fail if we skip past the end of the stream.
	if (fStream->skip(bytesToSkip) != bytesToSkip) {
	SkCodecPrintf("Failed to skip through buffered stream.\n");
	return false;
	}

	bytesInBuffer = 0;
	nextInputByte = fBuffer->bytes();
	return true;
	}
	#ifdef SK_CODEC_DECODES_JPEG_GAINMAPS
	const std::vector<SkJpegSegment>& getAllSegments() override {
	if (fScanner) {
	return fScanner->getSegments();
	}
	ScopedSkStreamRestorer streamRestorer(fStream);
	fScanner = std::make_unique<SkJpegSegmentScanner>(SkJpegSegmentScanner::kMarkerEndOfImage);
	while (!fScanner->isDone() && !fScanner->hadError()) {
	constexpr size_t kBufferSize = 1024;
	uint8_t buffer[kBufferSize];
	size_t bytesRead = fStream->read(buffer, kBufferSize);
	if (bytesRead == 0) {
	SkCodecPrintf("Unexpected EOF.\n");
	break;
	}
	fScanner->onBytes(buffer, bytesRead);
	}
	return fScanner->getSegments();
	}
	std::unique_ptr<SkStream> getSubsetStream(size_t offset, size_t size) override {
	ScopedSkStreamRestorer streamRestorer(fStream);
	if (!fStream->seek(offset)) {
	SkCodecPrintf("Failed to seek to subset stream position.\n");
	return nullptr;
	}
	sk_sp<SkData> data = SkData::MakeUninitialized(size);
	if (fStream->read(data->writable_data(), size) != size) {
	SkCodecPrintf("Failed to read subset stream data.\n");
	return nullptr;
	}
	return SkMemoryStream::Make(data);
	}
	#endif // SK_CODEC_DECODES_JPEG_GAINMAPS

	private:
	sk_sp<SkData> fBuffer;
	};

	////////////////////////////////////////////////////////////////////////////////////////////////////
	// SkJpegUnseekableSourceMgr

	#ifdef SK_CODEC_DECODES_JPEG_GAINMAPS
	/*
	* This class implements SkJpegSourceMgr for a stream that cannot seek or rewind. It scans the data
	* as it is presented to the decoder. This allows it to track the position of segments, so that it
	* can extract subsets at a specific offset (e.g, relative to the EndOfImage segment for JpegR or
	* relative to an MPF segment for MPF).
	*/
	class SkJpegUnseekableSourceMgr : public SkJpegSourceMgr {
	public:
	SkJpegUnseekableSourceMgr(SkStream* stream, size_t bufferSize) : SkJpegSourceMgr(stream) {
	fBuffer = SkData::MakeUninitialized(bufferSize);
	fScanner = std::make_unique<SkJpegSegmentScanner>(SkJpegSegmentScanner::kMarkerEndOfImage);
	}
	~SkJpegUnseekableSourceMgr() override {}

	void initSource(const uint8_t*& nextInputByte, size_t& bytesInBuffer) override {
	nextInputByte = fBuffer->bytes();
	bytesInBuffer = 0;
	}
	bool fillInputBuffer(const uint8_t*& nextInputByte, size_t& bytesInBuffer) override {
	if (!readToBufferAndScan(fBuffer->size())) {
	SkCodecPrintf("Failure filling unseekable input buffer.\n");
	return false;
	}
	nextInputByte = fBuffer->bytes();
	bytesInBuffer = fLastReadSize;
	return true;
	}
	bool skipInputBytes(size_t bytesToSkip,
	const uint8_t*& nextInputByte,
	size_t& bytesInBuffer) override {
	// Skip through the already-read (or already in memory) buffer.
	if (bytesToSkip <= bytesInBuffer) {
	nextInputByte += bytesToSkip;
	bytesInBuffer -= bytesToSkip;
	return true;
	}
	bytesToSkip -= bytesInBuffer;

	// Read the remaining bytes to skip into fBuffer and feed them into fScanner.
	while (bytesToSkip > 0) {
	if (!readToBufferAndScan(std::min(bytesToSkip, fBuffer->size()))) {
	SkCodecPrintf("Failure filling unseekable input buffer.\n");
	return false;
	}
	bytesToSkip -= fLastReadSize;
	}

	// Indicate to libjpeg that it it needs to call fillInputBuffer.
	bytesInBuffer = 0;
	nextInputByte = fBuffer->bytes();
	return true;
	}
	const std::vector<SkJpegSegment>& getAllSegments() override {
	while (!fScanner->isDone() && !fScanner->hadError()) {
	if (!readToBufferAndScan(fBuffer->size())) {
	SkCodecPrintf("Failure finishing unseekable input buffer.\n");
	break;
	}
	}
	return fScanner->getSegments();
	}
	std::unique_ptr<SkStream> getSubsetStream(size_t offset, size_t size) override {
	// This function should not be called before the scan has completed.
	SkASSERT(fScanner->isDone() \|\| fScanner->hadError());

	// If we have read past offset, we can never get that data back again.
	if (offset < fLastReadOffset) {
	SkCodecPrintf("Requested subset stream for data that is gone.\n");
	return nullptr;
	}

	// Allocate the memory to return.
	sk_sp<SkData> subsetData = SkData::MakeUninitialized(size);
	uint8_t* subsetDataCurrent = reinterpret_cast<uint8_t*>(subsetData->writable_data());

	// Determine the relationship between the offset we're reading from and \|fBuffer\|.
	size_t offsetIntoBuffer = offset - fLastReadOffset;
	if (offsetIntoBuffer >= fLastReadSize) {
	// We have to skip past \|fBuffer\| to get to \|offset\|.
	fLastReadOffset += fLastReadSize;
	fLastReadSize = 0;

	// Skip any additional bytes needed to get to \|offset\|.
	size_t bytesToSkip = offset - fLastReadOffset;
	while (bytesToSkip > 0) {
	size_t bytesSkipped = fStream->skip(bytesToSkip);
	if (bytesSkipped == 0) {
	SkCodecPrintf("Failed to skip bytes before subset.\n");
	return nullptr;
	}
	bytesToSkip -= bytesSkipped;
	fLastReadOffset += bytesSkipped;
	}
	} else {
	// This assert is to emphatically document the side of the branch we're on.
	SkASSERT(offsetIntoBuffer < fLastReadSize);

	// Some of the data we want to copy has already been read into \|fBuffer\|. Copy that data
	// to \|subsetData\|
	size_t bytesToReadFromBuffer = std::min(fLastReadSize - offsetIntoBuffer, size);
	memcpy(subsetDataCurrent, fBuffer->bytes() + offsetIntoBuffer, bytesToReadFromBuffer);
	size -= bytesToReadFromBuffer;
	subsetDataCurrent += bytesToReadFromBuffer;

	// If all of the data that we needed was in \|fBuffer\|, then return early.
	if (size == 0) {
	return SkMemoryStream::Make(subsetData);
	}
	// We will now have to read beyond \|fBuffer\|, so reset it.
	fLastReadOffset += fLastReadSize;
	fLastReadSize = 0;
	}

	// Read the remaining data from \|fStream\|.
	while (size > 0) {
	size_t bytesRead = fStream->read(subsetDataCurrent, size);
	if (bytesRead == 0) {
	SkCodecPrintf("Failed to read subset stream data.\n");
	return nullptr;
	}
	size -= bytesRead;
	subsetDataCurrent += bytesRead;
	fLastReadOffset += bytesRead;
	}

	return SkMemoryStream::Make(subsetData);
	}

	private:
	// Read the specified number of bytes into fBuffer and feed them to fScanner. The number of
	// bytes must not be larger than fBuffer's size.
	bool readToBufferAndScan(size_t bytesToRead) {
	SkASSERT(bytesToRead <= fBuffer->size());
	fLastReadOffset += fLastReadSize;
	fLastReadSize = fStream->read(fBuffer->writable_data(), bytesToRead);
	if (fLastReadSize == 0) {
	SkCodecPrintf("Hit end of file reading an unseekable stream.\n");
	return false;
	}
	fScanner->onBytes(fBuffer->bytes(), fLastReadSize);
	return true;
	}

	sk_sp<SkData> fBuffer;

	// The number of bytes that were most recently read into fBuffer (this can be less than the size
	// of fBuffer).
	size_t fLastReadSize = 0;

	// The offset into the stream (total number of bytes read) at the time of our most recent read
	// into fBuffer.
	size_t fLastReadOffset = 0;
	};

	sk_sp<SkData> SkJpegSourceMgr::copyParameters(const SkJpegSegment& segment,
	const void* signature,
	const size_t signatureLength) {
	// This functionality is only available for seekable streams.
	if (!fStream->hasPosition()) {
	return nullptr;
	}

	constexpr size_t kParameterLengthSize = SkJpegSegmentScanner::kParameterLengthSize;
	constexpr size_t kMarkerCodeSize = SkJpegSegmentScanner::kMarkerCodeSize;
	// If the segment's parameter length isn't long enough for the signature and the length,
	// early-out.
	if (segment.parameterLength < signatureLength + kParameterLengthSize) {
	return nullptr;
	}
	size_t sizeToRead = segment.parameterLength - signatureLength - kParameterLengthSize;

	// Seek to the start of the segment.
	if (!fStream->seek(segment.offset)) {
	SkCodecPrintf("Failed to seek to segment\n");
	return nullptr;
	}

	// Read the marker and verify it matches `segment`.
	uint8_t markerCode[kMarkerCodeSize] = {0};
	if (fStream->read(markerCode, kMarkerCodeSize) != kMarkerCodeSize) {
	SkCodecPrintf("Failed to read segment marker code\n");
	return nullptr;
	}
	SkASSERT(markerCode[0] == 0xFF);
	SkASSERT(markerCode[1] == segment.marker);

	// Read the parameter length and verify it matches `segment`.
	uint8_t parameterLength[kParameterLengthSize] = {0};
	if (fStream->read(parameterLength, kParameterLengthSize) != kParameterLengthSize) {
	SkCodecPrintf("Failed to read parameter length\n");
	return nullptr;
	}
	SkASSERT(256 * parameterLength[0] + parameterLength[1] == segment.parameterLength);

	// Check the next bytes against `signature`.
	auto segmentSignature = SkData::MakeUninitialized(signatureLength);
	if (fStream->read(segmentSignature->writable_data(), segmentSignature->size()) !=
	segmentSignature->size()) {
	SkCodecPrintf("Failed to read parameters\n");
	return nullptr;
	}
	if (memcmp(segmentSignature->data(), signature, signatureLength) != 0) {
	return nullptr;
	}

	// Finally, read the remainder of the segment.
	auto result = SkData::MakeUninitialized(sizeToRead);
	if (fStream->read(result->writable_data(), sizeToRead) != sizeToRead) {
	return nullptr;
	}

	return result;
	}
	#endif // SK_CODEC_DECODES_JPEG_GAINMAPS

	// static
	std::unique_ptr<SkJpegSourceMgr> SkJpegSourceMgr::Make(SkStream* stream, size_t bufferSize) {
	#ifdef SK_CODEC_DECODES_JPEG_GAINMAPS
	if (!stream->hasPosition()) {
	return std::make_unique<SkJpegUnseekableSourceMgr>(stream, bufferSize);
	}
	#endif
	if (stream->hasLength() && stream->getMemoryBase()) {
	return std::make_unique<SkJpegMemorySourceMgr>(stream);
	}
	return std::make_unique<SkJpegBufferedSourceMgr>(stream, bufferSize);
	}

	SkJpegSourceMgr::SkJpegSourceMgr(SkStream* stream) : fStream(stream) {}

	SkJpegSourceMgr::~SkJpegSourceMgr() = default;

	#else // SK_CODEC_DECODES_JPEG

	std::unique_ptr<SkJpegSourceMgr> SkJpegSourceMgr::Make(SkStream* stream, size_t bufferSize) {
	return nullptr;
	}

	SkJpegSourceMgr::SkJpegSourceMgr(SkStream* stream): fStream(nullptr) {}

	SkJpegSourceMgr::~SkJpegSourceMgr() = default;

	#endif