tor-browser

nsBMPDecoder.cpp (46294B)

---

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */

// This is a cross-platform BMP Decoder, which should work everywhere,
// including big-endian machines like the PowerPC.
//
// BMP is a format that has been extended multiple times. To understand the
// decoder you need to understand this history. The summary of the history
// below was determined from the following documents.
//
// - http://www.fileformat.info/format/bmp/egff.htm
// - http://www.fileformat.info/format/os2bmp/egff.htm
// - http://fileformats.archiveteam.org/wiki/BMP
// - http://fileformats.archiveteam.org/wiki/OS/2_BMP
// - https://en.wikipedia.org/wiki/BMP_file_format
// - https://upload.wikimedia.org/wikipedia/commons/c/c4/BMPfileFormat.png
//
// WINDOWS VERSIONS OF THE BMP FORMAT
// ----------------------------------
// WinBMPv1.
// - This version is no longer used and can be ignored.
//
// WinBMPv2.
// - First is a 14 byte file header that includes: the magic number ("BM"),
//   file size, and offset to the pixel data (|mDataOffset|).
// - Next is a 12 byte info header which includes: the info header size
//   (mBIHSize), width, height, number of color planes, and bits-per-pixel
//   (|mBpp|) which must be 1, 4, 8 or 24.
// - Next is the semi-optional color table, which has length 2^|mBpp| and has 3
//   bytes per value (BGR). The color table is required if |mBpp| is 1, 4, or 8.
// - Next is an optional gap.
// - Next is the pixel data, which is pointed to by |mDataOffset|.
//
// WinBMPv3. This is the most widely used version.
// - It changed the info header to 40 bytes by taking the WinBMPv2 info
//   header, enlargening its width and height fields, and adding more fields
//   including: a compression type (|mCompression|) and number of colors
//   (|mNumColors|).
// - The semi-optional color table is now 4 bytes per value (BGR0), and its
//   length is |mNumColors|, or 2^|mBpp| if |mNumColors| is zero.
// - |mCompression| can be RGB (i.e. no compression), RLE4 (if |mBpp|==4) or
//   RLE8 (if |mBpp|==8) values.
//
// WinBMPv3-NT. A variant of WinBMPv3.
// - It did not change the info header layout from WinBMPv3.
// - |mBpp| can now be 16 or 32, in which case |mCompression| can be RGB or the
//   new BITFIELDS value; in the latter case an additional 12 bytes of color
//   bitfields follow the info header.
//
// WinBMPv4.
// - It extended the info header to 108 bytes, including the 12 bytes of color
//   mask data from WinBMPv3-NT, plus alpha mask data, and also color-space and
//   gamma correction fields.
//
// WinBMPv5.
// - It extended the info header to 124 bytes, adding color profile data.
// - It also added an optional color profile table after the pixel data (and
//   another optional gap).
//
// WinBMPv3-ICO. This is a variant of WinBMPv3.
// - It's the BMP format used for BMP images within ICO files.
// - The only difference with WinBMPv3 is that if an image is 32bpp and has no
//   compression, then instead of treating the pixel data as 0RGB it is treated
//   as ARGB, but only if one or more of the A values are non-zero.
//
// Clipboard variants.
// - It's the BMP format used for BMP images captured from the clipboard.
// - It is missing the file header, containing the BM signature and the data
//   offset. Instead the data begins after the header.
// - If it uses BITFIELDS compression, then there is always an additional 12
//   bytes of data after the header that must be read. In WinBMPv4+, the masks
//   are supposed to be included in the header size, which are the values we use
//   for decoding purposes, but there is additional three masks following the
//   header which must be skipped to get to the pixel data.
//
// OS/2 VERSIONS OF THE BMP FORMAT
// -------------------------------
// OS2-BMPv1.
// - Almost identical to WinBMPv2; the differences are basically ignorable.
//
// OS2-BMPv2.
// - Similar to WinBMPv3.
// - The info header is 64 bytes but can be reduced to as little as 16; any
//   omitted fields are treated as zero. The first 40 bytes of these fields are
//   nearly identical to the WinBMPv3 info header; the remaining 24 bytes are
//   different.
// - Also adds compression types "Huffman 1D" and "RLE24", which we don't
//   support.
// - We treat OS2-BMPv2 files as if they are WinBMPv3 (i.e. ignore the extra 24
//   bytes in the info header), which in practice is good enough.

#include "ImageLogging.h"
#include "nsBMPDecoder.h"

#include <stdlib.h>

#include "mozilla/Attributes.h"
#include "mozilla/EndianUtils.h"
#include "mozilla/UniquePtrExtensions.h"

#include "RasterImage.h"
#include "SurfacePipeFactory.h"
#include "gfxPlatform.h"
#include <algorithm>

using namespace mozilla::gfx;

namespace mozilla {
namespace image {
namespace bmp {

struct Compression {
 enum { RGB = 0, RLE8 = 1, RLE4 = 2, BITFIELDS = 3 };
};

// RLE escape codes and constants.
struct RLE {
 enum {
   ESCAPE = 0,
   ESCAPE_EOL = 0,
   ESCAPE_EOF = 1,
   ESCAPE_DELTA = 2,

   SEGMENT_LENGTH = 2,
   DELTA_LENGTH = 2
 };
};

}  // namespace bmp

using namespace bmp;

static double FixedPoint2Dot30_To_Double(uint32_t aFixed) {
 constexpr double factor = 1.0 / 1073741824.0;  // 2^-30
 return double(aFixed) * factor;
}

static float FixedPoint16Dot16_To_Float(uint32_t aFixed) {
 constexpr double factor = 1.0 / 65536.0;  // 2^-16
 return double(aFixed) * factor;
}

static float CalRbgEndpointToQcms(const CalRgbEndpoint& aIn,
                                 qcms_CIE_xyY& aOut) {
 aOut.x = FixedPoint2Dot30_To_Double(aIn.mX);
 aOut.y = FixedPoint2Dot30_To_Double(aIn.mY);
 aOut.Y = FixedPoint2Dot30_To_Double(aIn.mZ);
 return FixedPoint16Dot16_To_Float(aIn.mGamma);
}

static void ReadCalRgbEndpoint(const char* aData, uint32_t aEndpointOffset,
                              uint32_t aGammaOffset, CalRgbEndpoint& aOut) {
 aOut.mX = LittleEndian::readUint32(aData + aEndpointOffset);
 aOut.mY = LittleEndian::readUint32(aData + aEndpointOffset + 4);
 aOut.mZ = LittleEndian::readUint32(aData + aEndpointOffset + 8);
 aOut.mGamma = LittleEndian::readUint32(aData + aGammaOffset);
}

/// Sets the pixel data in aDecoded to the given values.
/// @param aDecoded pointer to pixel to be set, will be incremented to point to
/// the next pixel.
static void SetPixel(uint32_t*& aDecoded, uint8_t aRed, uint8_t aGreen,
                    uint8_t aBlue, uint8_t aAlpha = 0xFF) {
 *aDecoded++ = gfxPackedPixelNoPreMultiply(aAlpha, aRed, aGreen, aBlue);
}

static void SetPixel(uint32_t*& aDecoded, uint8_t idx,
                    const UniquePtr<ColorTableEntry[]>& aColors) {
 SetPixel(aDecoded, aColors[idx].mRed, aColors[idx].mGreen,
          aColors[idx].mBlue);
}

/// Sets two (or one if aCount = 1) pixels
/// @param aDecoded where the data is stored. Will be moved 4 resp 8 bytes
/// depending on whether one or two pixels are written.
/// @param aData The values for the two pixels
/// @param aCount Current count. Is decremented by one or two.
static void Set4BitPixel(uint32_t*& aDecoded, uint8_t aData, uint32_t& aCount,
                        const UniquePtr<ColorTableEntry[]>& aColors) {
 uint8_t idx = aData >> 4;
 SetPixel(aDecoded, idx, aColors);
 if (--aCount > 0) {
   idx = aData & 0xF;
   SetPixel(aDecoded, idx, aColors);
   --aCount;
 }
}

static mozilla::LazyLogModule sBMPLog("BMPDecoder");

// The length of the mBIHSize field in the info header.
static const uint32_t BIHSIZE_FIELD_LENGTH = 4;

nsBMPDecoder::nsBMPDecoder(RasterImage* aImage, State aState, size_t aLength,
                          bool aForClipboard)
   : Decoder(aImage),
     mLexer(Transition::To(aState, aLength), Transition::TerminateSuccess()),
     mIsWithinICO(false),
     mIsForClipboard(aForClipboard),
     mMayHaveTransparency(false),
     mDoesHaveTransparency(false),
     mNumColors(0),
     mColors(nullptr),
     mBytesPerColor(0),
     mPreGapLength(0),
     mPixelRowSize(0),
     mCurrentRow(0),
     mCurrentPos(0),
     mAbsoluteModeNumPixels(0) {}

// Constructor for normal BMP files or from the clipboard.
nsBMPDecoder::nsBMPDecoder(RasterImage* aImage, bool aForClipboard)
   : nsBMPDecoder(aImage,
                  aForClipboard ? State::INFO_HEADER_SIZE : State::FILE_HEADER,
                  aForClipboard ? BIHSIZE_FIELD_LENGTH : FILE_HEADER_LENGTH,
                  aForClipboard) {}

// Constructor used for WinBMPv3-ICO files, which lack a file header.
nsBMPDecoder::nsBMPDecoder(RasterImage* aImage, uint32_t aDataOffset)
   : nsBMPDecoder(aImage, State::INFO_HEADER_SIZE, BIHSIZE_FIELD_LENGTH,
                  /* aForClipboard */ false) {
 SetIsWithinICO();

 // Even though the file header isn't present in this case, the dataOffset
 // field is set as if it is, and so we must increment mPreGapLength
 // accordingly.
 mPreGapLength += FILE_HEADER_LENGTH;

 // This is the one piece of data we normally get from a BMP file header, so
 // it must be provided via an argument.
 mH.mDataOffset = aDataOffset;
}

nsBMPDecoder::~nsBMPDecoder() {}

// Obtains the size of the compressed image resource.
int32_t nsBMPDecoder::GetCompressedImageSize() const {
 // In the RGB case mImageSize might not be set, so compute it manually.
 MOZ_ASSERT(mPixelRowSize != 0);
 return mH.mCompression == Compression::RGB ? mPixelRowSize * AbsoluteHeight()
                                            : mH.mImageSize;
}

nsresult nsBMPDecoder::BeforeFinishInternal() {
 if (!IsMetadataDecode() && !mImageData) {
   return NS_ERROR_FAILURE;  // No image; something went wrong.
 }

 return NS_OK;
}

nsresult nsBMPDecoder::FinishInternal() {
 // We shouldn't be called in error cases.
 MOZ_ASSERT(!HasError(), "Can't call FinishInternal on error!");

 // We should never make multiple frames.
 MOZ_ASSERT(GetFrameCount() <= 1, "Multiple BMP frames?");

 // Send notifications if appropriate.
 if (!IsMetadataDecode() && HasSize()) {
   // We should have image data.
   MOZ_ASSERT(mImageData);

   // If it was truncated, fill in the missing pixels as black.
   while (mCurrentRow > 0) {
     uint32_t* dst = RowBuffer();
     while (mCurrentPos < mH.mWidth) {
       SetPixel(dst, 0, 0, 0);
       mCurrentPos++;
     }
     mCurrentPos = 0;
     FinishRow();
   }

   MOZ_ASSERT_IF(mDoesHaveTransparency, mMayHaveTransparency);

   // We have transparency if we either detected some in the image itself
   // (i.e., |mDoesHaveTransparency| is true) or we're in an ICO, which could
   // mean we have an AND mask that provides transparency (i.e., |mIsWithinICO|
   // is true).
   // XXX(seth): We can tell when we create the decoder if the AND mask is
   // present, so we could be more precise about this.
   const Opacity opacity = mDoesHaveTransparency || mIsWithinICO
                               ? Opacity::SOME_TRANSPARENCY
                               : Opacity::FULLY_OPAQUE;

   PostFrameStop(opacity);
   PostDecodeDone();
 }

 return NS_OK;
}

// ----------------------------------------
// Actual Data Processing
// ----------------------------------------

void BitFields::Value::Set(uint32_t aMask) {
 mMask = aMask;

 // Handle this exceptional case first. The chosen values don't matter
 // (because a mask of zero will always give a value of zero) except that
 // mBitWidth:
 // - shouldn't be zero, because that would cause an infinite loop in Get();
 // - shouldn't be 5 or 8, because that could cause a false positive match in
 //   IsR5G5B5() or IsR8G8B8().
 if (mMask == 0x0) {
   mRightShift = 0;
   mBitWidth = 1;
   return;
 }

 // Find the rightmost 1.
 uint8_t i;
 for (i = 0; i < 32; i++) {
   if (mMask & (1 << i)) {
     break;
   }
 }
 mRightShift = i;

 // Now find the leftmost 1 in the same run of 1s. (If there are multiple runs
 // of 1s -- which isn't valid -- we'll behave as if only the lowest run was
 // present, which seems reasonable.)
 for (i = i + 1; i < 32; i++) {
   if (!(mMask & (1 << i))) {
     break;
   }
 }
 mBitWidth = i - mRightShift;
}

MOZ_ALWAYS_INLINE uint8_t BitFields::Value::Get(uint32_t aValue) const {
 // Extract the unscaled value.
 uint32_t v = (aValue & mMask) >> mRightShift;

 // Idea: to upscale v precisely we need to duplicate its bits, possibly
 // repeatedly, possibly partially in the last case, from bit 7 down to bit 0
 // in v2. For example:
 //
 // - mBitWidth=1:  v2 = v<<7 | v<<6 | ... | v<<1 | v>>0     k -> kkkkkkkk
 // - mBitWidth=2:  v2 = v<<6 | v<<4 | v<<2 | v>>0          jk -> jkjkjkjk
 // - mBitWidth=3:  v2 = v<<5 | v<<2 | v>>1                ijk -> ijkijkij
 // - mBitWidth=4:  v2 = v<<4 | v>>0                      hijk -> hijkhijk
 // - mBitWidth=5:  v2 = v<<3 | v>>2                     ghijk -> ghijkghi
 // - mBitWidth=6:  v2 = v<<2 | v>>4                    fghijk -> fghijkfg
 // - mBitWidth=7:  v2 = v<<1 | v>>6                   efghijk -> efghijke
 // - mBitWidth=8:  v2 = v>>0                         defghijk -> defghijk
 // - mBitWidth=9:  v2 = v>>1                        cdefghijk -> cdefghij
 // - mBitWidth=10: v2 = v>>2                       bcdefghijk -> bcdefghi
 // - mBitWidth=11: v2 = v>>3                      abcdefghijk -> abcdefgh
 // - etc.
 //
 uint8_t v2 = 0;
 int32_t i;  // must be a signed integer
 for (i = 8 - mBitWidth; i > 0; i -= mBitWidth) {
   v2 |= v << uint32_t(i);
 }
 v2 |= v >> uint32_t(-i);
 return v2;
}

MOZ_ALWAYS_INLINE uint8_t BitFields::Value::GetAlpha(uint32_t aValue,
                                                    bool& aHasAlphaOut) const {
 if (mMask == 0x0) {
   return 0xff;
 }
 aHasAlphaOut = true;
 return Get(aValue);
}

MOZ_ALWAYS_INLINE uint8_t BitFields::Value::Get5(uint32_t aValue) const {
 MOZ_ASSERT(mBitWidth == 5);
 uint32_t v = (aValue & mMask) >> mRightShift;
 return (v << 3u) | (v >> 2u);
}

MOZ_ALWAYS_INLINE uint8_t BitFields::Value::Get8(uint32_t aValue) const {
 MOZ_ASSERT(mBitWidth == 8);
 uint32_t v = (aValue & mMask) >> mRightShift;
 return v;
}

void BitFields::SetR5G5B5() {
 mRed.Set(0x7c00);
 mGreen.Set(0x03e0);
 mBlue.Set(0x001f);
}

void BitFields::SetR8G8B8() {
 mRed.Set(0xff0000);
 mGreen.Set(0xff00);
 mBlue.Set(0x00ff);
}

bool BitFields::IsR5G5B5() const {
 return mRed.mBitWidth == 5 && mGreen.mBitWidth == 5 && mBlue.mBitWidth == 5 &&
        mAlpha.mMask == 0x0;
}

bool BitFields::IsR8G8B8() const {
 return mRed.mBitWidth == 8 && mGreen.mBitWidth == 8 && mBlue.mBitWidth == 8 &&
        mAlpha.mMask == 0x0;
}

uint32_t* nsBMPDecoder::RowBuffer() { return mRowBuffer.get() + mCurrentPos; }

void nsBMPDecoder::ClearRowBufferRemainder() {
 int32_t len = mH.mWidth - mCurrentPos;
 memset(RowBuffer(), mMayHaveTransparency ? 0 : 0xFF, len * sizeof(uint32_t));
}

void nsBMPDecoder::FinishRow() {
 mPipe.WriteBuffer(mRowBuffer.get());
 Maybe<SurfaceInvalidRect> invalidRect = mPipe.TakeInvalidRect();
 if (invalidRect) {
   PostInvalidation(invalidRect->mInputSpaceRect,
                    Some(invalidRect->mOutputSpaceRect));
 }
 mCurrentRow--;
}

LexerResult nsBMPDecoder::DoDecode(SourceBufferIterator& aIterator,
                                  IResumable* aOnResume) {
 MOZ_ASSERT(!HasError(), "Shouldn't call DoDecode after error!");

 return mLexer.Lex(
     aIterator, aOnResume,
     [=](State aState, const char* aData, size_t aLength) {
       switch (aState) {
         case State::FILE_HEADER:
           return ReadFileHeader(aData, aLength);
         case State::INFO_HEADER_SIZE:
           return ReadInfoHeaderSize(aData, aLength);
         case State::INFO_HEADER_REST:
           return ReadInfoHeaderRest(aData, aLength);
         case State::BITFIELDS:
           return ReadBitfields(aData, aLength);
         case State::SKIP_TO_COLOR_PROFILE:
           return Transition::ContinueUnbuffered(State::SKIP_TO_COLOR_PROFILE);
         case State::FOUND_COLOR_PROFILE:
           return Transition::To(State::COLOR_PROFILE,
                                 mH.mColorSpace.mProfile.mLength);
         case State::COLOR_PROFILE:
           return ReadColorProfile(aData, aLength);
         case State::ALLOCATE_SURFACE:
           return AllocateSurface();
         case State::COLOR_TABLE:
           return ReadColorTable(aData, aLength);
         case State::GAP:
           return SkipGap();
         case State::AFTER_GAP:
           return AfterGap();
         case State::PIXEL_ROW:
           return ReadPixelRow(aData);
         case State::RLE_SEGMENT:
           return ReadRLESegment(aData);
         case State::RLE_DELTA:
           return ReadRLEDelta(aData);
         case State::RLE_ABSOLUTE:
           return ReadRLEAbsolute(aData, aLength);
         default:
           MOZ_CRASH("Unknown State");
       }
     });
}

LexerTransition<nsBMPDecoder::State> nsBMPDecoder::ReadFileHeader(
   const char* aData, size_t aLength) {
 mPreGapLength += aLength;

 bool signatureOk = aData[0] == 'B' && aData[1] == 'M';
 if (!signatureOk) {
   return Transition::TerminateFailure();
 }

 // We ignore the filesize (aData + 2) and reserved (aData + 6) fields.

 mH.mDataOffset = LittleEndian::readUint32(aData + 10);

 return Transition::To(State::INFO_HEADER_SIZE, BIHSIZE_FIELD_LENGTH);
}

// We read the info header in two steps: (a) read the mBIHSize field to
// determine how long the header is; (b) read the rest of the header.
LexerTransition<nsBMPDecoder::State> nsBMPDecoder::ReadInfoHeaderSize(
   const char* aData, size_t aLength) {
 mH.mBIHSize = LittleEndian::readUint32(aData);

 // Data offset can be wrong so fix it using the BIH size.
 if (!mIsForClipboard && mH.mDataOffset < mPreGapLength + mH.mBIHSize) {
   mH.mDataOffset = mPreGapLength + mH.mBIHSize;
 }

 mPreGapLength += aLength;

 bool bihSizeOk = mH.mBIHSize == InfoHeaderLength::WIN_V2 ||
                  mH.mBIHSize == InfoHeaderLength::WIN_V3 ||
                  mH.mBIHSize == InfoHeaderLength::WIN_V4 ||
                  mH.mBIHSize == InfoHeaderLength::WIN_V5 ||
                  (mH.mBIHSize >= InfoHeaderLength::OS2_V2_MIN &&
                   mH.mBIHSize <= InfoHeaderLength::OS2_V2_MAX);
 if (!bihSizeOk) {
   return Transition::TerminateFailure();
 }
 // ICO BMPs must have a WinBMPv3 header. nsICODecoder should have already
 // terminated decoding if this isn't the case.
 MOZ_ASSERT_IF(mIsWithinICO, mH.mBIHSize == InfoHeaderLength::WIN_V3);

 return Transition::To(State::INFO_HEADER_REST,
                       mH.mBIHSize - BIHSIZE_FIELD_LENGTH);
}

LexerTransition<nsBMPDecoder::State> nsBMPDecoder::ReadInfoHeaderRest(
   const char* aData, size_t aLength) {
 mPreGapLength += aLength;

 // |mWidth| and |mHeight| may be signed (Windows) or unsigned (OS/2). We just
 // read as unsigned because in practice that's good enough.
 if (mH.mBIHSize == InfoHeaderLength::WIN_V2) {
   mH.mWidth = LittleEndian::readUint16(aData + 0);
   mH.mHeight = LittleEndian::readUint16(aData + 2);
   // We ignore the planes (aData + 4) field; it should always be 1.
   mH.mBpp = LittleEndian::readUint16(aData + 6);
 } else {
   mH.mWidth = LittleEndian::readUint32(aData + 0);
   mH.mHeight = LittleEndian::readUint32(aData + 4);
   // We ignore the planes (aData + 4) field; it should always be 1.
   mH.mBpp = LittleEndian::readUint16(aData + 10);

   // For OS2-BMPv2 the info header may be as little as 16 bytes, so be
   // careful for these fields.
   mH.mCompression = aLength >= 16 ? LittleEndian::readUint32(aData + 12) : 0;
   mH.mImageSize = aLength >= 20 ? LittleEndian::readUint32(aData + 16) : 0;
   // We ignore the xppm (aData + 20) and yppm (aData + 24) fields.
   mH.mNumColors = aLength >= 32 ? LittleEndian::readUint32(aData + 28) : 0;
   // We ignore the important_colors (aData + 36) field.

   // Read color management properties we may need later.
   mH.mCsType =
       aLength >= 56
           ? static_cast<InfoColorSpace>(LittleEndian::readUint32(aData + 52))
           : InfoColorSpace::SRGB;
   mH.mCsIntent = aLength >= 108 ? static_cast<InfoColorIntent>(
                                       LittleEndian::readUint32(aData + 104))
                                 : InfoColorIntent::IMAGES;

   switch (mH.mCsType) {
     case InfoColorSpace::CALIBRATED_RGB:
       if (aLength >= 104) {
         ReadCalRgbEndpoint(aData, 56, 92, mH.mColorSpace.mCalibrated.mRed);
         ReadCalRgbEndpoint(aData, 68, 96, mH.mColorSpace.mCalibrated.mGreen);
         ReadCalRgbEndpoint(aData, 80, 100, mH.mColorSpace.mCalibrated.mBlue);
       } else {
         mH.mCsType = InfoColorSpace::SRGB;
       }
       break;
     case InfoColorSpace::EMBEDDED:
       if (aLength >= 116) {
         mH.mColorSpace.mProfile.mOffset =
             LittleEndian::readUint32(aData + 108);
         mH.mColorSpace.mProfile.mLength =
             LittleEndian::readUint32(aData + 112);
       } else {
         mH.mCsType = InfoColorSpace::SRGB;
       }
       break;
     case InfoColorSpace::LINKED:
     case InfoColorSpace::SRGB:
     case InfoColorSpace::WIN:
     default:
       // Nothing to be done at this time.
       break;
   }

   // For WinBMPv4, WinBMPv5 and (possibly) OS2-BMPv2 there are additional
   // fields in the info header which we ignore, with the possible exception
   // of the color bitfields (see below).
 }

 // The height for BMPs embedded inside an ICO includes spaces for the AND
 // mask even if it is not present, thus we need to adjust for that here.
 if (mIsWithinICO) {
   // XXX(seth): Should we really be writing the absolute value from
   // the BIH below? Seems like this could be problematic for inverted BMPs.
   mH.mHeight = abs(mH.mHeight) / 2;
 }

 // Run with MOZ_LOG=BMPDecoder:5 set to see this output.
 MOZ_LOG(sBMPLog, LogLevel::Debug,
         ("BMP: bihsize=%u, %d x %d, bpp=%u, compression=%u, colors=%u, "
          "data-offset=%u\n",
          mH.mBIHSize, mH.mWidth, mH.mHeight, uint32_t(mH.mBpp),
          mH.mCompression, mH.mNumColors, mH.mDataOffset));

 // BMPs with negative width are invalid. Also, reject extremely wide images
 // to keep the math sane. And reject INT_MIN as a height because you can't
 // get its absolute value (because -INT_MIN is one more than INT_MAX).
 const int32_t k64KWidth = 0x0000FFFF;
 bool sizeOk =
     0 <= mH.mWidth && mH.mWidth <= k64KWidth && mH.mHeight != INT_MIN;
 if (!sizeOk) {
   return Transition::TerminateFailure();
 }

 // Check mBpp and mCompression.
 bool bppCompressionOk =
     (mH.mCompression == Compression::RGB &&
      (mH.mBpp == 1 || mH.mBpp == 4 || mH.mBpp == 8 || mH.mBpp == 16 ||
       mH.mBpp == 24 || mH.mBpp == 32)) ||
     (mH.mCompression == Compression::RLE8 && mH.mBpp == 8) ||
     (mH.mCompression == Compression::RLE4 && mH.mBpp == 4) ||
     (mH.mCompression == Compression::BITFIELDS &&
      // For BITFIELDS compression we require an exact match for one of the
      // WinBMP BIH sizes; this clearly isn't an OS2 BMP.
      (mH.mBIHSize == InfoHeaderLength::WIN_V3 ||
       mH.mBIHSize == InfoHeaderLength::WIN_V4 ||
       mH.mBIHSize == InfoHeaderLength::WIN_V5) &&
      (mH.mBpp == 16 || mH.mBpp == 32));
 if (!bppCompressionOk) {
   return Transition::TerminateFailure();
 }

 // Initialize our current row to the top of the image.
 mCurrentRow = AbsoluteHeight();

 // Round it up to the nearest byte count, then pad to 4-byte boundary.
 // Compute this even for a metadate decode because GetCompressedImageSize()
 // relies on it.
 mPixelRowSize = (mH.mBpp * mH.mWidth + 7) / 8;
 uint32_t surplus = mPixelRowSize % 4;
 if (surplus != 0) {
   mPixelRowSize += 4 - surplus;
 }

 size_t bitFieldsLengthStillToRead = 0;
 if (mH.mCompression == Compression::BITFIELDS) {
   // Need to read bitfields.
   if (mH.mBIHSize >= InfoHeaderLength::WIN_V4) {
     // Bitfields are present in the info header, so we can read them
     // immediately.
     mBitFields.ReadFromHeader(aData + 36, /* aReadAlpha = */ true);

     // If this came from the clipboard, then we know that even if the header
     // explicitly includes the bitfield masks, we need to add an additional
     // offset for the start of the RGB data.
     if (mIsForClipboard) {
       mH.mDataOffset += BitFields::LENGTH;
     }
   } else {
     // Bitfields are present after the info header, so we will read them in
     // ReadBitfields().
     bitFieldsLengthStillToRead = BitFields::LENGTH;
   }
 } else if (mH.mBpp == 16) {
   // No bitfields specified; use the default 5-5-5 values.
   mBitFields.SetR5G5B5();
 } else if (mH.mBpp == 32) {
   // No bitfields specified; use the default 8-8-8 values.
   mBitFields.SetR8G8B8();
 }

 return Transition::To(State::BITFIELDS, bitFieldsLengthStillToRead);
}

void BitFields::ReadFromHeader(const char* aData, bool aReadAlpha) {
 mRed.Set(LittleEndian::readUint32(aData + 0));
 mGreen.Set(LittleEndian::readUint32(aData + 4));
 mBlue.Set(LittleEndian::readUint32(aData + 8));
 if (aReadAlpha) {
   mAlpha.Set(LittleEndian::readUint32(aData + 12));
 }
}

LexerTransition<nsBMPDecoder::State> nsBMPDecoder::ReadBitfields(
   const char* aData, size_t aLength) {
 mPreGapLength += aLength;

 // If aLength is zero there are no bitfields to read, or we already read them
 // in ReadInfoHeader().
 if (aLength != 0) {
   mBitFields.ReadFromHeader(aData, /* aReadAlpha = */ false);
 }

 // Note that RLE-encoded BMPs might be transparent because the 'delta' mode
 // can skip pixels and cause implicit transparency.
 mMayHaveTransparency = mIsWithinICO || mH.mCompression == Compression::RLE8 ||
                        mH.mCompression == Compression::RLE4 ||
                        (mH.mCompression == Compression::BITFIELDS &&
                         mBitFields.mAlpha.IsPresent());
 if (mMayHaveTransparency) {
   PostHasTransparency();
 }

 // Post our size to the superclass.
 PostSize(mH.mWidth, AbsoluteHeight());
 if (WantsFrameCount()) {
   PostFrameCount(/* aFrameCount */ 1);
 }
 if (HasError()) {
   return Transition::TerminateFailure();
 }

 // We've now read all the headers. If we're doing a metadata decode, we're
 // done.
 if (IsMetadataDecode()) {
   return Transition::TerminateSuccess();
 }

 // Set up the color table, if present; it'll be filled in by ReadColorTable().
 if (mH.mBpp <= 8) {
   mNumColors = 1 << mH.mBpp;
   if (0 < mH.mNumColors && mH.mNumColors < mNumColors) {
     mNumColors = mH.mNumColors;
   }

   // Always allocate and zero 256 entries, even though mNumColors might be
   // smaller, because the file might erroneously index past mNumColors.
   mColors = MakeUniqueFallible<ColorTableEntry[]>(256);
   if (NS_WARN_IF(!mColors)) {
     return Transition::TerminateFailure();
   }
   memset(mColors.get(), 0, 256 * sizeof(ColorTableEntry));

   // OS/2 Bitmaps have no padding byte.
   mBytesPerColor = (mH.mBIHSize == InfoHeaderLength::WIN_V2) ? 3 : 4;
 }

 if (mCMSMode != CMSMode::Off) {
   switch (mH.mCsType) {
     case InfoColorSpace::EMBEDDED:
       return SeekColorProfile(aLength);
     case InfoColorSpace::CALIBRATED_RGB:
       PrepareCalibratedColorProfile();
       break;
     case InfoColorSpace::SRGB:
     case InfoColorSpace::WIN:
       MOZ_LOG(sBMPLog, LogLevel::Debug, ("using sRGB color profile\n"));
       if (mColors) {
         // We will transform the color table instead of the output pixels.
         mTransform = GetCMSsRGBTransform(SurfaceFormat::R8G8B8);
       } else {
         mTransform = GetCMSsRGBTransform(SurfaceFormat::OS_RGBA);
       }
       break;
     case InfoColorSpace::LINKED:
     default:
       // Not supported, no color management.
       MOZ_LOG(sBMPLog, LogLevel::Debug, ("color space type not provided\n"));
       break;
   }
 }

 return Transition::To(State::ALLOCATE_SURFACE, 0);
}

void nsBMPDecoder::PrepareCalibratedColorProfile() {
 // BMP does not define a white point. Use the same as sRGB. This matches what
 // Chrome does as well.
 qcms_CIE_xyY white_point = qcms_white_point_sRGB();

 qcms_CIE_xyYTRIPLE primaries;
 float redGamma =
     CalRbgEndpointToQcms(mH.mColorSpace.mCalibrated.mRed, primaries.red);
 float greenGamma =
     CalRbgEndpointToQcms(mH.mColorSpace.mCalibrated.mGreen, primaries.green);
 float blueGamma =
     CalRbgEndpointToQcms(mH.mColorSpace.mCalibrated.mBlue, primaries.blue);

 // Explicitly verify the profile because sometimes the values from the BMP
 // header are just garbage.
 mInProfile = qcms_profile_create_rgb_with_gamma_set(
     white_point, primaries, redGamma, greenGamma, blueGamma);
 if (mInProfile && qcms_profile_is_bogus(mInProfile)) {
   // Bad profile, just use sRGB instead. Release the profile here, so that
   // our destructor doesn't assume we are the owner for the transform.
   qcms_profile_release(mInProfile);
   mInProfile = nullptr;
 }

 if (mInProfile) {
   MOZ_LOG(sBMPLog, LogLevel::Debug, ("using calibrated RGB color profile\n"));
   PrepareColorProfileTransform();
 } else {
   MOZ_LOG(sBMPLog, LogLevel::Debug,
           ("failed to create calibrated RGB color profile, using sRGB\n"));
   if (mColors) {
     // We will transform the color table instead of the output pixels.
     mTransform = GetCMSsRGBTransform(SurfaceFormat::R8G8B8);
   } else {
     mTransform = GetCMSsRGBTransform(SurfaceFormat::OS_RGBA);
   }
 }
}

void nsBMPDecoder::PrepareColorProfileTransform() {
 if (!mInProfile || !GetCMSOutputProfile()) {
   return;
 }

 qcms_data_type inType;
 qcms_data_type outType;
 if (mColors) {
   // We will transform the color table instead of the output pixels.
   inType = QCMS_DATA_RGB_8;
   outType = QCMS_DATA_RGB_8;
 } else {
   inType = gfxPlatform::GetCMSOSRGBAType();
   outType = inType;
 }

 qcms_intent intent;
 switch (mH.mCsIntent) {
   case InfoColorIntent::BUSINESS:
     intent = QCMS_INTENT_SATURATION;
     break;
   case InfoColorIntent::GRAPHICS:
     intent = QCMS_INTENT_RELATIVE_COLORIMETRIC;
     break;
   case InfoColorIntent::ABS_COLORIMETRIC:
     intent = QCMS_INTENT_ABSOLUTE_COLORIMETRIC;
     break;
   case InfoColorIntent::IMAGES:
   default:
     intent = QCMS_INTENT_PERCEPTUAL;
     break;
 }

 mTransform = qcms_transform_create(mInProfile, inType, GetCMSOutputProfile(),
                                    outType, intent);
 if (!mTransform) {
   MOZ_LOG(sBMPLog, LogLevel::Debug,
           ("failed to create color profile transform\n"));
 }
}

LexerTransition<nsBMPDecoder::State> nsBMPDecoder::SeekColorProfile(
   size_t aLength) {
 // The offset needs to be at least after the color table.
 uint32_t offset = mH.mColorSpace.mProfile.mOffset;
 if (offset <= mH.mBIHSize + aLength + mNumColors * mBytesPerColor ||
     mH.mColorSpace.mProfile.mLength == 0) {
   return Transition::To(State::ALLOCATE_SURFACE, 0);
 }

 // We have already read the header and bitfields.
 offset -= mH.mBIHSize + aLength;

 // We need to skip ahead to search for the embedded color profile. We want
 // to return to this point once we read it.
 MOZ_ASSERT(!mReturnIterator.isSome());
 mReturnIterator = mLexer.Clone(*mIterator, SIZE_MAX);
 if (!mReturnIterator) {
   return Transition::TerminateFailure();
 }

 return Transition::ToUnbuffered(State::FOUND_COLOR_PROFILE,
                                 State::SKIP_TO_COLOR_PROFILE, offset);
}

LexerTransition<nsBMPDecoder::State> nsBMPDecoder::ReadColorProfile(
   const char* aData, size_t aLength) {
 mInProfile = qcms_profile_from_memory(aData, aLength);
 if (mInProfile) {
   MOZ_LOG(sBMPLog, LogLevel::Debug, ("using embedded color profile\n"));
   PrepareColorProfileTransform();
 }

 // Jump back to where we left off.
 MOZ_ASSERT(mReturnIterator.isSome());
 mIterator = std::move(mReturnIterator);
 return Transition::To(State::ALLOCATE_SURFACE, 0);
}

LexerTransition<nsBMPDecoder::State> nsBMPDecoder::AllocateSurface() {
 SurfaceFormat format;
 SurfacePipeFlags pipeFlags = SurfacePipeFlags();

 if (mMayHaveTransparency) {
   format = SurfaceFormat::OS_RGBA;
   if (!(GetSurfaceFlags() & SurfaceFlags::NO_PREMULTIPLY_ALPHA)) {
     pipeFlags |= SurfacePipeFlags::PREMULTIPLY_ALPHA;
   }
 } else {
   format = SurfaceFormat::OS_RGBX;
 }

 if (mH.mHeight >= 0) {
   // BMPs store their rows in reverse order, so we may need to flip.
   pipeFlags |= SurfacePipeFlags::FLIP_VERTICALLY;
 }

 mRowBuffer.reset(new (fallible) uint32_t[mH.mWidth]);
 if (!mRowBuffer) {
   return Transition::TerminateFailure();
 }

 // Only give the color transform to the SurfacePipe if we are not transforming
 // the color table in advance.
 qcms_transform* transform = mColors ? nullptr : mTransform;

 Maybe<SurfacePipe> pipe = SurfacePipeFactory::CreateSurfacePipe(
     this, Size(), OutputSize(), FullFrame(), format, format, Nothing(),
     transform, pipeFlags);
 if (!pipe) {
   return Transition::TerminateFailure();
 }

 mPipe = std::move(*pipe);
 ClearRowBufferRemainder();

 // We might want to back track to here if we have a corrupt bmp that points
 // into the color table for image data, so save an iterator at this point.
 MOZ_ASSERT(!mReturnIterator.isSome());
 mReturnIterator = mLexer.Clone(*mIterator, SIZE_MAX);
 if (!mReturnIterator) {
   return Transition::TerminateFailure();
 }

 return Transition::To(State::COLOR_TABLE, mNumColors * mBytesPerColor);
}

LexerTransition<nsBMPDecoder::State> nsBMPDecoder::ReadColorTable(
   const char* aData, size_t aLength) {
 MOZ_ASSERT_IF(aLength != 0, mNumColors > 0 && mColors);

 mPreGapLength += aLength;

 for (uint32_t i = 0; i < mNumColors; i++) {
   // The format is BGR or BGR0.
   mColors[i].mBlue = uint8_t(aData[0]);
   mColors[i].mGreen = uint8_t(aData[1]);
   mColors[i].mRed = uint8_t(aData[2]);
   aData += mBytesPerColor;
 }

 // If we have a color table and a transform, we can avoid transforming each
 // pixel by doing the table in advance. We color manage every entry in the
 // table, even if it is smaller in case the BMP is malformed and overruns
 // its stated color range.
 if (mColors && mTransform) {
   qcms_transform_data(mTransform, mColors.get(), mColors.get(), 256);
 }

 // If we are decoding a BMP from the clipboard, we did not know the data
 // offset in advance. It is just defined as after the header and color table.
 if (mIsForClipboard) {
   mH.mDataOffset += mPreGapLength;
 }

 // We know how many bytes we've read so far (mPreGapLength) and we know the
 // offset of the pixel data (mH.mDataOffset), so we can determine the length
 // of the gap (possibly zero) between the color table and the pixel data.
 //
 // If the gap is negative the file must be malformed (e.g. mH.mDataOffset
 // points into the middle of the color palette instead of past the end).
 if (mPreGapLength > mH.mDataOffset) {
   // Allow corrupt bmp that say the image data starts in the color table, but
   // if the image data offset is even before the color tables thats too far
   // back and give up.
   if (mPreGapLength - aLength > mH.mDataOffset) {
     return Transition::TerminateFailure();
   }
   MOZ_ASSERT(mReturnIterator.isSome());
   mIterator = std::move(mReturnIterator);
   uint32_t gapLength = mH.mDataOffset - (mPreGapLength - aLength);
   return Transition::ToUnbuffered(State::AFTER_GAP, State::GAP, gapLength);
 }

 mReturnIterator.reset();

 uint32_t gapLength = mH.mDataOffset - mPreGapLength;
 return Transition::ToUnbuffered(State::AFTER_GAP, State::GAP, gapLength);
}

LexerTransition<nsBMPDecoder::State> nsBMPDecoder::SkipGap() {
 return Transition::ContinueUnbuffered(State::GAP);
}

LexerTransition<nsBMPDecoder::State> nsBMPDecoder::AfterGap() {
 // If there are no pixels we can stop.
 //
 // XXX: normally, if there are no pixels we will have stopped decoding before
 // now, outside of this decoder. However, if the BMP is within an ICO file,
 // it's possible that the ICO claimed the image had a non-zero size while the
 // BMP claims otherwise. This test is to catch that awkward case. If we ever
 // come up with a more general solution to this ICO-and-BMP-disagree-on-size
 // problem, this test can be removed.
 if (mH.mWidth == 0 || mH.mHeight == 0) {
   return Transition::TerminateSuccess();
 }

 bool hasRLE = mH.mCompression == Compression::RLE8 ||
               mH.mCompression == Compression::RLE4;
 return hasRLE ? Transition::To(State::RLE_SEGMENT, RLE::SEGMENT_LENGTH)
               : Transition::To(State::PIXEL_ROW, mPixelRowSize);
}

LexerTransition<nsBMPDecoder::State> nsBMPDecoder::ReadPixelRow(
   const char* aData) {
 MOZ_ASSERT(mCurrentRow > 0);
 MOZ_ASSERT(mCurrentPos == 0);

 const uint8_t* src = reinterpret_cast<const uint8_t*>(aData);
 uint32_t* dst = RowBuffer();
 uint32_t lpos = mH.mWidth;
 switch (mH.mBpp) {
   case 1:
     while (lpos > 0) {
       int8_t bit;
       uint8_t idx;
       for (bit = 7; bit >= 0 && lpos > 0; bit--) {
         idx = (*src >> bit) & 1;
         SetPixel(dst, idx, mColors);
         --lpos;
       }
       ++src;
     }
     break;

   case 4:
     while (lpos > 0) {
       Set4BitPixel(dst, *src, lpos, mColors);
       ++src;
     }
     break;

   case 8:
     while (lpos > 0) {
       SetPixel(dst, *src, mColors);
       --lpos;
       ++src;
     }
     break;

   case 16:
     if (mBitFields.IsR5G5B5()) {
       // Specialize this common case.
       while (lpos > 0) {
         uint16_t val = LittleEndian::readUint16(src);
         SetPixel(dst, mBitFields.mRed.Get5(val), mBitFields.mGreen.Get5(val),
                  mBitFields.mBlue.Get5(val));
         --lpos;
         src += 2;
       }
     } else {
       bool anyHasAlpha = false;
       while (lpos > 0) {
         uint16_t val = LittleEndian::readUint16(src);
         SetPixel(dst, mBitFields.mRed.Get(val), mBitFields.mGreen.Get(val),
                  mBitFields.mBlue.Get(val),
                  mBitFields.mAlpha.GetAlpha(val, anyHasAlpha));
         --lpos;
         src += 2;
       }
       if (anyHasAlpha) {
         MOZ_ASSERT(mMayHaveTransparency);
         mDoesHaveTransparency = true;
       }
     }
     break;

   case 24:
     while (lpos > 0) {
       SetPixel(dst, src[2], src[1], src[0]);
       --lpos;
       src += 3;
     }
     break;

   case 32:
     if (mH.mCompression == Compression::RGB && mIsWithinICO &&
         mH.mBpp == 32) {
       // This is a special case only used for 32bpp WinBMPv3-ICO files, which
       // could be in either 0RGB or ARGB format. We start by assuming it's
       // an 0RGB image. If we hit a non-zero alpha value, then we know it's
       // actually an ARGB image, and change tack accordingly.
       // (Note: a fully-transparent ARGB image is indistinguishable from a
       // 0RGB image, and we will render such an image as a 0RGB image, i.e.
       // opaquely. This is unlikely to be a problem in practice.)
       while (lpos > 0) {
         if (!mDoesHaveTransparency && src[3] != 0) {
           // Up until now this looked like an 0RGB image, but we now know
           // it's actually an ARGB image. Which means every pixel we've seen
           // so far has been fully transparent. So we go back and redo them.

           // Tell the SurfacePipe to go back to the start.
           mPipe.ResetToFirstRow();

           // Redo the complete rows we've already done.
           MOZ_ASSERT(mCurrentPos == 0);
           int32_t currentRow = mCurrentRow;
           mCurrentRow = AbsoluteHeight();
           ClearRowBufferRemainder();
           while (mCurrentRow > currentRow) {
             FinishRow();
           }

           // Reset the row pointer back to where we started.
           dst = RowBuffer() + (mH.mWidth - lpos);

           MOZ_ASSERT(mMayHaveTransparency);
           mDoesHaveTransparency = true;
         }

         // If mDoesHaveTransparency is false, treat this as an 0RGB image.
         // Otherwise, treat this as an ARGB image.
         SetPixel(dst, src[2], src[1], src[0],
                  mDoesHaveTransparency ? src[3] : 0xff);
         src += 4;
         --lpos;
       }
     } else if (mBitFields.IsR8G8B8()) {
       // Specialize this common case.
       while (lpos > 0) {
         uint32_t val = LittleEndian::readUint32(src);
         SetPixel(dst, mBitFields.mRed.Get8(val), mBitFields.mGreen.Get8(val),
                  mBitFields.mBlue.Get8(val));
         --lpos;
         src += 4;
       }
     } else {
       bool anyHasAlpha = false;
       while (lpos > 0) {
         uint32_t val = LittleEndian::readUint32(src);
         SetPixel(dst, mBitFields.mRed.Get(val), mBitFields.mGreen.Get(val),
                  mBitFields.mBlue.Get(val),
                  mBitFields.mAlpha.GetAlpha(val, anyHasAlpha));
         --lpos;
         src += 4;
       }
       if (anyHasAlpha) {
         MOZ_ASSERT(mMayHaveTransparency);
         mDoesHaveTransparency = true;
       }
     }
     break;

   default:
     MOZ_CRASH("Unsupported color depth; earlier check didn't catch it?");
 }

 FinishRow();
 return mCurrentRow == 0 ? Transition::TerminateSuccess()
                         : Transition::To(State::PIXEL_ROW, mPixelRowSize);
}

LexerTransition<nsBMPDecoder::State> nsBMPDecoder::ReadRLESegment(
   const char* aData) {
 if (mCurrentRow == 0) {
   return Transition::TerminateSuccess();
 }

 uint8_t byte1 = uint8_t(aData[0]);
 uint8_t byte2 = uint8_t(aData[1]);

 if (byte1 != RLE::ESCAPE) {
   // Encoded mode consists of two bytes: byte1 specifies the number of
   // consecutive pixels to be drawn using the color index contained in
   // byte2.
   //
   // Work around bitmaps that specify too many pixels.
   uint32_t pixelsNeeded = std::min<uint32_t>(mH.mWidth - mCurrentPos, byte1);
   if (pixelsNeeded) {
     uint32_t* dst = RowBuffer();
     mCurrentPos += pixelsNeeded;
     if (mH.mCompression == Compression::RLE8) {
       do {
         SetPixel(dst, byte2, mColors);
         pixelsNeeded--;
       } while (pixelsNeeded);
     } else {
       do {
         Set4BitPixel(dst, byte2, pixelsNeeded, mColors);
       } while (pixelsNeeded);
     }
   }
   return Transition::To(State::RLE_SEGMENT, RLE::SEGMENT_LENGTH);
 }

 if (byte2 == RLE::ESCAPE_EOL) {
   ClearRowBufferRemainder();
   mCurrentPos = 0;
   FinishRow();
   return mCurrentRow == 0
              ? Transition::TerminateSuccess()
              : Transition::To(State::RLE_SEGMENT, RLE::SEGMENT_LENGTH);
 }

 if (byte2 == RLE::ESCAPE_EOF) {
   return Transition::TerminateSuccess();
 }

 if (byte2 == RLE::ESCAPE_DELTA) {
   return Transition::To(State::RLE_DELTA, RLE::DELTA_LENGTH);
 }

 // Absolute mode. |byte2| gives the number of pixels. The length depends on
 // whether it's 4-bit or 8-bit RLE. Also, the length must be even (and zero
 // padding is used to achieve this when necessary).
 MOZ_ASSERT(mAbsoluteModeNumPixels == 0);
 mAbsoluteModeNumPixels = byte2;
 uint32_t length = byte2;
 if (mH.mCompression == Compression::RLE4) {
   length = (length + 1) / 2;  // halve, rounding up
 }
 if (length & 1) {
   length++;
 }
 return Transition::To(State::RLE_ABSOLUTE, length);
}

LexerTransition<nsBMPDecoder::State> nsBMPDecoder::ReadRLEDelta(
   const char* aData) {
 // Delta encoding makes it possible to skip pixels making part of the image
 // transparent.
 MOZ_ASSERT(mMayHaveTransparency);
 mDoesHaveTransparency = true;

 // Clear the skipped pixels. (This clears to the end of the row,
 // which is perfect if there's a Y delta and harmless if not).
 ClearRowBufferRemainder();

 // Handle the XDelta.
 mCurrentPos += uint8_t(aData[0]);
 if (mCurrentPos > mH.mWidth) {
   mCurrentPos = mH.mWidth;
 }

 // Handle the Y Delta.
 int32_t yDelta = std::min<int32_t>(uint8_t(aData[1]), mCurrentRow);
 if (yDelta > 0) {
   // Commit the current row (the first of the skipped rows).
   FinishRow();

   // Clear and commit the remaining skipped rows. We want to be careful not
   // to change mCurrentPos here.
   memset(mRowBuffer.get(), 0, mH.mWidth * sizeof(uint32_t));
   for (int32_t line = 1; line < yDelta; line++) {
     FinishRow();
   }
 }

 return mCurrentRow == 0
            ? Transition::TerminateSuccess()
            : Transition::To(State::RLE_SEGMENT, RLE::SEGMENT_LENGTH);
}

LexerTransition<nsBMPDecoder::State> nsBMPDecoder::ReadRLEAbsolute(
   const char* aData, size_t aLength) {
 uint32_t n = mAbsoluteModeNumPixels;
 mAbsoluteModeNumPixels = 0;

 if (mCurrentPos + n > uint32_t(mH.mWidth)) {
   // Some DIB RLE8 encoders count a padding byte as the absolute mode
   // pixel number at the end of the row.
   if (mH.mCompression == Compression::RLE8 && n > 0 && (n & 1) == 0 &&
       mCurrentPos + n - uint32_t(mH.mWidth) == 1 && aLength > 0 &&
       aData[aLength - 1] == 0) {
     n--;
   } else {
     // Bad data. Stop decoding; at least part of the image may have been
     // decoded.
     return Transition::TerminateSuccess();
   }
 }

 // In absolute mode, n represents the number of pixels that follow, each of
 // which contains the color index of a single pixel.
 uint32_t* dst = RowBuffer();
 uint32_t iSrc = 0;
 uint32_t* oldPos = dst;
 if (mH.mCompression == Compression::RLE8) {
   while (n > 0) {
     SetPixel(dst, aData[iSrc], mColors);
     n--;
     iSrc++;
   }
 } else {
   while (n > 0) {
     Set4BitPixel(dst, aData[iSrc], n, mColors);
     iSrc++;
   }
 }
 mCurrentPos += dst - oldPos;

 // We should read all the data (unless the last byte is zero padding).
 MOZ_ASSERT(iSrc == aLength - 1 || iSrc == aLength);

 return Transition::To(State::RLE_SEGMENT, RLE::SEGMENT_LENGTH);
}

}  // namespace image
}  // namespace mozilla