dom/media/BitWriter.cpp

   1 /* This Source Code Form is subject to the terms of the Mozilla Public
   2  * License, v. 2.0. If a copy of the MPL was not distributed with this
   3  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   4
   5 #include "BitWriter.h"
   6
   7 #include <stdint.h>
   8
   9 #include "MediaData.h"
  10 #include "mozilla/MathAlgorithms.h"
  11
  12 namespace mozilla {
  13
  14 constexpr uint8_t golombLen[256] = {
  15     1,  3,  3,  5,  5,  5,  5,  7,  7,  7,  7,  7,  7,  7,  7,  9,  9,  9,  9,
  16     9,  9,  9,  9,  9,  9,  9,  9,  9,  9,  9,  9,  11, 11, 11, 11, 11, 11, 11,
  17     11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11,
  18     11, 11, 11, 11, 11, 11, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13,
  19     13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13,
  20     13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13,
  21     13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 15, 15, 15, 15, 15, 15,
  22     15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
  23     15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
  24     15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
  25     15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
  26     15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
  27     15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
  28     15, 15, 15, 15, 15, 15, 15, 15, 17,
  29 };
  30
  31 BitWriter::BitWriter(MediaByteBuffer* aBuffer) : mBuffer(aBuffer) {}
  32
  33 BitWriter::~BitWriter() = default;
  34
  35 void BitWriter::WriteBits(uint64_t aValue, size_t aBits) {
  36   MOZ_ASSERT(aBits <= sizeof(uint64_t) * 8);
  37
  38   while (aBits) {
  39     if (mBitIndex == 0) {
  40       mBuffer->AppendElement(0);
  41     }
  42
  43     const uint8_t clearMask = ~(~0u << (8 - mBitIndex));
  44     uint8_t mask = 0;
  45
  46     if (mBitIndex + aBits > 8) {
  47       // Not enough bits in the current byte to write all the bits
  48       // required, we'll process what we can and continue with the left over.
  49       const uint8_t leftOverBits = mBitIndex + aBits - 8;
  50       const uint64_t leftOver = aValue & (~uint64_t(0) >> (8 - mBitIndex));
  51       mask = aValue >> leftOverBits;
  52
  53       mBitIndex = 8;
  54       aValue = leftOver;
  55       aBits = leftOverBits;
  56     } else {
  57       const uint8_t offset = 8 - mBitIndex - aBits;
  58       mask = aValue << offset;
  59
  60       mBitIndex += aBits;
  61       aBits = 0;
  62     }
  63
  64     mBuffer->ElementAt(mPosition) |= mask & clearMask;
  65
  66     if (mBitIndex == 8) {
  67       mPosition++;
  68       mBitIndex = 0;
  69     }
  70   }
  71 }
  72
  73 void BitWriter::WriteUE(uint32_t aValue) {
  74   MOZ_ASSERT(aValue <= (UINT32_MAX - 1));
  75
  76   if (aValue < 256) {
  77     WriteBits(aValue + 1, golombLen[aValue]);
  78   } else {
  79     const uint32_t e = FloorLog2(aValue + 1);
  80     WriteBits(aValue + 1, e * 2 + 1);
  81   }
  82 }
  83
  84 void BitWriter::WriteULEB128(uint64_t aValue) {
  85   // See https://en.wikipedia.org/wiki/LEB128#Encode_unsigned_integer
  86   do {
  87     uint8_t byte = aValue & 0x7F;
  88     aValue >>= 7;
  89     WriteBit(aValue != 0);
  90     WriteBits(byte, 7);
  91   } while (aValue != 0);
  92 }
  93
  94 void BitWriter::CloseWithRbspTrailing() {
  95   WriteBit(true);
  96   WriteBits(0, (8 - mBitIndex) & 7);
  97 }
  98
  99 void BitWriter::AdvanceBytes(uint32_t aByteOffset) {
 100   MOZ_DIAGNOSTIC_ASSERT(mBitIndex == 0);
 101   mPosition += aByteOffset;
 102 }
 103
 104 }  // namespace mozilla