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