media/base/bit_reader_core.cc

   1 // Copyright 2014 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "media/base/bit_reader_core.h"
   6
   7 #include <base/port.h>
   8 #include <base/sys_byteorder.h>
   9
  10 namespace {
  11 const int kRegWidthInBits = sizeof(uint64) * 8;
  12 }
  13
  14 namespace media {
  15
  16 BitReaderCore::ByteStreamProvider::ByteStreamProvider() {
  17 }
  18
  19 BitReaderCore::ByteStreamProvider::~ByteStreamProvider() {
  20 }
  21
  22 BitReaderCore::BitReaderCore(ByteStreamProvider* byte_stream_provider)
  23     : byte_stream_provider_(byte_stream_provider),
  24       bits_read_(0),
  25       nbits_(0),
  26       reg_(0),
  27       nbits_next_(0),
  28       reg_next_(0) {
  29 }
  30
  31 BitReaderCore::~BitReaderCore() {
  32 }
  33
  34 bool BitReaderCore::ReadFlag(bool* flag) {
  35   if (nbits_ == 0 && !Refill(1))
  36     return false;
  37
  38   *flag = (reg_ & (GG_UINT64_C(1) << (kRegWidthInBits - 1))) != 0;
  39   reg_ <<= 1;
  40   nbits_--;
  41   bits_read_++;
  42   return true;
  43 }
  44
  45 int BitReaderCore::PeekBitsMsbAligned(int num_bits, uint64* out) {
  46   // Try to have at least |num_bits| in the bit register.
  47   if (nbits_ < num_bits)
  48     Refill(num_bits);
  49
  50   *out = reg_;
  51   return nbits_;
  52 }
  53
  54 bool BitReaderCore::SkipBits(int num_bits) {
  55   DCHECK_GE(num_bits, 0);
  56   DVLOG_IF(0, num_bits > 100)
  57       << "BitReader::SkipBits inefficient for large skips";
  58
  59   uint64 dummy;
  60   while (num_bits >= kRegWidthInBits) {
  61     if (!ReadBitsInternal(kRegWidthInBits, &dummy))
  62       return false;
  63     num_bits -= kRegWidthInBits;
  64   }
  65   return ReadBitsInternal(num_bits, &dummy);
  66 }
  67
  68 int BitReaderCore::bits_read() const {
  69   return bits_read_;
  70 }
  71
  72 bool BitReaderCore::ReadBitsInternal(int num_bits, uint64* out) {
  73   DCHECK_GE(num_bits, 0);
  74
  75   if (num_bits == 0) {
  76     *out = 0;
  77     return true;
  78   }
  79
  80   if (num_bits > nbits_ && !Refill(num_bits)) {
  81     // Any subsequent ReadBits should fail:
  82     // empty the current bit register for that purpose.
  83     nbits_ = 0;
  84     reg_ = 0;
  85     return false;
  86   }
  87
  88   bits_read_ += num_bits;
  89
  90   if (num_bits == kRegWidthInBits) {
  91     // Special case needed since for example for a 64 bit integer "a"
  92     // "a << 64" is not defined by the C/C++ standard.
  93     *out = reg_;
  94     reg_ = 0;
  95     nbits_ = 0;
  96     return true;
  97   }
  98
  99   *out = reg_ >> (kRegWidthInBits - num_bits);
 100   reg_ <<= num_bits;
 101   nbits_ -= num_bits;
 102   return true;
 103 }
 104
 105 bool BitReaderCore::Refill(int min_nbits) {
 106   DCHECK_LE(min_nbits, kRegWidthInBits);
 107
 108   // Transfer from the next to the current register.
 109   RefillCurrentRegister();
 110   if (min_nbits <= nbits_)
 111     return true;
 112   DCHECK_EQ(nbits_next_, 0);
 113   DCHECK_EQ(reg_next_, 0u);
 114
 115   // Max number of bytes to refill.
 116   int max_nbytes = sizeof(reg_next_);
 117
 118   // Refill.
 119   const uint8* byte_stream_window;
 120   int window_size =
 121       byte_stream_provider_->GetBytes(max_nbytes, &byte_stream_window);
 122   DCHECK_GE(window_size, 0);
 123   DCHECK_LE(window_size, max_nbytes);
 124   if (window_size == 0)
 125     return false;
 126
 127   reg_next_ = 0;
 128   memcpy(&reg_next_, byte_stream_window, window_size);
 129   reg_next_ = base::NetToHost64(reg_next_);
 130   nbits_next_ = window_size * 8;
 131
 132   // Transfer from the next to the current register.
 133   RefillCurrentRegister();
 134
 135   return (nbits_ >= min_nbits);
 136 }
 137
 138 void BitReaderCore::RefillCurrentRegister() {
 139   // No refill possible if the destination register is full
 140   // or the source register is empty.
 141   if (nbits_ == kRegWidthInBits || nbits_next_ == 0)
 142     return;
 143
 144   reg_ |= (reg_next_ >> nbits_);
 145
 146   int free_nbits = kRegWidthInBits - nbits_;
 147   if (free_nbits >= nbits_next_) {
 148     nbits_ += nbits_next_;
 149     reg_next_ = 0;
 150     nbits_next_ = 0;
 151     return;
 152   }
 153
 154   nbits_ += free_nbits;
 155   reg_next_ <<= free_nbits;
 156   nbits_next_ -= free_nbits;
 157 }
 158
 159 }  // namespace media