media/base/bit_reader.cc

   1 // Copyright (c) 2012 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.h"
   6
   7 namespace media {
   8
   9 BitReader::BitReader(const uint8* data, off_t size)
  10     : data_(data), bytes_left_(size), num_remaining_bits_in_curr_byte_(0) {
  11   DCHECK(data_ != NULL && bytes_left_ > 0);
  12
  13   UpdateCurrByte();
  14 }
  15
  16 BitReader::~BitReader() {}
  17
  18 bool BitReader::SkipBits(int num_bits) {
  19   DCHECK_GE(num_bits, 0);
  20   DLOG_IF(INFO, num_bits > 100)
  21       << "BitReader::SkipBits inefficient for large skips";
  22
  23   // Skip any bits in the current byte waiting to be processed, then
  24   // process full bytes until less than 8 bits remaining.
  25   while (num_bits > 0 && num_bits > num_remaining_bits_in_curr_byte_) {
  26     num_bits -= num_remaining_bits_in_curr_byte_;
  27     num_remaining_bits_in_curr_byte_ = 0;
  28     UpdateCurrByte();
  29
  30     // If there is no more data remaining, only return true if we
  31     // skipped all that were requested.
  32     if (num_remaining_bits_in_curr_byte_ == 0)
  33       return (num_bits == 0);
  34   }
  35
  36   // Less than 8 bits remaining to skip. Use ReadBitsInternal to verify
  37   // that the remaining bits we need exist, and adjust them as necessary
  38   // for subsequent operations.
  39   uint64 not_needed;
  40   return ReadBitsInternal(num_bits, &not_needed);
  41 }
  42
  43 int BitReader::bits_available() const {
  44   return 8 * bytes_left_ + num_remaining_bits_in_curr_byte_;
  45 }
  46
  47 bool BitReader::ReadBitsInternal(int num_bits, uint64* out) {
  48   DCHECK_LE(num_bits, 64);
  49
  50   *out = 0;
  51
  52   while (num_remaining_bits_in_curr_byte_ != 0 && num_bits != 0) {
  53     int bits_to_take = std::min(num_remaining_bits_in_curr_byte_, num_bits);
  54
  55     *out <<= bits_to_take;
  56     *out += curr_byte_ >> (num_remaining_bits_in_curr_byte_ - bits_to_take);
  57     num_bits -= bits_to_take;
  58     num_remaining_bits_in_curr_byte_ -= bits_to_take;
  59     curr_byte_ &= (1 << num_remaining_bits_in_curr_byte_) - 1;
  60
  61     if (num_remaining_bits_in_curr_byte_ == 0)
  62       UpdateCurrByte();
  63   }
  64
  65   return num_bits == 0;
  66 }
  67
  68 void BitReader::UpdateCurrByte() {
  69   DCHECK_EQ(num_remaining_bits_in_curr_byte_, 0);
  70
  71   if (bytes_left_ == 0)
  72     return;
  73
  74   // Load a new byte and advance pointers.
  75   curr_byte_ = *data_;
  76   ++data_;
  77   --bytes_left_;
  78   num_remaining_bits_in_curr_byte_ = 8;
  79 }
  80
  81 }  // namespace media