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 #include <algorithm>
   8
   9 namespace media {
  10
  11 BitReader::BitReader(const uint8* data, off_t size)
  12     : data_(data), bytes_left_(size), num_remaining_bits_in_curr_byte_(0) {
  13   DCHECK(data_ != NULL && bytes_left_ > 0);
  14
  15   UpdateCurrByte();
  16 }
  17
  18 BitReader::~BitReader() {}
  19
  20 bool BitReader::SkipBits(int num_bits) {
  21   DCHECK_GE(num_bits, 0);
  22   DVLOG_IF(0, num_bits > 100)
  23       << "BitReader::SkipBits inefficient for large skips";
  24
  25   // Skip any bits in the current byte waiting to be processed, then
  26   // process full bytes until less than 8 bits remaining.
  27   while (num_bits > 0 && num_bits > num_remaining_bits_in_curr_byte_) {
  28     num_bits -= num_remaining_bits_in_curr_byte_;
  29     num_remaining_bits_in_curr_byte_ = 0;
  30     UpdateCurrByte();
  31
  32     // If there is no more data remaining, only return true if we
  33     // skipped all that were requested.
  34     if (num_remaining_bits_in_curr_byte_ == 0)
  35       return (num_bits == 0);
  36   }
  37
  38   // Less than 8 bits remaining to skip. Use ReadBitsInternal to verify
  39   // that the remaining bits we need exist, and adjust them as necessary
  40   // for subsequent operations.
  41   uint64 not_needed;
  42   return ReadBitsInternal(num_bits, &not_needed);
  43 }
  44
  45 int BitReader::bits_available() const {
  46   return 8 * bytes_left_ + num_remaining_bits_in_curr_byte_;
  47 }
  48
  49 bool BitReader::ReadBitsInternal(int num_bits, uint64* out) {
  50   DCHECK_LE(num_bits, 64);
  51
  52   *out = 0;
  53
  54   while (num_remaining_bits_in_curr_byte_ != 0 && num_bits != 0) {
  55     int bits_to_take = std::min(num_remaining_bits_in_curr_byte_, num_bits);
  56
  57     *out <<= bits_to_take;
  58     *out += curr_byte_ >> (num_remaining_bits_in_curr_byte_ - bits_to_take);
  59     num_bits -= bits_to_take;
  60     num_remaining_bits_in_curr_byte_ -= bits_to_take;
  61     curr_byte_ &= (1 << num_remaining_bits_in_curr_byte_) - 1;
  62
  63     if (num_remaining_bits_in_curr_byte_ == 0)
  64       UpdateCurrByte();
  65   }
  66
  67   return num_bits == 0;
  68 }
  69
  70 void BitReader::UpdateCurrByte() {
  71   DCHECK_EQ(num_remaining_bits_in_curr_byte_, 0);
  72
  73   if (bytes_left_ == 0)
  74     return;
  75
  76   // Load a new byte and advance pointers.
  77   curr_byte_ = *data_;
  78   ++data_;
  79   --bytes_left_;
  80   num_remaining_bits_in_curr_byte_ = 8;
  81 }
  82
  83 }  // namespace media