82 lines
2.3 KiB
C++
82 lines
2.3 KiB
C++
// Copyright (c) 2012 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#include "media/base/bit_reader.h"
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namespace media {
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BitReader::BitReader(const uint8* data, off_t size)
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: data_(data), bytes_left_(size), num_remaining_bits_in_curr_byte_(0) {
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DCHECK(data_ != NULL && bytes_left_ > 0);
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UpdateCurrByte();
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}
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BitReader::~BitReader() {}
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bool BitReader::SkipBits(int num_bits) {
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DCHECK_GE(num_bits, 0);
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DLOG_IF(INFO, num_bits > 100)
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<< "BitReader::SkipBits inefficient for large skips";
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// Skip any bits in the current byte waiting to be processed, then
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// process full bytes until less than 8 bits remaining.
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while (num_bits > 0 && num_bits > num_remaining_bits_in_curr_byte_) {
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num_bits -= num_remaining_bits_in_curr_byte_;
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num_remaining_bits_in_curr_byte_ = 0;
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UpdateCurrByte();
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// If there is no more data remaining, only return true if we
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// skipped all that were requested.
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if (num_remaining_bits_in_curr_byte_ == 0)
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return (num_bits == 0);
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}
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// Less than 8 bits remaining to skip. Use ReadBitsInternal to verify
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// that the remaining bits we need exist, and adjust them as necessary
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// for subsequent operations.
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uint64 not_needed;
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return ReadBitsInternal(num_bits, ¬_needed);
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}
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int BitReader::bits_available() const {
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return 8 * bytes_left_ + num_remaining_bits_in_curr_byte_;
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}
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bool BitReader::ReadBitsInternal(int num_bits, uint64* out) {
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DCHECK_LE(num_bits, 64);
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*out = 0;
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while (num_remaining_bits_in_curr_byte_ != 0 && num_bits != 0) {
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int bits_to_take = std::min(num_remaining_bits_in_curr_byte_, num_bits);
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*out <<= bits_to_take;
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*out += curr_byte_ >> (num_remaining_bits_in_curr_byte_ - bits_to_take);
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num_bits -= bits_to_take;
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num_remaining_bits_in_curr_byte_ -= bits_to_take;
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curr_byte_ &= (1 << num_remaining_bits_in_curr_byte_) - 1;
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if (num_remaining_bits_in_curr_byte_ == 0)
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UpdateCurrByte();
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}
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return num_bits == 0;
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}
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void BitReader::UpdateCurrByte() {
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DCHECK_EQ(num_remaining_bits_in_curr_byte_, 0);
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if (bytes_left_ == 0)
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return;
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// Load a new byte and advance pointers.
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curr_byte_ = *data_;
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++data_;
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--bytes_left_;
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num_remaining_bits_in_curr_byte_ = 8;
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}
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} // namespace media
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