126 lines
4.6 KiB
C++
126 lines
4.6 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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#ifndef MEDIA_BASE_BIT_READER_H_
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#define MEDIA_BASE_BIT_READER_H_
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#include <stdint.h>
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#include <sys/types.h>
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#include "packager/base/logging.h"
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namespace shaka {
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namespace media {
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/// A class to read bit streams.
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class BitReader {
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public:
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/// Initialize the BitReader object to read a data buffer.
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/// @param data points to the beginning of the buffer.
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/// @param size is the buffer size in bytes.
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BitReader(const uint8_t* data, off_t size);
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~BitReader();
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/// Read a number of bits from stream.
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/// @param num_bits specifies the number of bits to read. It cannot be larger
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/// than the number of bits the type can hold.
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/// @param[out] out stores the output. The type @b T has to be a primitive
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/// integer type.
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/// @return false if the given number of bits cannot be read (not enough
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/// bits in the stream), true otherwise. When false is returned, the
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/// stream will enter a state where further ReadBits/SkipBits
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/// operations will always return false unless @a num_bits is 0.
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template <typename T>
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bool ReadBits(int num_bits, T* out) {
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DCHECK_LE(num_bits, static_cast<int>(sizeof(T) * 8));
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uint64_t temp;
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bool ret = ReadBitsInternal(num_bits, &temp);
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*out = static_cast<T>(temp);
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return ret;
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}
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// Explicit T=bool overload to make MSVC happy.
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bool ReadBits(int num_bits, bool* out) {
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DCHECK_EQ(num_bits, 1);
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uint64_t temp;
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bool ret = ReadBitsInternal(num_bits, &temp);
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*out = temp != 0;
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return ret;
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}
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/// Skip a number of bits from stream.
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/// @param num_bits specifies the number of bits to be skipped.
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/// @return false if the given number of bits cannot be skipped (not enough
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/// bits in the stream), true otherwise. When false is returned, the
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/// stream will enter a state where further ReadXXX/SkipXXX
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/// operations will always return false unless |num_bits/bytes| is 0.
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bool SkipBits(int num_bits);
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/// Read one bit then skip the number of bits specified if that bit matches @a
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/// condition.
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/// @param condition indicates when the number of bits should be skipped.
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/// @param num_bits specifies the number of bits to be skipped.
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/// @return false if the one bit cannot be read (not enough bits in the
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/// stream) or if the bit is set but the given number of bits cannot
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/// be skipped (not enough bits in the stream), true otherwise. When
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/// false is returned, the stream will enter a state where further
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/// ReadXXX/SkipXXX operations will always return false.
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bool SkipBitsConditional(bool condition, int num_bits) {
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bool condition_read = true;
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if (!ReadBits(1, &condition_read))
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return false;
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return condition_read == condition ? SkipBits(num_bits) : true;
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}
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/// Skip a number of bytes from stream. The current posision should be byte
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/// aligned, otherwise a false is returned and bytes are not skipped.
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/// @param num_bytes specifies the number of bytes to be skipped.
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/// @return false if the current position is not byte aligned or if the given
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/// number of bytes cannot be skipped (not enough bytes in the
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/// stream), true otherwise.
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bool SkipBytes(int num_bytes);
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/// @return The number of bits available for reading.
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int 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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/// @return The current bit position.
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int bit_position() const { return 8 * initial_size_ - bits_available(); }
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private:
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// Help function used by ReadBits to avoid inlining the bit reading logic.
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bool ReadBitsInternal(int num_bits, uint64_t* out);
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// Advance to the next byte, loading it into curr_byte_.
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// If the num_remaining_bits_in_curr_byte_ is 0 after this function returns,
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// the stream has reached the end.
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void UpdateCurrByte();
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// Pointer to the next unread (not in curr_byte_) byte in the stream.
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const uint8_t* data_;
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// Initial size of the input data.
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// TODO(kqyang): Use size_t instead of off_t instead.
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off_t initial_size_;
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// Bytes left in the stream (without the curr_byte_).
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off_t bytes_left_;
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// Contents of the current byte; first unread bit starting at position
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// 8 - num_remaining_bits_in_curr_byte_ from MSB.
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uint8_t curr_byte_;
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// Number of bits remaining in curr_byte_
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int num_remaining_bits_in_curr_byte_;
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private:
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DISALLOW_COPY_AND_ASSIGN(BitReader);
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};
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} // namespace media
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} // namespace shaka
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#endif // MEDIA_BASE_BIT_READER_H_
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