chromium / chromium / src / 3a262825ef5da3d7f10097627fe01a9e6bb2c3bc / . / media / base / bit_reader_core.h

// Copyright 2014 The Chromium Authors. All rights reserved. | |

// Use of this source code is governed by a BSD-style license that can be | |

// found in the LICENSE file. | |

#ifndef MEDIA_BASE_BIT_READER_CORE_H_ | |

#define MEDIA_BASE_BIT_READER_CORE_H_ | |

#include <stdint.h> | |

#include "base/logging.h" | |

#include "base/macros.h" | |

#include "media/base/media_export.h" | |

namespace media { | |

class MEDIA_EXPORT BitReaderCore { | |

public: | |

class ByteStreamProvider { | |

public: | |

ByteStreamProvider(); | |

virtual ~ByteStreamProvider(); | |

// Consume at most the following |max_n| bytes of the stream | |

// and return the number n of bytes actually consumed. | |

// Set |*array| to point to a memory buffer containing those n bytes. | |

// Note: |*array| must be valid until the next call to GetBytes | |

// but there is no guarantee it is valid after. | |

virtual int GetBytes(int max_n, const uint8_t** array) = 0; | |

}; | |

// Lifetime of |byte_stream_provider| must be longer than BitReaderCore. | |

explicit BitReaderCore(ByteStreamProvider* byte_stream_provider); | |

~BitReaderCore(); | |

// Read one bit from the stream and return it as a boolean in |*out|. | |

// Remark: we do not use the template version for reading a bool | |

// since it generates some optimization warnings during compilation | |

// on Windows platforms. | |

bool ReadBits(int num_bits, bool* out) { | |

DCHECK_EQ(num_bits, 1); | |

return ReadFlag(out); | |

} | |

// Read |num_bits| next bits from stream and return in |*out|, first bit | |

// from the stream starting at |num_bits| position in |*out|, | |

// bits of |*out| whose position is strictly greater than |num_bits| | |

// are all set to zero. | |

// Notes: | |

// - |num_bits| cannot be larger than the bits the type can hold. | |

// - From the above description, passing a signed type in |T| does not | |

// mean the first bit read from the stream gives the sign of the value. | |

// Return false if the given number of bits cannot be read (not enough | |

// bits in the stream), true otherwise. When return false, the stream will | |

// enter a state where further ReadBits/SkipBits operations will always | |

// return false unless |num_bits| is 0. The type |T| has to be a primitive | |

// integer type. | |

template<typename T> bool ReadBits(int num_bits, T* out) { | |

DCHECK_LE(num_bits, static_cast<int>(sizeof(T) * 8)); | |

uint64_t temp = 0; | |

bool ret = ReadBitsInternal(num_bits, &temp); | |

if (ret) | |

*out = static_cast<T>(temp); | |

return ret; | |

} | |

// Read one bit from the stream and return it as a boolean in |*flag|. | |

bool ReadFlag(bool* flag); | |

// Retrieve some bits without actually consuming them. | |

// Bits returned in |*out| are shifted so the most significant bit contains | |

// the next bit that can be read from the stream. | |

// Return the number of bits actually written in |out|. | |

// Note: |num_bits| is just a suggestion of how many bits the caller | |

// wish to get in |*out| and must be less than 64: | |

// - The number of bits returned can be more than |num_bits|. | |

// - However, it will be strictly less than |num_bits| | |

// if and only if there are not enough bits left in the stream. | |

int PeekBitsMsbAligned(int num_bits, uint64_t* out); | |

// Skip |num_bits| next bits from stream. Return false if the given number of | |

// bits cannot be skipped (not enough bits in the stream), true otherwise. | |

// When return false, the stream will enter a state where further | |

// ReadBits/ReadFlag/SkipBits operations | |

// will always return false unless |num_bits| is 0. | |

bool SkipBits(int num_bits); | |

// Returns the number of bits read so far. | |

int bits_read() const; | |

private: | |

// This function can skip any number of bits but is more efficient | |

// for small numbers. Return false if the given number of bits cannot be | |

// skipped (not enough bits in the stream), true otherwise. | |

bool SkipBitsSmall(int num_bits); | |

// Help function used by ReadBits to avoid inlining the bit reading logic. | |

bool ReadBitsInternal(int num_bits, uint64_t* out); | |

// Refill bit registers to have at least |min_nbits| bits available. | |

// Return true if the mininimum bit count condition is met after the refill. | |

bool Refill(int min_nbits); | |

// Refill the current bit register from the next bit register. | |

void RefillCurrentRegister(); | |

ByteStreamProvider* const byte_stream_provider_; | |

// Number of bits read so far. | |

int bits_read_; | |

// Number of bits in |reg_| that have not been consumed yet. | |

// Note: bits are consumed from MSB to LSB. | |

int nbits_; | |

uint64_t reg_; | |

// Number of bits in |reg_next_| that have not been consumed yet. | |

// Note: bits are consumed from MSB to LSB. | |

int nbits_next_; | |

uint64_t reg_next_; | |

DISALLOW_COPY_AND_ASSIGN(BitReaderCore); | |

}; | |

} // namespace media | |

#endif // MEDIA_BASE_BIT_READER_CORE_H_ |