// 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 "base/basictypes.h"
#include "base/logging.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** 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 temp;
bool ret = ReadBitsInternal(num_bits, &temp);
*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* 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:
// Help function used by ReadBits to avoid inlining the bit reading logic.
bool ReadBitsInternal(int num_bits, uint64* 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 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 reg_next_;
DISALLOW_COPY_AND_ASSIGN(BitReaderCore);
};
} // namespace media
#endif // MEDIA_BASE_BIT_READER_CORE_H_