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- // 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/check_op.h"
- #include "base/memory/raw_ptr.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(const BitReaderCore&) = delete;
- BitReaderCore& operator=(const BitReaderCore&) = delete;
- ~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();
- const raw_ptr<ByteStreamProvider> 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_;
- };
- } // namespace media
- #endif // MEDIA_BASE_BIT_READER_CORE_H_
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