1 // Copyright 2014 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 #ifndef BIT_READER_CORE_H_ 6 #define BIT_READER_CORE_H_ 7 8 #include <stdint.h> 9 10 #include "base/logging.h" 11 #include "base/macros.h" 12 13 namespace media { 14 15 class BitReaderCore { 16 public: 17 class ByteStreamProvider { 18 public: 19 ByteStreamProvider(); 20 virtual ~ByteStreamProvider(); 21 22 // Consume at most the following |max_n| bytes of the stream 23 // and return the number n of bytes actually consumed. 24 // Set |*array| to point to a memory buffer containing those n bytes. 25 // Note: |*array| must be valid until the next call to GetBytes 26 // but there is no guarantee it is valid after. 27 virtual int GetBytes(int max_n, const uint8_t** array) = 0; 28 }; 29 30 // Lifetime of |byte_stream_provider| must be longer than BitReaderCore. 31 explicit BitReaderCore(ByteStreamProvider* byte_stream_provider); 32 ~BitReaderCore(); 33 34 // Read one bit from the stream and return it as a boolean in |*out|. 35 // Remark: we do not use the template version for reading a bool 36 // since it generates some optimization warnings during compilation 37 // on Windows platforms. ReadBits(int num_bits,bool * out)38 bool ReadBits(int num_bits, bool* out) { 39 DCHECK_EQ(num_bits, 1); 40 return ReadFlag(out); 41 } 42 43 // Read |num_bits| next bits from stream and return in |*out|, first bit 44 // from the stream starting at |num_bits| position in |*out|, 45 // bits of |*out| whose position is strictly greater than |num_bits| 46 // are all set to zero. 47 // Notes: 48 // - |num_bits| cannot be larger than the bits the type can hold. 49 // - From the above description, passing a signed type in |T| does not 50 // mean the first bit read from the stream gives the sign of the value. 51 // Return false if the given number of bits cannot be read (not enough 52 // bits in the stream), true otherwise. When return false, the stream will 53 // enter a state where further ReadBits/SkipBits operations will always 54 // return false unless |num_bits| is 0. The type |T| has to be a primitive 55 // integer type. ReadBits(int num_bits,T * out)56 template<typename T> bool ReadBits(int num_bits, T* out) { 57 DCHECK_LE(num_bits, static_cast<int>(sizeof(T) * 8)); 58 uint64_t temp; 59 bool ret = ReadBitsInternal(num_bits, &temp); 60 *out = static_cast<T>(temp); 61 return ret; 62 } 63 64 // Read one bit from the stream and return it as a boolean in |*flag|. 65 bool ReadFlag(bool* flag); 66 67 // Retrieve some bits without actually consuming them. 68 // Bits returned in |*out| are shifted so the most significant bit contains 69 // the next bit that can be read from the stream. 70 // Return the number of bits actually written in |out|. 71 // Note: |num_bits| is just a suggestion of how many bits the caller 72 // wish to get in |*out| and must be less than 64: 73 // - The number of bits returned can be more than |num_bits|. 74 // - However, it will be strictly less than |num_bits| 75 // if and only if there are not enough bits left in the stream. 76 int PeekBitsMsbAligned(int num_bits, uint64_t* out); 77 78 // Skip |num_bits| next bits from stream. Return false if the given number of 79 // bits cannot be skipped (not enough bits in the stream), true otherwise. 80 // When return false, the stream will enter a state where further 81 // ReadBits/ReadFlag/SkipBits operations 82 // will always return false unless |num_bits| is 0. 83 bool SkipBits(int num_bits); 84 85 // Returns the number of bits read so far. 86 int bits_read() const; 87 88 private: 89 // This function can skip any number of bits but is more efficient 90 // for small numbers. Return false if the given number of bits cannot be 91 // skipped (not enough bits in the stream), true otherwise. 92 bool SkipBitsSmall(int num_bits); 93 94 // Help function used by ReadBits to avoid inlining the bit reading logic. 95 bool ReadBitsInternal(int num_bits, uint64_t* out); 96 97 // Refill bit registers to have at least |min_nbits| bits available. 98 // Return true if the mininimum bit count condition is met after the refill. 99 bool Refill(int min_nbits); 100 101 // Refill the current bit register from the next bit register. 102 void RefillCurrentRegister(); 103 104 ByteStreamProvider* const byte_stream_provider_; 105 106 // Number of bits read so far. 107 int bits_read_; 108 109 // Number of bits in |reg_| that have not been consumed yet. 110 // Note: bits are consumed from MSB to LSB. 111 int nbits_; 112 uint64_t reg_; 113 114 // Number of bits in |reg_next_| that have not been consumed yet. 115 // Note: bits are consumed from MSB to LSB. 116 int nbits_next_; 117 uint64_t reg_next_; 118 119 DISALLOW_COPY_AND_ASSIGN(BitReaderCore); 120 }; 121 122 } // namespace media 123 124 #endif // BIT_READER_CORE_H_ 125