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 // Note: ported from Chromium commit head: 1323b9c 5 6 #ifndef BIT_READER_CORE_H_ 7 #define BIT_READER_CORE_H_ 8 9 #include <stdint.h> 10 11 #include "base/logging.h" 12 #include "base/macros.h" 13 14 namespace media { 15 16 class BitReaderCore { 17 public: 18 class ByteStreamProvider { 19 public: 20 ByteStreamProvider(); 21 virtual ~ByteStreamProvider(); 22 23 // Consume at most the following |max_n| bytes of the stream 24 // and return the number n of bytes actually consumed. 25 // Set |*array| to point to a memory buffer containing those n bytes. 26 // Note: |*array| must be valid until the next call to GetBytes 27 // but there is no guarantee it is valid after. 28 virtual int GetBytes(int max_n, const uint8_t** array) = 0; 29 }; 30 31 // Lifetime of |byte_stream_provider| must be longer than BitReaderCore. 32 explicit BitReaderCore(ByteStreamProvider* byte_stream_provider); 33 ~BitReaderCore(); 34 35 // Read one bit from the stream and return it as a boolean in |*out|. 36 // Remark: we do not use the template version for reading a bool 37 // since it generates some optimization warnings during compilation 38 // on Windows platforms. 39 bool ReadBits(int num_bits, bool* out) { 40 DCHECK_EQ(num_bits, 1); 41 return ReadFlag(out); 42 } 43 44 // Read |num_bits| next bits from stream and return in |*out|, first bit 45 // from the stream starting at |num_bits| position in |*out|, 46 // bits of |*out| whose position is strictly greater than |num_bits| 47 // are all set to zero. 48 // Notes: 49 // - |num_bits| cannot be larger than the bits the type can hold. 50 // - From the above description, passing a signed type in |T| does not 51 // mean the first bit read from the stream gives the sign of the value. 52 // Return false if the given number of bits cannot be read (not enough 53 // bits in the stream), true otherwise. When return false, the stream will 54 // enter a state where further ReadBits/SkipBits operations will always 55 // return false unless |num_bits| is 0. The type |T| has to be a primitive 56 // integer type. 57 template<typename T> bool ReadBits(int num_bits, T* out) { 58 DCHECK_LE(num_bits, static_cast<int>(sizeof(T) * 8)); 59 uint64_t temp; 60 bool ret = ReadBitsInternal(num_bits, &temp); 61 *out = static_cast<T>(temp); 62 return ret; 63 } 64 65 // Read one bit from the stream and return it as a boolean in |*flag|. 66 bool ReadFlag(bool* flag); 67 68 // Retrieve some bits without actually consuming them. 69 // Bits returned in |*out| are shifted so the most significant bit contains 70 // the next bit that can be read from the stream. 71 // Return the number of bits actually written in |out|. 72 // Note: |num_bits| is just a suggestion of how many bits the caller 73 // wish to get in |*out| and must be less than 64: 74 // - The number of bits returned can be more than |num_bits|. 75 // - However, it will be strictly less than |num_bits| 76 // if and only if there are not enough bits left in the stream. 77 int PeekBitsMsbAligned(int num_bits, uint64_t* out); 78 79 // Skip |num_bits| next bits from stream. Return false if the given number of 80 // bits cannot be skipped (not enough bits in the stream), true otherwise. 81 // When return false, the stream will enter a state where further 82 // ReadBits/ReadFlag/SkipBits operations 83 // will always return false unless |num_bits| is 0. 84 bool SkipBits(int num_bits); 85 86 // Returns the number of bits read so far. 87 int bits_read() const; 88 89 private: 90 // This function can skip any number of bits but is more efficient 91 // for small numbers. Return false if the given number of bits cannot be 92 // skipped (not enough bits in the stream), true otherwise. 93 bool SkipBitsSmall(int num_bits); 94 95 // Help function used by ReadBits to avoid inlining the bit reading logic. 96 bool ReadBitsInternal(int num_bits, uint64_t* out); 97 98 // Refill bit registers to have at least |min_nbits| bits available. 99 // Return true if the mininimum bit count condition is met after the refill. 100 bool Refill(int min_nbits); 101 102 // Refill the current bit register from the next bit register. 103 void RefillCurrentRegister(); 104 105 ByteStreamProvider* const byte_stream_provider_; 106 107 // Number of bits read so far. 108 int bits_read_; 109 110 // Number of bits in |reg_| that have not been consumed yet. 111 // Note: bits are consumed from MSB to LSB. 112 int nbits_; 113 uint64_t reg_; 114 115 // Number of bits in |reg_next_| that have not been consumed yet. 116 // Note: bits are consumed from MSB to LSB. 117 int nbits_next_; 118 uint64_t reg_next_; 119 120 DISALLOW_COPY_AND_ASSIGN(BitReaderCore); 121 }; 122 123 } // namespace media 124 125 #endif // BIT_READER_CORE_H_ 126
