1 // Copyright (c) 2012 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 MEDIA_BASE_BIT_READER_H_ 6 #define MEDIA_BASE_BIT_READER_H_ 7 8 #include <sys/types.h> 9 10 #include "base/basictypes.h" 11 #include "base/logging.h" 12 #include "media/base/media_export.h" 13 14 namespace media { 15 16 // A class to read bit streams. 17 class MEDIA_EXPORT BitReader { 18 public: 19 // Initialize the reader to start reading at |data|, |size| being size 20 // of |data| in bytes. 21 BitReader(const uint8* data, off_t size); 22 ~BitReader(); 23 24 // Read |num_bits| next bits from stream and return in |*out|, first bit 25 // from the stream starting at |num_bits| position in |*out|. 26 // |num_bits| cannot be larger than the bits the type can hold. 27 // Return false if the given number of bits cannot be read (not enough 28 // bits in the stream), true otherwise. When return false, the stream will 29 // enter a state where further ReadBits/SkipBits operations will always 30 // return false unless |num_bits| is 0. The type |T| has to be a primitive 31 // integer type. 32 template<typename T> bool ReadBits(int num_bits, T *out) { 33 DCHECK_LE(num_bits, static_cast<int>(sizeof(T) * 8)); 34 uint64 temp; 35 bool ret = ReadBitsInternal(num_bits, &temp); 36 *out = static_cast<T>(temp); 37 return ret; 38 } 39 40 // Skip |num_bits| next bits from stream. Return false if the given number of 41 // bits cannot be skipped (not enough bits in the stream), true otherwise. 42 // When return false, the stream will enter a state where further ReadBits/ 43 // SkipBits operations will always return false unless |num_bits| is 0. 44 bool SkipBits(int num_bits); 45 46 // Returns the number of bits available for reading. 47 int bits_available() const; 48 49 private: 50 // Help function used by ReadBits to avoid inlining the bit reading logic. 51 bool ReadBitsInternal(int num_bits, uint64* out); 52 53 // Advance to the next byte, loading it into curr_byte_. 54 // If the num_remaining_bits_in_curr_byte_ is 0 after this function returns, 55 // the stream has reached the end. 56 void UpdateCurrByte(); 57 58 // Pointer to the next unread (not in curr_byte_) byte in the stream. 59 const uint8* data_; 60 61 // Bytes left in the stream (without the curr_byte_). 62 off_t bytes_left_; 63 64 // Contents of the current byte; first unread bit starting at position 65 // 8 - num_remaining_bits_in_curr_byte_ from MSB. 66 uint8 curr_byte_; 67 68 // Number of bits remaining in curr_byte_ 69 int num_remaining_bits_in_curr_byte_; 70 71 private: 72 DISALLOW_COPY_AND_ASSIGN(BitReader); 73 }; 74 75 } // namespace media 76 77 #endif // MEDIA_BASE_BIT_READER_H_ 78
