media/base/bit_reader.h

   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_