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| 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 CHROME_BROWSER_NET_BIT_STREAM_READER_H_ |
| 6 #define CHROME_BROWSER_NET_BIT_STREAM_READER_H_ |
| 7 |
| 8 #include <stdint.h> |
| 9 |
| 10 #include "base/strings/string_piece.h" |
| 11 |
| 12 namespace internal { |
| 13 |
| 14 // A class for reading individual bits from a packed buffer. Bits are read |
| 15 // MSB-first from the stream. |
| 16 // It is limited to 64-bit reads, 4GB streams and is inefficient as a design |
| 17 // choice. This class should not be used frequently. |
| 18 // |
| 19 // It is meant for data that is is packed across bytes, necessitating the need |
| 20 // to read a variable number of bits across a byte boundary. |
| 21 class BitStreamReader { |
| 22 public: |
| 23 explicit BitStreamReader(const base::StringPiece& source); |
| 24 |
| 25 // Reads unary-encoded number into |out|. Returns true if |
| 26 // there was at least one bit to read, false otherwise. |
| 27 bool ReadUnaryEncoding(uint64_t* out); |
| 28 |
| 29 // Reads |num_bits| (up to 64) into |out|. |out| is filled from the MSB to the |
| 30 // LSB. If |num_bits| is less than 64, the most significant |64 - num_bits| |
| 31 // bits are unused and left as zeros. Returns true if the stream had the |
| 32 // requested |num_bits|, false otherwise. |
| 33 bool ReadBits(uint8_t num_bits, uint64_t* out); |
| 34 |
| 35 // Returns the number of bits left in the stream. |
| 36 uint64_t BitsLeft() const; |
| 37 |
| 38 private: |
| 39 // Reads a single bit. Within a byte, the bits are read from the MSB to the |
| 40 // LSB. |
| 41 uint8_t ReadBit(); |
| 42 |
| 43 const base::StringPiece source_; |
| 44 |
| 45 // Index of the byte currently being read from. |
| 46 size_t current_byte_; |
| 47 |
| 48 // Index of the last bit read within |current_byte_|. Since bits are read |
| 49 // from the MSB to the LSB, this value is initialized to 7 and decremented |
| 50 // after each read. |
| 51 int8 current_bit_; |
| 52 |
| 53 DISALLOW_COPY_AND_ASSIGN(BitStreamReader); |
| 54 }; |
| 55 |
| 56 } // namespace internal |
| 57 |
| 58 #endif // CHROME_BROWSER_NET_BIT_STREAM_READER_H_ |
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