components/packed_ct_ev_whitelist/bit_stream_reader.h

   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 COMPONENTS_PACKED_CT_EV_WHITELIST_BIT_STREAM_READER_H_
   6 #define COMPONENTS_PACKED_CT_EV_WHITELIST_BIT_STREAM_READER_H_
   7
   8 #include <stdint.h>
   9
  10 #include "base/strings/string_piece.h"
  11
  12 namespace packed_ct_ev_whitelist {
  13 namespace internal {
  14
  15 // A class for reading individual bits from a packed buffer. Bits are read
  16 // MSB-first from the stream.
  17 // It is limited to 64-bit reads, 4GB streams and is inefficient as a design
  18 // choice. This class should not be used frequently.
  19 //
  20 // It is meant for data that is is packed across bytes, necessitating the need
  21 // to read a variable number of bits across a byte boundary.
  22 class BitStreamReader {
  23  public:
  24   explicit BitStreamReader(const base::StringPiece& source);
  25
  26   // Reads unary-encoded number into |out|. Returns true if
  27   // there was at least one bit to read, false otherwise.
  28   bool ReadUnaryEncoding(uint64_t* out);
  29
  30   // Reads |num_bits| (up to 64) into |out|. |out| is filled from the MSB to the
  31   // LSB. If |num_bits| is less than 64, the most significant |64 - num_bits|
  32   // bits are unused and left as zeros. Returns true if the stream had the
  33   // requested |num_bits|, false otherwise.
  34   bool ReadBits(uint8_t num_bits, uint64_t* out);
  35
  36   // Returns the number of bits left in the stream.
  37   uint64_t BitsLeft() const;
  38
  39  private:
  40   // Reads a single bit. Within a byte, the bits are read from the MSB to the
  41   // LSB.
  42   uint8_t ReadBit();
  43
  44   // Reads a single byte.
  45   // Precondition: The stream must be byte-aligned (current_bit_ == 7) before
  46   // calling this function.
  47   uint8_t ReadByte();
  48
  49   const base::StringPiece source_;
  50
  51   // Index of the byte currently being read from.
  52   size_t current_byte_;
  53
  54   // Index of the last bit read within |current_byte_|. Since bits are read
  55   // from the MSB to the LSB, this value is initialized to 7 and decremented
  56   // after each read.
  57   int8 current_bit_;
  58
  59   DISALLOW_COPY_AND_ASSIGN(BitStreamReader);
  60 };
  61
  62 }  // namespace internal
  63 }  // namespace packed_ct_ev_whitelist
  64
  65 #endif  // COMPONENTS_PACKED_CT_EV_WHITELIST_BIT_STREAM_READER_H_