components/packed_ct_ev_whitelist/bit_stream_reader.cc

   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 #include "components/packed_ct_ev_whitelist/bit_stream_reader.h"
   6
   7 #include "base/big_endian.h"
   8 #include "base/logging.h"
   9 #include "base/numerics/safe_conversions.h"
  10
  11 namespace packed_ct_ev_whitelist {
  12 namespace internal {
  13
  14 BitStreamReader::BitStreamReader(const base::StringPiece& source)
  15     : source_(source), current_byte_(0), current_bit_(7) {
  16   DCHECK_LT(source_.length(), UINT32_MAX);
  17 }
  18
  19 bool BitStreamReader::ReadUnaryEncoding(uint64_t* out) {
  20   if (BitsLeft() == 0)
  21     return false;
  22
  23   *out = 0;
  24   while ((BitsLeft() > 0) && ReadBit())
  25     ++(*out);
  26
  27   return true;
  28 }
  29
  30 bool BitStreamReader::ReadBits(uint8_t num_bits, uint64_t* out) {
  31   DCHECK_LE(num_bits, 64);
  32
  33   if (BitsLeft() < num_bits)
  34     return false;
  35
  36   *out = 0;
  37
  38   for (; num_bits && (current_bit_ != 7); --num_bits)
  39     (*out) |= (static_cast<uint64_t>(ReadBit()) << (num_bits - 1));
  40   for (; num_bits / 8; num_bits -= 8)
  41     (*out) |= (static_cast<uint64_t>(ReadByte()) << (num_bits - 8));
  42   for (; num_bits; --num_bits)
  43     (*out) |= (static_cast<uint64_t>(ReadBit()) << (num_bits - 1));
  44
  45   return true;
  46 }
  47
  48 uint64_t BitStreamReader::BitsLeft() const {
  49   if (current_byte_ == source_.length())
  50     return 0;
  51   DCHECK_GT(source_.length(), current_byte_);
  52   return (source_.length() - (current_byte_ + 1)) * 8 + current_bit_ + 1;
  53 }
  54
  55 uint8_t BitStreamReader::ReadBit() {
  56   DCHECK_GT(BitsLeft(), 0u);
  57   DCHECK(current_bit_ < 8 && current_bit_ >= 0);
  58   uint8_t res =
  59       (source_.data()[current_byte_] & (1 << current_bit_)) >> current_bit_;
  60   current_bit_--;
  61   if (current_bit_ < 0) {
  62     current_byte_++;
  63     current_bit_ = 7;
  64   }
  65
  66   return res;
  67 }
  68
  69 uint8_t BitStreamReader::ReadByte() {
  70   DCHECK_GT(BitsLeft(), 7u);
  71   DCHECK_EQ(current_bit_, 7);
  72
  73   return source_.data()[current_byte_++];
  74
  75 }
  76
  77 }  // namespace internal
  78 }  // namespace packed_ct_ev_whitelist