Add a new BitBuffer class to webrtc base.

Provides a read-only interface for reading byte and bit-sized data from
an underlying buffer in network/big-endian order. Also provides a method
for reading exponential golomb encoded values, which will be useful in
H.264 packet parsing (separate CL).

BUG=
R=pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/49719004

Cr-Commit-Position: refs/heads/master@{#9046}

webrtc/base/bitbuffer.cc

@@ -0,0 +1,154 @@
+/*
+ *  Copyright 2015 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "webrtc/base/bitbuffer.h"
+
+#include <limits>
+
+#include "webrtc/base/checks.h"
+
+namespace {
+
+// Returns the lowest (right-most) |bit_count| bits in |byte|.
+uint8 LowestBits(uint8 byte, size_t bit_count) {
+  DCHECK_LE(bit_count, 8u);
+  uint8 mask_shift = 8 - static_cast<uint8>(bit_count);
+  return byte & (0xFF >> mask_shift);
+}
+
+// Returns the highest (left-most) |bit_count| bits in |byte|, shifted to the
+// lowest bits (to the right).
+uint8 HighestBits(uint8 byte, size_t bit_count) {
+  DCHECK_LE(bit_count, 8u);
+  uint8 shift = 8 - static_cast<uint8>(bit_count);
+  uint8 mask = 0xFF << shift;
+  return (byte & mask) >> shift;
+}
+
+}  // namespace
+
+namespace rtc {
+
+BitBuffer::BitBuffer(const uint8* bytes, size_t byte_count)
+    : bytes_(bytes), byte_count_(byte_count), byte_offset_(), bit_offset_() {
+  DCHECK(static_cast<uint64>(byte_count_) <=
+         std::numeric_limits<uint32>::max());
+}
+
+uint64 BitBuffer::RemainingBitCount() const {
+  return (static_cast<uint64>(byte_count_) - byte_offset_) * 8 - bit_offset_;
+}
+
+bool BitBuffer::ReadUInt8(uint8* val) {
+  uint32 bit_val;
+  if (!ReadBits(&bit_val, sizeof(uint8) * 8)) {
+    return false;
+  }
+  DCHECK(bit_val <= std::numeric_limits<uint8>::max());
+  *val = static_cast<uint8>(bit_val);
+  return true;
+}
+
+bool BitBuffer::ReadUInt16(uint16* val) {
+  uint32 bit_val;
+  if (!ReadBits(&bit_val, sizeof(uint16) * 8)) {
+    return false;
+  }
+  DCHECK(bit_val <= std::numeric_limits<uint16>::max());
+  *val = static_cast<uint16>(bit_val);
+  return true;
+}
+
+bool BitBuffer::ReadUInt32(uint32* val) {
+  return ReadBits(val, sizeof(uint32) * 8);
+}
+
+bool BitBuffer::PeekBits(uint32* val, size_t bit_count) {
+  if (!val || bit_count > RemainingBitCount() || bit_count > 32) {
+    return false;
+  }
+  const uint8* bytes = bytes_ + byte_offset_;
+  size_t remaining_bits_in_current_byte = 8 - bit_offset_;
+  uint32 bits = LowestBits(*bytes++, remaining_bits_in_current_byte);
+  // If we're reading fewer bits than what's left in the current byte, just
+  // return the portion of this byte that we need.
+  if (bit_count < remaining_bits_in_current_byte) {
+    *val = HighestBits(bits, bit_offset_ + bit_count);
+    return true;
+  }
+  // Otherwise, subtract what we've read from the bit count and read as many
+  // full bytes as we can into bits.
+  bit_count -= remaining_bits_in_current_byte;
+  while (bit_count >= 8) {
+    bits = (bits << 8) | *bytes++;
+    bit_count -= 8;
+  }
+  // Whatever we have left is smaller than a byte, so grab just the bits we need
+  // and shift them into the lowest bits.
+  if (bit_count > 0) {
+    bits <<= bit_count;
+    bits |= HighestBits(*bytes, bit_count);
+  }
+  *val = bits;
+  return true;
+}
+
+bool BitBuffer::ReadBits(uint32* val, size_t bit_count) {
+  return PeekBits(val, bit_count) && ConsumeBits(bit_count);
+}
+
+bool BitBuffer::ConsumeBytes(size_t byte_count) {
+  return ConsumeBits(byte_count * 8);
+}
+
+bool BitBuffer::ConsumeBits(size_t bit_count) {
+  if (bit_count > RemainingBitCount()) {
+    return false;
+  }
+
+  byte_offset_ += (bit_offset_ + bit_count) / 8;
+  bit_offset_ = (bit_offset_ + bit_count) % 8;
+  return true;
+}
+
+bool BitBuffer::ReadExponentialGolomb(uint32* val) {
+  if (!val) {
+    return false;
+  }
+  // Store off the current byte/bit offset, in case we want to restore them due
+  // to a failed parse.
+  size_t original_byte_offset = byte_offset_;
+  size_t original_bit_offset = bit_offset_;
+
+  // Count the number of leading 0 bits by peeking/consuming them one at a time.
+  size_t zero_bit_count = 0;
+  uint32 peeked_bit;
+  while (PeekBits(&peeked_bit, 1) && peeked_bit == 0) {
+    zero_bit_count++;
+    ConsumeBits(1);
+  }
+
+  // We should either be at the end of the stream, or the next bit should be 1.
+  DCHECK(!PeekBits(&peeked_bit, 1) || peeked_bit == 1);
+
+  // The bit count of the value is the number of zeros + 1. Make sure that many
+  // bits fits in a uint32 and that we have enough bits left for it, and then
+  // read the value.
+  size_t value_bit_count = zero_bit_count + 1;
+  if (value_bit_count > 32 || !ReadBits(val, value_bit_count)) {
+    byte_offset_ = original_byte_offset;
+    bit_offset_ = original_bit_offset;
+    return false;
+  }
+  *val -= 1;
+  return true;
+}
+
+}  // namespace rtc