Split the bit reader functionalities from the byte stream provider.

media/base/bit_reader_core.cc

+// Copyright (c) 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "media/base/bit_reader_core.h"
+
+#include <base/sys_byteorder.h>
+
+namespace media {
+
+BitReaderCore::ByteStreamProvider::ByteStreamProvider() {
+}
+
+BitReaderCore::ByteStreamProvider::~ByteStreamProvider() {
+}
+
+BitReaderCore::BitReaderCore(ByteStreamProvider* byte_stream_provider)
+    : byte_stream_provider_(byte_stream_provider),
+      bit_count_(0),
+      nbits_(0),
+      reg_(0),
+      nbits_next_(0),
+      reg_next_(0) {
+}
+
+BitReaderCore::~BitReaderCore() {
+}
+
+bool BitReaderCore::ReadFlag(bool* flag) {
+  if (nbits_ == 0 && !Refill(1))
+    return false;
+
+  *flag = (static_cast<SignedRegType>(reg_) < 0);
+  reg_ <<= 1;
+  nbits_--;
+  bit_count_++;
+  return true;
+}
+
+bool BitReaderCore::ReadUE(uint32* out) {
+  // Get the number of leading zeros.
+  int zero_count = -1;
+  bool is_one;
+  do {
+    if (nbits_ == 0 && !Refill(1))
+      return false;
+    is_one = (static_cast<SignedRegType>(reg_) < 0);
+    reg_ <<= 1;
+    nbits_--;
+    zero_count++;
+  } while(!is_one);
+  bit_count_ += (zero_count + 1);
+
+  // If zero_count is greater than 31, the calculated value will overflow.
+  if (zero_count > 31)
+    return false;
+
+  // Read the actual value.
+  uint32 base = (1 << zero_count) - 1;
+  uint32 offset;
+  if (!ReadBits(zero_count, &offset))
+    return false;
+  *out = base + offset;
+
+  return true;
+}
+
+bool BitReaderCore::SkipBits(int num_bits) {
+  DCHECK_GE(num_bits, 0);
+  DVLOG_IF(0, num_bits > 100)
+      << "BitReader::SkipBits inefficient for large skips";
+
+  uint64 dummy;
+  while (num_bits >= 32) {
+    if (!ReadBitsInternal(32, &dummy))
+      return false;
+    num_bits -= 32;
+  }
+  return ReadBitsInternal(num_bits, &dummy);
+}
+
+int BitReaderCore::GetBitCount() const {
+  return bit_count_;
+}
+
+int BitReaderCore::bits_available() const {
+  return nbits_ + nbits_next_;
+}
+
+bool BitReaderCore::HasMoreRBSPData() {
+  // Try to have at least 9 bits (strictly more than 1 byte).
+  if (nbits_ < 9)
+    Refill(9);
+
+  // Not on last byte.
+  if (nbits_ > 8)
+    return true;
+
+  // Last byte, look for stop bit;
+  // We have more RBSP data if the last non-zero bit we find is not the
+  // first available bit.
+  return (reg_ << 1) != 0;
+}
+
+bool BitReaderCore::ReadBitsInternal(int num_bits, uint64* out) {
+  DCHECK_GE(num_bits, 0);
+
+  if (num_bits == 0) {
+    *out = 0;
+    return true;
+  }
+
+  if (num_bits > nbits_ && !Refill(num_bits)) {
+    // Any subsequent ReadBits should fail:
+    // empty the current bit register for that purposer.
+    nbits_ = 0;
+    reg_ = 0;
+    return false;
+  }
+
+  bit_count_ += num_bits;
+
+  if (num_bits == sizeof(RegType) * CHAR_BIT) {
+    // Special case needed since for example for a 64 bit integer "a"
+    // "a << 64" is not defined by the C/C++ standard.
+    *out = reg_;
+    reg_ = 0;
+    nbits_ = 0;
+    return true;
+  }
+
+  *out = reg_ >> (sizeof(RegType) * CHAR_BIT - num_bits);
+  reg_ <<= num_bits;
+  nbits_ -= num_bits;
+  return true;
+}
+
+bool BitReaderCore::Refill(int min_nbits) {
+  DCHECK_LE(min_nbits, static_cast<int>(sizeof(RegType) * CHAR_BIT));
+
+  // Transfer from the next to the current register.
+  RefillCurrentRegister();
+  if (min_nbits <= nbits_)
+    return true;
+  DCHECK_EQ(nbits_next_, 0);
+  DCHECK_EQ(reg_next_, 0u);
+
+  // Strict minimum number of bytes to refill.
+  int min_nbytes = (min_nbits - nbits_ + CHAR_BIT - 1) / CHAR_BIT;
+
+  // Max number of bytes to refill.
+  int max_nbytes = sizeof(RegType);
+
+  // Refill.
+  const uint8* byte_stream_window;
+  int window_size =
+      byte_stream_provider_->GetBytes(
+          min_nbytes, max_nbytes, &byte_stream_window);
+  if (window_size == 0)
+    return false;
+
+  reg_next_ = 0;
+  memcpy(&reg_next_, byte_stream_window, window_size);
+  reg_next_ = base::NetToHost64(reg_next_);
+  nbits_next_ = window_size * CHAR_BIT;
+
+  // Transfer from the next to the current register.
+  RefillCurrentRegister();
+
+  return (nbits_ >= min_nbits);
+}
+
+void BitReaderCore::RefillCurrentRegister() {
+  // No refill possible if the destination register is full
+  // or the source register is empty.
+  if (nbits_ == sizeof(RegType) * CHAR_BIT || nbits_next_ == 0)
+    return;
+
+  reg_ |= (reg_next_ >> nbits_);
+
+  int free_nbits = static_cast<int>(sizeof(RegType) * CHAR_BIT) - nbits_;
+  if (free_nbits >= nbits_next_) {
+    nbits_ += nbits_next_;
+    reg_next_ = 0;
+    nbits_next_ = 0;
+  } else {
+    nbits_ += free_nbits;
+    reg_next_ <<= free_nbits;
+    nbits_next_ -= free_nbits;
+  }
+}
+
+}  // namespace media