Split the bit reader functionalities from the byte stream provider.

BUG=None

Review URL: https://codereview.chromium.org/112343011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244959 0039d316-1c4b-4281-b951-d872f2087c98

media/base/bit_reader.cc

@@ -4,80 +4,31 @@
 
 #include "media/base/bit_reader.h"
 
-#include <algorithm>
-
 namespace media {
 
-BitReader::BitReader(const uint8* data, off_t size)
-    : data_(data), bytes_left_(size), num_remaining_bits_in_curr_byte_(0) {
-  DCHECK(data_ != NULL && bytes_left_ > 0);
-
-  UpdateCurrByte();
+BitReader::BitReader(const uint8* data, int size)
+    : initial_size_(size),
+      data_(data),
+      bytes_left_(size),
+      bit_reader_core_(this) {
+  DCHECK(data != NULL);
+  DCHECK_GE(size, 0);
 }
 
 BitReader::~BitReader() {}
 
-bool BitReader::SkipBits(int num_bits) {
-  DCHECK_GE(num_bits, 0);
-  DVLOG_IF(0, num_bits > 100)
-      << "BitReader::SkipBits inefficient for large skips";
-
-  // Skip any bits in the current byte waiting to be processed, then
-  // process full bytes until less than 8 bits remaining.
-  while (num_bits > 0 && num_bits > num_remaining_bits_in_curr_byte_) {
-    num_bits -= num_remaining_bits_in_curr_byte_;
-    num_remaining_bits_in_curr_byte_ = 0;
-    UpdateCurrByte();
-
-    // If there is no more data remaining, only return true if we
-    // skipped all that were requested.
-    if (num_remaining_bits_in_curr_byte_ == 0)
-      return (num_bits == 0);
-  }
-
-  // Less than 8 bits remaining to skip. Use ReadBitsInternal to verify
-  // that the remaining bits we need exist, and adjust them as necessary
-  // for subsequent operations.
-  uint64 not_needed;
-  return ReadBitsInternal(num_bits, &not_needed);
-}
-
-int BitReader::bits_available() const {
-  return 8 * bytes_left_ + num_remaining_bits_in_curr_byte_;
-}
-
-bool BitReader::ReadBitsInternal(int num_bits, uint64* out) {
-  DCHECK_LE(num_bits, 64);
-
-  *out = 0;
-
-  while (num_remaining_bits_in_curr_byte_ != 0 && num_bits != 0) {
-    int bits_to_take = std::min(num_remaining_bits_in_curr_byte_, num_bits);
-
-    *out <<= bits_to_take;
-    *out += curr_byte_ >> (num_remaining_bits_in_curr_byte_ - bits_to_take);
-    num_bits -= bits_to_take;
-    num_remaining_bits_in_curr_byte_ -= bits_to_take;
-    curr_byte_ &= (1 << num_remaining_bits_in_curr_byte_) - 1;
-
-    if (num_remaining_bits_in_curr_byte_ == 0)
-      UpdateCurrByte();
-  }
-
-  return num_bits == 0;
-}
-
-void BitReader::UpdateCurrByte() {
-  DCHECK_EQ(num_remaining_bits_in_curr_byte_, 0);
+int BitReader::GetBytes(int max_nbytes, const uint8** out) {
+  DCHECK_GE(max_nbytes, 0);
+  DCHECK(out);
 
-  if (bytes_left_ == 0)
-    return;
+  int nbytes = max_nbytes;
+  if (nbytes > bytes_left_)
+    nbytes = bytes_left_;
 
-  // Load a new byte and advance pointers.
-  curr_byte_ = *data_;
-  ++data_;
-  --bytes_left_;
-  num_remaining_bits_in_curr_byte_ = 8;
+  *out = data_;
+  data_ += nbytes;
+  bytes_left_ -= nbytes;
+  return nbytes;
 }
 
 }  // namespace media

media/base/bit_reader.h

@@ -5,70 +5,57 @@
 #ifndef MEDIA_BASE_BIT_READER_H_
 #define MEDIA_BASE_BIT_READER_H_
 
-#include <sys/types.h>
-
 #include "base/basictypes.h"
-#include "base/logging.h"
+#include "base/compiler_specific.h"
+#include "base/memory/scoped_ptr.h"
+#include "media/base/bit_reader_core.h"
 #include "media/base/media_export.h"
 
 namespace media {
 
-// A class to read bit streams.
-class MEDIA_EXPORT BitReader {
+class MEDIA_EXPORT BitReader
+    : NON_EXPORTED_BASE(private BitReaderCore::ByteStreamProvider)  {
  public:
   // Initialize the reader to start reading at |data|, |size| being size
   // of |data| in bytes.
-  BitReader(const uint8* data, off_t size);
-  ~BitReader();
-
-  // Read |num_bits| next bits from stream and return in |*out|, first bit
-  // from the stream starting at |num_bits| position in |*out|.
-  // |num_bits| cannot be larger than the bits the type can hold.
-  // Return false if the given number of bits cannot be read (not enough
-  // bits in the stream), true otherwise. When return false, the stream will
-  // enter a state where further ReadBits/SkipBits operations will always
-  // return false unless |num_bits| is 0. The type |T| has to be a primitive
-  // integer type.
-  template<typename T> bool ReadBits(int num_bits, T *out) {
-    DCHECK_LE(num_bits, static_cast<int>(sizeof(T) * 8));
-    uint64 temp;
-    bool ret = ReadBitsInternal(num_bits, &temp);
-    *out = static_cast<T>(temp);
-    return ret;
+  BitReader(const uint8* data, int size);
+  virtual ~BitReader();
+
+  template<typename T> bool ReadBits(int num_bits, T* out) {
+    return bit_reader_core_.ReadBits(num_bits, out);
+  }
+
+  bool ReadFlag(bool* flag) {
+    return bit_reader_core_.ReadFlag(flag);
+  }
+
+  bool SkipBits(int num_bits) {
+    return bit_reader_core_.SkipBits(num_bits);
   }
 
-  // Skip |num_bits| next bits from stream. Return false if the given number of
-  // bits cannot be skipped (not enough bits in the stream), true otherwise.
-  // When return false, the stream will enter a state where further ReadBits/
-  // SkipBits operations will always return false unless |num_bits| is 0.
-  bool SkipBits(int num_bits);
+  int bits_available() const {
+    return initial_size_ * 8 - bits_read();
+  }
 
-  // Returns the number of bits available for reading.
-  int bits_available() const;
+  int bits_read() const {
+    return bit_reader_core_.bits_read();
+  }
 
  private:
-  // Help function used by ReadBits to avoid inlining the bit reading logic.
-  bool ReadBitsInternal(int num_bits, uint64* out);
+  // BitReaderCore::ByteStreamProvider implementation.
+  virtual int GetBytes(int max_n, const uint8** out) OVERRIDE;
 
-  // Advance to the next byte, loading it into curr_byte_.
-  // If the num_remaining_bits_in_curr_byte_ is 0 after this function returns,
-  // the stream has reached the end.
-  void UpdateCurrByte();
+  // Total number of bytes that was initially passed to BitReader.
+  const int initial_size_;
 
-  // Pointer to the next unread (not in curr_byte_) byte in the stream.
+  // Pointer to the next unread byte in the stream.
   const uint8* data_;
 
-  // Bytes left in the stream (without the curr_byte_).
-  off_t bytes_left_;
+  // Bytes left in the stream.
+  int bytes_left_;
 
-  // Contents of the current byte; first unread bit starting at position
-  // 8 - num_remaining_bits_in_curr_byte_ from MSB.
-  uint8 curr_byte_;
+  BitReaderCore bit_reader_core_;
 
-  // Number of bits remaining in curr_byte_
-  int num_remaining_bits_in_curr_byte_;
-
- private:
   DISALLOW_COPY_AND_ASSIGN(BitReader);
 };
 

media/base/bit_reader_core.cc

@@ -0,0 +1,159 @@
+// Copyright 2014 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/port.h>
+#include <base/sys_byteorder.h>
+
+namespace {
+const int kRegWidthInBits = sizeof(uint64) * 8;
+}
+
+namespace media {
+
+BitReaderCore::ByteStreamProvider::ByteStreamProvider() {
+}
+
+BitReaderCore::ByteStreamProvider::~ByteStreamProvider() {
+}
+
+BitReaderCore::BitReaderCore(ByteStreamProvider* byte_stream_provider)
+    : byte_stream_provider_(byte_stream_provider),
+      bits_read_(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 = (reg_ & (GG_UINT64_C(1) << (kRegWidthInBits - 1))) != 0;
+  reg_ <<= 1;
+  nbits_--;
+  bits_read_++;
+  return true;
+}
+
+int BitReaderCore::PeekBitsMsbAligned(int num_bits, uint64* out) {
+  // Try to have at least |num_bits| in the bit register.
+  if (nbits_ < num_bits)
+    Refill(num_bits);
+
+  *out = reg_;
+  return nbits_;
+}
+
+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 >= kRegWidthInBits) {
+    if (!ReadBitsInternal(kRegWidthInBits, &dummy))
+      return false;
+    num_bits -= kRegWidthInBits;
+  }
+  return ReadBitsInternal(num_bits, &dummy);
+}
+
+int BitReaderCore::bits_read() const {
+  return bits_read_;
+}
+
+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 purpose.
+    nbits_ = 0;
+    reg_ = 0;
+    return false;
+  }
+
+  bits_read_ += num_bits;
+
+  if (num_bits == kRegWidthInBits) {
+    // 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_ >> (kRegWidthInBits - num_bits);
+  reg_ <<= num_bits;
+  nbits_ -= num_bits;
+  return true;
+}
+
+bool BitReaderCore::Refill(int min_nbits) {
+  DCHECK_LE(min_nbits, kRegWidthInBits);
+
+  // 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);
+
+  // Max number of bytes to refill.
+  int max_nbytes = sizeof(reg_next_);
+
+  // Refill.
+  const uint8* byte_stream_window;
+  int window_size =
+      byte_stream_provider_->GetBytes(max_nbytes, &byte_stream_window);
+  DCHECK_GE(window_size, 0);
+  DCHECK_LE(window_size, max_nbytes);
+  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 * 8;
+
+  // 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_ == kRegWidthInBits || nbits_next_ == 0)
+    return;
+
+  reg_ |= (reg_next_ >> nbits_);
+
+  int free_nbits = kRegWidthInBits - nbits_;
+  if (free_nbits >= nbits_next_) {
+    nbits_ += nbits_next_;
+    reg_next_ = 0;
+    nbits_next_ = 0;
+    return;
+  }
+
+  nbits_ += free_nbits;
+  reg_next_ <<= free_nbits;
+  nbits_next_ -= free_nbits;
+}
+
+}  // namespace media