Add HE AAC support to ISO BMFF.

media/base/bit_reader.h

+// Copyright (c) 2012 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.
+
+#ifndef MEDIA_BASE_BIT_READER_H_
+#define MEDIA_BASE_BIT_READER_H_
+
+#include <sys/types.h>
+#include <algorithm>
+#include <climits>
+
+#include "base/basictypes.h"
+#include "base/logging.h"
+#include "media/base/media_export.h"
+
+
+namespace media {
+
+// A class to read bit streams.
+// Classes inherited this class can override its UpdateCurrByte function
+// to support specific escape mechanism, which is widely used by streaming
+// formats like H.264 Annex B.
+class MEDIA_EXPORT BitReader {
+ public:
+  BitReader();
+  BitReader(const uint8* data, off_t size);
+  virtual ~BitReader();
+
+  // Initialize the reader to start reading at |data|, |size| being size
+  // of |data| in bytes.
+  void Initialize(const uint8* data, off_t size);
+
+  // 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 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(num_bits <= static_cast<int>(sizeof(T) * CHAR_BIT));
+
+    *out = 0;
+    position_ += num_bits;
+
+    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 = (*out << bits_to_take) +
+          (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;

[Pawel Osciak, 2012/07/16 21:27:09]

Could you please explain why this more complicated version than the original
one? This version looks like it's doing more operations than the original,
perhaps I'm missing something?

= Old version:
(2 comps + 2 subs + 1 shift) * num_bytes + 3 subs + 2 shifts + 1 AND + 1 OR
for 1 byte: 2 comps, 5 subs, 3 shifts, 1 AND, 1 OR = 12 ops
for 2 bytes: 4 comps, 7 subs, 4 shifts, 1 AND, 1 OR = 17 ops
etc.

= New version:
(4 comps + 4 subs + 3 shifts + 1 add + 1 AND) * num_bytes + 1 comp
for 1 byte: 4 comps, 4 subs, 3 shifts, 1 add, 1 AND, 1 comp = 14 ops
for 2 bytes: 8 comps, 8 subs, 6 shifts, 2 adds, 2 AND, 1 comp = 27 ops
etc.

(hopefully didn't make a mistake counting, also not counting number of registers
needed)

Old version:
bool H264Parser::H264BitReader::ReadBits(int num_bits, int *out) {
  int bits_left = num_bits;
  *out = 0;
  DCHECK(num_bits <= 31);

  while (num_remaining_bits_in_curr_byte_ < bits_left) {
    // Take all that's left in current byte, shift to make space for the rest.
    *out = (curr_byte_ << (bits_left - num_remaining_bits_in_curr_byte_));
    bits_left -= num_remaining_bits_in_curr_byte_;

    if (!UpdateCurrByte())
      return false;
  }

  *out |= (curr_byte_ >> (num_remaining_bits_in_curr_byte_ - bits_left));
  *out &= ((1 << num_bits) - 1);
  num_remaining_bits_in_curr_byte_ -= bits_left;

  return true;
}

+
+      if (num_remaining_bits_in_curr_byte_ == 0)
+        UpdateCurrByte();
+    }
+
+    if (num_bits == 0)
+      return true;
+
+    *out = 0;
+    num_remaining_bits_in_curr_byte_ = 0;
+    bytes_left_ = 0;
+
+    return false;
+  }
+
+  bool SkipBits(int num_bits);
+
+  // Return the current position in the stream in unit of bit.
+  // This includes the skipped escape bytes if there are any.
+  off_t Tell() const;
+
+  // Return the number of bits left in the stream.
+  // This doesn't take any escape sequence into account.
+  off_t NumBitsLeft() const;
+
+  bool HasMoreData() const;
+
+ protected:
+  // Advance to the next byte, loading it into curr_byte_.
+  // This function can be overridden to support specific escape mechanism.
+  // If the num_remaining_bits_in_curr_byte_ is 0 after this function returns,
+  // the stream has reached the end.
+  virtual void UpdateCurrByte();
+
+  // Pointer to the next unread (not in curr_byte_) byte in the stream.
+  const uint8* data_;
+
+  // Bytes left in the stream (without the curr_byte_).
+  off_t bytes_left_;
+
+  // Current position in bits.
+  off_t position_;
+
+  // Contents of the current byte; first unread bit starting at position
+  // 8 - num_remaining_bits_in_curr_byte_ from MSB.
+  uint8 curr_byte_;
+
+  // Number of bits remaining in curr_byte_
+  int num_remaining_bits_in_curr_byte_;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(BitReader);
+};
+
+}  // namespace media
+
+#endif  // MEDIA_BASE_BIT_READER_H_