SeekableBuffer to implement a window of buffer which has short seeking ability.

media/base/seekable_buffer.cc

+// Copyright (c) 2009 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/seekable_buffer.h"
+
+#include <algorithm>
+
+#include "base/logging.h"
+#include "base/stl_util-inl.h"
+
+namespace media {
+
+SeekableBuffer::SeekableBuffer(size_t backward_capacity,
+                               size_t forward_capacity)
+    : current_buffer_offset_(0),
+      backward_capacity_(backward_capacity),
+      backward_bytes_(0),
+      forward_capacity_(forward_capacity),
+      forward_bytes_(0) {
+  current_buffer_ = buffers_.begin();
+}
+
+SeekableBuffer::~SeekableBuffer() {
+  STLDeleteElements(&buffers_);
+}
+
+size_t SeekableBuffer::Read(size_t size, uint8* data) {
+  DCHECK(data);
+  return InternalRead(size, data);
+}
+
+bool SeekableBuffer::Append(size_t size, const uint8* data) {
+  // Since the forward capacity is only used to check the criteria for buffer
+  // full, we will always append data to the buffer.
+  Buffer* buffer = new Buffer(size);
+  memcpy(buffer->data.get(), data, size);
+  buffers_.push_back(buffer);
+
+  // After we have written the first buffer, update the |current_buffer_| to
+  // point to it.
+  if (current_buffer_ == buffers_.end()) {
+    DCHECK_EQ(0, forward_bytes_);
+    current_buffer_ = buffers_.begin();
+  }
+
+  // Update the |forward_bytes_| counter since we have more bytes.
+  forward_bytes_ += size;
+
+  // Advise the user to stop append if the amount of forward bytes exceeds
+  // the forward capacity. A false return value means the user should stop
+  // appending more data to this buffer.
+  if (forward_bytes_ >= forward_capacity_)
+    return false;
+  return true;
+}
+
+bool SeekableBuffer::Seek(int32 offset) {
+  if (offset > 0)
+    return SeekForward(offset);
+  else if (offset < 0)
+    return SeekBackward(-offset);
+  return true;
+}
+
+bool SeekableBuffer::SeekForward(size_t size) {
+  // Perform seeking forward only if we have enough bytes in the queue.
+  if (size > forward_bytes_)
+    return false;
+
+  // Do a read of |size| bytes.
+  size_t taken = InternalRead(size, NULL);
+  DCHECK_EQ(taken, size);
+  return true;
+}
+
+bool SeekableBuffer::SeekBackward(size_t size) {
+  if (size > backward_bytes_)
+    return false;
+  // Record the number of bytes taken.
+  size_t taken = 0;
+  // Loop until we taken enough bytes and rewind by the desired |size|.
+  while (taken < size) {
+    // |current_buffer_| can never be invalid when we are in this loop. It can
+    // only be invalid before any data is appended, this case should be handled
+    // by checks before we enter this loop.
+    DCHECK(current_buffer_ != buffers_.end());
+
+    // We try to at most |size| bytes in the backward direction, we also have
+    // to account for the offset we are in the current buffer, take the minimum
+    // between the two to determine the amount of bytes to take from the
+    // current buffer.
+    size_t consumed = std::min(size - taken, current_buffer_offset_);
+
+    // Decreases the offset in the current buffer since we are rewinding.
+    current_buffer_offset_ -= consumed;
+
+    // Increase the amount of bytes taken in the backward direction, this
+    // determines when to stop the loop.
+    taken += consumed;
+
+    // Forward bytes increases, and backward bytes decreases by the amount
+    // consumed in the current buffer.
+    forward_bytes_ += consumed;
+    backward_bytes_ -= consumed;
+    DCHECK_GE(backward_bytes_, 0u);
+
+    // The current buffer pointed by current iterator has been consumed,
+    // move the iterator backward so it points to the previous buffer.
+    if (current_buffer_offset_ == 0) {
+      if (current_buffer_ == buffers_.begin())
+        break;
+      // Move the iterator backward.
+      --current_buffer_;
+      // Set the offset into the current buffer to be the buffer size as we
+      // are preparing for rewind for next iteration.
+      current_buffer_offset_ = (*current_buffer_)->size;
+    }
+  }
+  DCHECK_EQ(taken, size);
+  return true;
+}
+
+void SeekableBuffer::EvictBackwardBuffers() {
+  // Advances the iterator until we hit the current pointer.
+  while (backward_bytes_ > backward_capacity_) {
+    BufferQueue::iterator i = buffers_.begin();
+    if (i == current_buffer_)
+      break;
+    Buffer* buffer = *i;
+    backward_bytes_ -= buffer->size;
+    DCHECK_GE(backward_bytes_, 0u);
+
+    delete *i;
+    buffers_.erase(i);
+  }
+}
+
+size_t SeekableBuffer::InternalRead(size_t size, uint8* data) {
+  // Counts how many bytes are actually read from the buffer queue.
+  size_t taken = 0;
+
+  while (taken < size) {
+    // |current_buffer_| is valid since the first time this buffer is appended
+    // with data.
+    if (current_buffer_ == buffers_.end()) {
+      DCHECK_EQ(0, forward_bytes_);
+      break;
+    }
+    Buffer* buffer = *current_buffer_;
+
+    // Find the right amount to copy from the current buffer referenced by
+    // |buffer|. We shall copy no more than |size| bytes in total and each
+    // single step copied no more than the current buffer size.
+    size_t copied = std::min(size - taken,
+                             buffer->size - current_buffer_offset_);
+
+    // |data| is NULL if we are seeking forward, thus there's no need to copy.
+    if (data)
+      memcpy(data + taken, buffer->data.get() + current_buffer_offset_, copied);
+
+    // Increase total number of bytes copied, which regulates when to end this
+    // loop.
+    taken += copied;
+
+    // We have read |copied| bytes from the current buffer, advances the offset.
+    current_buffer_offset_ += copied;
+
+    // We have less forward bytes and more backward bytes, update these counters
+    // by |copied|.
+    forward_bytes_ -= copied;
+    backward_bytes_ += copied;
+    DCHECK_GE(forward_bytes_, 0u);
+
+    // The buffer has been consumed.
+    if (current_buffer_offset_ == buffer->size) {
+      BufferQueue::iterator next = current_buffer_;
+      ++next;
+      // If we are at the last buffer, don't advance.
+      if (next == buffers_.end())
+        break;
+
+      // Advances the iterator.
+      current_buffer_ = next;
+      current_buffer_offset_ = 0;
+    }
+  }
+  EvictBackwardBuffers();
+  return taken;
+}
+
+}  // namespace media