Fire canplaythrough event at the proper time for audio/video

media/base/download_rate_monitor.h

+// Copyright (c) 2011 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_DOWNLOAD_RATE_MONITOR_H_
+#define MEDIA_BASE_DOWNLOAD_RATE_MONITOR_H_
+
+#include <deque>
+#include <stdio.h>

[scherkus (not reviewing), 2011/11/09 02:55:15]

what's stdio.h used for?

[vrk (LEFT CHROMIUM), 2011/11/11 02:51:06]

Uhh, debug printfs! Removed :)

+
+#include "base/bind.h"
+#include "base/callback.h"
+#include "base/time.h"
+
+namespace media {
+
+// Measures download speed based on the rate at which a media file is buffering.
+// Signals when it believes the media file can be played back without needing to
+// pause to buffer.
+class DownloadRateMonitor {

[scherkus (not reviewing), 2011/11/09 02:55:15]

class looks like a pretty good candidate for unit tests :)

if you want to wait until we iron out some details that's OK

[vrk (LEFT CHROMIUM), 2011/11/11 02:51:06]

I agree!
Yup will do!

+ public:
+  typedef base::Callback<void()> CanPlayThroughCB;
+
+  DownloadRateMonitor();
+
+  // Begin measuring download rate. The monitor will run |canplaythrough_cb|
+  // when it believes the media can be played back without needing to pause to
+  // buffer. |media_bitrate| is the bitrate of the video.
+  void Start(const CanPlayThroughCB& canplaythrough_cb, int media_bitrate);

[scherkus (not reviewing), 2011/11/09 02:55:15]

is there no stop?

what happens if I call these other methods without calling Start()?

[vrk (LEFT CHROMIUM), 2011/11/11 02:51:06]

Added Stop(), added comments about Start() and Stop() to the relevant methods in
the header. Let me know if I should add more comments!

+
+  // Notifies the monitor of the current number of bytes buffered by the media
+  // file at what timestamp. The monitor expects subsequent calls to
+  // SetBufferedBytes to have monotonically nondecreasing values.
+  void SetBufferedBytes(int64 buffered_bytes, const base::Time& timestamp);
+
+  // Notifies the monitor when the media file has paused or continued
+  // downloading data.
+  void SetNetworkActivity(bool is_downloading_data);
+
+  // Sets the size of the media file in bytes.
+  void SetTotalBytes(int64 total_bytes) { total_bytes_ = total_bytes; }
+
+  // Sets whether the media file is a fully loaded source.
+  void SetLoaded(bool loaded) { loaded_ = loaded; }
+
+ private:
+  // Represents a span of time in which the media was buffering data.
+  class Sample {
+   public:
+    Sample();
+
+    // The buffering state at a point in time. The first value represents the
+    // number of bytes buffered by the media player, the second value represents
+    // the time at which the buffering measurement was taken.
+    typedef std::pair<int64, base::Time> Point;

[scherkus (not reviewing), 2011/11/09 02:55:15]

favour a two-member struct over std::pair

struct Point {
  base::Time time;
  int64 bytes_buffered;
};

[vrk (LEFT CHROMIUM), 2011/11/11 02:51:06]

Done.

+
+    // Set the start point of the data sample.
+    void SetStartPoint(const Point& new_start);
+
+    // Set the end point of the data sample.
+    void SetEndPoint(const Point& new_end);
+
+    const Point& start_point() const { return start_; }

[acolwell GONE FROM CHROMIUM, 2011/11/09 00:50:22]

nit accessor style: set_start(), set_end(), start(), end()

[vrk (LEFT CHROMIUM), 2011/11/11 02:51:06]

Done.

+    const Point& end_point() const { return end_; }
+
+    // Returns the bytes downloaded per second in this sample. Returns -1.0 if
+    // sample is invalid.
+    float bytes_per_second() const;
+
+    // Returns the seconds elapsed in this sample. Returns -1.0 if the sample
+    // has not begun yet.
+    float seconds_elapsed() const;
+
+    // Returns bytes downloaded in this sample.
+    int64 bytes_downloaded() const;
+
+    // Returns true if Sample is null, false otherwise.

[acolwell GONE FROM CHROMIUM, 2011/11/09 00:50:22]

How about "Returns true if Sample has not been initialized."? This matches the
description in base::Time::is_null().

[vrk (LEFT CHROMIUM), 2011/11/11 02:51:06]

Done.

+    bool is_null() const;
+
+    // Resets the sample to an uninitialized state.
+    void Reset();
+
+    // Restarts the sample to begin its measurement at what was previously its
+    // end point.
+    void RestartAtEndPoint();
+
+   private:
+    Point start_;
+    Point end_;
+  };
+
+  int64 bytes_downloaded_in_window() const;
+  float seconds_elapsed_in_window() const;
+
+  // Updates window with latest sample if it is ready.
+  void UpdateSampleWindow();
+
+  // Returns an approximation of the current download rate in bytes per second.
+  // Returns -1.0 if unknown.
+  float ApproximateDownloadByteRate() const;
+
+  // Helper method that returns true if the monitor believes it should fire the
+  // |canplaythrough_cb_|.
+  bool ShouldNotifyCanPlayThrough();
+
+  // Examines the download rate and fires the |canplaythrough_cb_| callback if
+  // the monitor deems it the right time.
+  void NotifyCanPlayThroughIfNeeded();
+
+  // Callback to run when the monitor believes the media can play through
+  // without needing to pause to buffer.
+  CanPlayThroughCB canplaythrough_cb_;
+
+  // Indicates whether the monitor has run the |canplaythrough_cb_|.
+  bool has_notified_can_play_through_;
+
+  // Measurements used to approximate download speed.
+  Sample current_sample_;
+  std::deque<Sample> sample_window_;
+
+  // True if actively downloading bytes, false otherwise.
+  bool is_downloading_data_;
+
+  // Total number of bytes in the media file, 0 if unknown or undefined.
+  int64 total_bytes_;
+
+  // Amount of bytes buffered.
+  int64 buffered_bytes_;
+
+  // True if the media file is a fully loaded source, e.g. file:// protocol.
+  bool loaded_;
+
+  // Bitrate of the media file, 0 if unknown.
+  int bitrate_;
+
+  // True if monitor has been started, false otherwise.
+  bool started_;
+
+  DISALLOW_COPY_AND_ASSIGN(DownloadRateMonitor);
+};
+
+}  // namespace media
+
+#endif  // MEDIA_BASE_DOWNLOAD_RATE_MONITOR_H_