TextRenderer only pushes new cues downstream

media/base/text_ranges.h

+// 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.
+
+#ifndef MEDIA_BASE_TEXT_RANGES_H_
+#define MEDIA_BASE_TEXT_RANGES_H_
+
+#include <map>
+
+#include "base/macros.h"
+#include "base/time/time.h"
+#include "media/base/media_export.h"
+
+namespace media {
+
+// Helper class used by the TextRenderer to filter out text cues that
+// have already been passed downstream.
+class MEDIA_EXPORT TextRanges {
+ public:
+  TextRanges();
+  ~TextRanges();
+
+  // Reset the current range pointer, such that we bind to a new range
+  // (either one that exists already, or one that is freshly-created)
+  // during the next AddCue().
+  void Reset();
+
+  // Given a cue with starting timestamp |start_time|, add its start
+  // time to the time ranges. (Note that following a Reset(), cue
+  // times are assumed to be monotonically increasing.) If this time
+  // has already been added to the time ranges, then AddCue() returns
+  // false and clients should not push the cue downstream. Otherwise,
+  // the time is added to the time ranges and AddCue() returns true,
+  // meaning that the cue should be pushed downstream.
+  bool AddCue(base::TimeDelta start_time);
+
+  // Returns a count of the number of time ranges, intended for use by
+  // the unit test module to vet proper time range merge behavior.
+  size_t RangeCountForTesting() const;
+
+ private:
+  // Describes a range of times for cues that have already been
+  // pushed downstream.
+  class Range {
+   public:
+    // Initialize last_time count.
+    void ResetCount(base::TimeDelta start_time);
+
+    // Set last_time and associated counts.
+    void SetLastTime(base::TimeDelta last_time);
+
+    // Adjust time range state to mark the cue as having been seen,
+    // returning true if we have not seen |start_time| already and
+    // false otherwise.
+    bool AddCue(base::TimeDelta start_time);
+
+    // Returns the value of the last time in the range.
+    base::TimeDelta last_time() const;
+
+   private:
+    // The last timestamp of this range.
+    base::TimeDelta last_time_;
+
+    // The number of cues we have detected so far, for this range,
+    // whose timestamp matches last_time.
+    int max_count_;
+
+    // The number of cues we have seen since the most recent Reset(),
+    // whose timestamp matches last_time.
+    int count_;
+  };
+
+  typedef std::map<base::TimeDelta, Range> RangeMap;
+
+  // NewRange() is used to create a new time range when AddCue() is
+  // called immediately following a Reset(), and no existing time
+  // range contains the indicated |start_time| of the cue.
+  void NewRange(base::TimeDelta start_time);
+
+  // Coalesce curr_range with the range that immediately follows.
+  void Merge(Range& curr_range, const RangeMap::iterator& next_range_itr);
+
+  // The collection of time ranges, each of which is bounded
+  // (inclusive) by the key and Range::last_time.
+  RangeMap range_map_;
+
+  // The time range to which we bind following a Reset().
+  RangeMap::iterator curr_range_itr_;
+
+  DISALLOW_COPY_AND_ASSIGN(TextRanges);
+};
+
+}  // namespace media
+
+#endif  // MEDIA_BASE_TEXT_RANGES_H_