Make the timestamp unroll offset consistent accross PES pids.

media/formats/mp2t/timestamp_unroller.cc

+// 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/formats/mp2t/timestamp_unroller.h"
+
+#include "base/logging.h"
+
+namespace media {
+namespace mp2t {
+
+TimestampUnroller::TimestampUnroller()
+    : is_previous_timestamp_valid_(false),
+      previous_unrolled_timestamp_(0) {
+}
+
+TimestampUnroller::~TimestampUnroller() {
+}
+
+int64 TimestampUnroller::GetUnrolledTimestamp(int64 timestamp) {
+  // Mpeg2 TS timestamps have an accuracy of 33 bits.
+  const int nbits = 33;
+
+  // |timestamp| has a precision of |nbits|
+  // so make sure the highest bits are set to 0.
+  DCHECK_EQ((timestamp >> nbits), 0);
+
+  if (!is_previous_timestamp_valid_) {
+    previous_unrolled_timestamp_ = timestamp;
+    is_previous_timestamp_valid_ = true;
+    return timestamp;
+  }
+
+  // |timestamp| is known modulo 2^33, so estimate the highest bits
+  // to minimize the discontinuity with the previous unrolled timestamp.
+  // Three possibilities are considered to estimate the missing high bits
+  // of |timestamp|. If the bits of the previous unrolled timestamp are
+  // {b63, b62, ..., b0} and bits of |timestamp| are {0, ..., 0, a32, ..., a0}
+  // then the 3 possibilities are:
+  // 1) t1 = {b63, ..., b33, a32, ..., a0} (apply the same offset multiple
+  //    of 2^33 than the one used for the previous timestamps)

[wolenetz, 2014/09/16 17:56:15]

nit: s/than/as

+  // 2) t0 = t1 - 2^33
+  // 3) t2 = t1 + 2^33
+  //
+  // A few remarks:
+  // - the purpose of the timestamp unroller is only to unroll timestamps
+  // in such a way timestamp continuity is satisfied. It can generate negative
+  // values during that process.
+  // - possible overflows are not considered here since 64 bits on a 90kHz
+  // timescale is way enough to represent several years of playback.
+  int64 previous_unrolled_time_high =
+      (previous_unrolled_timestamp_ >> nbits);
+  int64 time0 = ((previous_unrolled_time_high - 1) << nbits) | timestamp;
+  int64 time1 = ((previous_unrolled_time_high + 0) << nbits) | timestamp;
+  int64 time2 = ((previous_unrolled_time_high + 1) << nbits) | timestamp;
+
+  // Select the min absolute difference with the current time
+  // so as to ensure time continuity.
+  int64 diff0 = time0 - previous_unrolled_timestamp_;
+  int64 diff1 = time1 - previous_unrolled_timestamp_;
+  int64 diff2 = time2 - previous_unrolled_timestamp_;
+  if (diff0 < 0)
+    diff0 = -diff0;
+  if (diff1 < 0)
+    diff1 = -diff1;
+  if (diff2 < 0)
+    diff2 = -diff2;
+
+  int64 unrolled_time;
+  int64 min_diff;
+  if (diff1 < diff0) {
+    unrolled_time = time1;
+    min_diff = diff1;
+  } else {
+    unrolled_time = time0;
+    min_diff = diff0;
+  }
+  if (diff2 < min_diff)
+    unrolled_time = time2;
+
+  // Update the state of the timestamp unroller.
+  previous_unrolled_timestamp_ = unrolled_time;
+
+  return unrolled_time;
+}
+
+void TimestampUnroller::Reset() {
+  is_previous_timestamp_valid_ = false;
+  previous_unrolled_timestamp_ = 0;
+}
+
+}  // namespace mp2t
+}  // namespace media