Unwrap picture ids in the RtpFrameReferencerFinder.

webrtc/modules/video_coding/sequence_number_util.h

 /*
  *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 73 matching lines @@
 // can be set. The unwrapped value is not allowed to wrap.
 template <typename T, T M = 0>
 class SeqNumUnwrapper {
   // Use '<' instead of rtc::SafeLt to avoid crbug.com/753488
   static_assert(
       std::is_unsigned<T>::value &&
           std::numeric_limits<T>::max() < std::numeric_limits<uint64_t>::max(),
       "Type unwrapped must be an unsigned integer smaller than uint64_t.");
 
  public:
-  SeqNumUnwrapper() : last_unwrapped_(0) {}
+  // We want a value that is close to 2^63 for two reasons. Firstly, we
+  // can unwrap wrapping numbers in either direction, and secondly, we avoid
+  // causing a crash on bad input. We also want the default value to be somewhat
+  // human readable, a power of 10 for example.
+  static constexpr uint64_t kDefaultStartValue = 10000000000000000000UL;
+
+  SeqNumUnwrapper() : last_unwrapped_(kDefaultStartValue) {}
   explicit SeqNumUnwrapper(uint64_t start_at) : last_unwrapped_(start_at) {}
 
   uint64_t Unwrap(T value) {
     if (!last_value_)
       last_value_.emplace(value);
 
     uint64_t unwrapped = 0;
     if (AheadOrAt<T, M>(value, *last_value_)) {
       unwrapped = last_unwrapped_ + ForwardDiff<T, M>(*last_value_, value);
       RTC_CHECK_GE(unwrapped, last_unwrapped_);
     } else {
       unwrapped = last_unwrapped_ - ReverseDiff<T, M>(*last_value_, value);
       RTC_CHECK_LT(unwrapped, last_unwrapped_);
     }
 
     *last_value_ = value;
     last_unwrapped_ = unwrapped;
     return last_unwrapped_;
   }
 
  private:
   uint64_t last_unwrapped_;
   rtc::Optional<T> last_value_;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_VIDEO_CODING_SEQUENCE_NUMBER_UTIL_H_