Function FindCorrespondence

src/tracks/ui/TimeShiftHandle.cpp:585–653

   using Correspondence = std::unordered_map< Track*, Track* >;

   bool FindCorrespondence(
      Correspondence &correspondence,
      TrackList &trackList, Track &capturedTrack, Track &track,
      ClipMoveState &state)
   {
      if (state.shifters.empty())
         // Shift + Dragging hasn't yet supported vertical movement
         return false;

      // Accumulate new pairs for the correspondence, and merge them
      // into the given correspondence only on success
      Correspondence newPairs;

      auto sameType = [&](auto pTrack){
         return capturedTrack.SameKindAs(*pTrack);
      };
      if (!sameType(&track))
         return false;

      // All tracks of the same kind as the captured track
      auto range = trackList.Any() + sameType;

      // Find how far this track would shift down among those (signed)
      const auto myPosition =
         std::distance(range.first, trackList.Find(&capturedTrack));
      const auto otherPosition =
         std::distance(range.first, trackList.Find(&track));
      auto diff = otherPosition - myPosition;

      // Point to destination track for first of range, when diff >= 0
      // Otherwise the loop below iterates -diff times checking that initial
      // members of the range are not the shifting tracks
      auto iter = range.first.advance(diff >= 0 ? diff : 0);

      for (auto pTrack : range) {
         auto &pShifter = state.shifters[pTrack];
         if (!pShifter->MovingIntervals().empty()) {
            // One of the interesting tracks

            auto pOther = *iter;
            if (diff < 0 || !pOther)
               // No corresponding track
               return false;

            if (!pShifter->MayMigrateTo(*pOther))
               // Rejected for other reason
               return false;

            if (correspondence.count(pTrack))
               // Don't overwrite the given correspondence
               return false;

            newPairs[pTrack] = pOther;
         }

         if (diff < 0)
            ++diff; // Still consuming initial tracks
         else