cache: began base implementation of "shifting" the cache

Should be a fairly important optimization to make.

app/render/playbackcache.cpp

@@ -61,6 +61,30 @@ bool PlaybackCache::IsFullyValidated() const
   return invalidated_.isEmpty();
 }
 
+void PlaybackCache::Shift(const rational &from, const rational &to)
+{
+  // An region between `from` and `to` will be inserted or spliced out
+  TimeRangeList ranges_to_shift = invalidated_.Intersects(TimeRange(from, RATIONAL_MAX));
+
+  // Remove everything from the minimum point
+  invalidated_.RemoveTimeRange(TimeRange(qMin(from, to), RATIONAL_MAX));
+
+  // Shift everything in our ranges to shift list
+  //
+  // `diff` is POSITIVE when moving forward -> and NEGATIVE when moving backward <-
+  rational diff = to - from;
+  foreach (const TimeRange& r, ranges_to_shift) {
+    invalidated_.InsertTimeRange(r + diff);
+  }
+
+  ShiftEvent(from, to);
+
+  if (diff > rational()) {
+    // If shifting forward, add this section to the invalidated region
+    Invalidate(TimeRange(from, to));
+  }
+}
+
 void PlaybackCache::Validate(const TimeRange &r)
 {
   invalidated_.RemoveTimeRange(r);
@@ -81,4 +105,8 @@ void PlaybackCache::InvalidateEvent(const TimeRange &)
 {
 }
 
+void PlaybackCache::ShiftEvent(const rational &, const rational &)
+{
+}
+
 OLIVE_NAMESPACE_EXIT