Keyframe::GetRepeatFraction(): Binary search, skipping when constant

The old implementation did a linear scan over the values.  This was
slow with slowly changing keyframes.  This new implementation skips
over constant (when rounded) segments and performs binary search
in (possibly long) interpolated segments to find the X coordinates
where a change occurs quickly.

src/KeyFrame.cpp

@@ -379,40 +379,123 @@ void Keyframe::SetJsonValue(Json::Value root) {
 // Get the fraction that represents how many times this value is repeated in the curve
 // This is depreciated and will be removed soon.
 Fraction Keyframe::GetRepeatFraction(int64_t index) const {
-	// Is index a valid point?
-	if (index >= 1 && (index + 1) < GetLength()) {
-		int64_t current_value = GetLong(index);
-		int64_t previous_repeats = 0;
-		int64_t next_repeats = 0;
-
-		// Loop backwards and look for the next unique value
-		for (int64_t i = index; i > 0; --i) {
-			if (GetLong(i) == current_value) {
-				// Found same value
-				previous_repeats++;
-			} else {
-				// Found non repeating value, no more repeats found
-				break;
-			}
-		}
-
-		// Loop forwards and look for the next unique value
-		for (int64_t i = index + 1; i < GetLength(); ++i) {
-			if (GetLong(i) == current_value) {
-				// Found same value
-				next_repeats++;
-			} else {
-				// Found non repeating value, no more repeats found
-				break;
-			}
-		}
-
-		int64_t total_repeats = previous_repeats + next_repeats;
-		return Fraction(previous_repeats, total_repeats);
-	}
-	else
-		// return a blank coordinate
+	// Frame numbers (index) outside of the "defined" range of this
+	// keyframe result in a 1/1 default value.
+	if (index < 1 || (index + 1) >= GetLength()) {
 		return Fraction(1,1);
+	}
+	assert(Points.size() > 1); // Due to ! ((index + 1) >= GetLength) there are at least two points!
+
+	// First, get the value at the given frame and the closest point
+	// to the right.
+	int64_t const current_value = GetLong(index);
+	std::vector<Point>::const_iterator const candidate =
+		std::lower_bound(begin(Points), end(Points), static_cast<double>(index), IsPointBeforeX);
+	assert(candidate != end(Points)); // Due to the (index + 1) >= GetLength check above!
+
+	// Calculate how many of the next values are going to be the same:
+	int64_t next_repeats = 0;
+	std::vector<Point>::const_iterator i = candidate;
+	// If the index (frame number) is the X coordinate of the closest
+	// point, then look at the segment to the right; the "current"
+	// segement is not interesting because we're already at the last
+	// value of it.
+	if (i->co.X == index) {
+		++i;
+	}
+	// Skip over "constant" (when rounded) segments.
+	bool all_constant = true;
+	for (; i != end(Points); ++i) {
+		if (current_value != round(i->co.Y)) {
+			all_constant = false;
+			break;
+		}
+	}
+	if (! all_constant) {
+		// Found a point which defines a segment which will give a
+		// different value than the current value.  This means we
+		// moved at least one segment to the right, thus we cannot be
+		// at the first point.
+		assert(i != begin(Points));
+		Point const left = *(i - 1);
+		Point const right = *i;
+		// Binary search for the first value which is different from
+		// the current value.
+		bool const increasing = current_value < round(i->co.Y);
+		int64_t start = left.co.X;
+		int64_t stop = right.co.X;
+		while (start < stop) {
+			int64_t const mid = (start + stop + 1) / 2;
+			double const value = InterpolateBetween(left, right, mid, 0.01);
+			bool const smaller = round(value) <= current_value;
+			bool const larger = round(value) >= current_value;
+			if ((increasing && smaller) || (!increasing && larger)) {
+				start = mid;
+			} else {
+				stop = mid - 1;
+			}
+		}
+		next_repeats = start - index;
+	} else {
+		// All values to the right are the same!
+		next_repeats = Points.back().co.X - index;
+	}
+
+	// Now look to the left, to the previous values.
+	all_constant = true;
+	i = candidate;
+	if (i != begin(Points)) {
+		// The binary search below assumes i to be the left point;
+		// candidate is the right point of the current segment
+		// though. So change this if possible. If this branch is NOT
+		// taken, then we're at/before the first point and all is
+		// constant!
+		--i;
+	}
+	int64_t previous_repeats = 0;
+	// Skip over constant (when rounded) segments!
+	for (; i != begin(Points); --i) {
+		if (current_value != round(i->co.Y)) {
+			all_constant = false;
+			break;
+		}
+	}
+	// Special case when skipped until the first point, but the first
+	// point is actually different.  Will not happen if index is
+	// before the first point!
+	if (current_value != round(i->co.Y)) {
+		assert(i != candidate);
+		all_constant = false;
+	}
+	if (! all_constant) {
+		// There are at least two points, and we're not at the end,
+		// thus the following is safe!
+		Point const left = *i;
+		Point const right = *(i + 1);
+		// Binary search for the last value (seen from the left to
+		// right) to be different than the current value.
+		bool const increasing = current_value > round(left.co.Y);
+		int64_t start = left.co.X;
+		int64_t stop = right.co.X;
+		while (start < stop) {
+			int64_t const mid = (start + stop + 1) / 2;
+			double const value = InterpolateBetween(left, right, mid, 0.01);
+			bool const smaller = round(value) < current_value;
+			bool const larger = round(value) > current_value;
+			if ((increasing && smaller) || (!increasing && larger)) {
+				start = mid;
+			} else {
+				stop = mid - 1;
+			}
+		}
+		previous_repeats = index - start;
+	} else {
+		// Every previous value is the same (rounded) as the current
+		// value.
+		previous_repeats = index;
+	}
+	int64_t total_repeats = previous_repeats + next_repeats;
+	return Fraction(previous_repeats, total_repeats);
 }
 
 // Get the change in Y value (from the previous Y value)