lerpAngle function

double lerpAngle(double start, double end, double fraction, { double range = 1, bool shortestPath = true, })

Imagine a circle with a range of 0, range. If shortestPath is true, the shortest path between start and end will be used. If shortestPath is false, the longest path between start and end will be used.

In case of 0...1 range, and start = 0 and end = 0.25, if shortestPath is true, the result will be 0.25. If shortestPath is false, the result will be 0.375.

Implementation

double lerpAngle(double start, double end, double fraction, {double range = 1, bool shortestPath = true}) {
  final dA = (end - start).abs(); // always inside the range (0.1 .. 0.7, 0.7 .. 0.1 will return the same value)
  final dB = range - dA; // always outside the range
  final shortestCrossesRange = shortestPath && dB < dA && dB != 0;
  final longerCrossesRange = !shortestPath && dB > dA && start != end;
  if (shortestCrossesRange || longerCrossesRange) {
    // crossing the range is shorter
    if (end > start) {
      // eg. start 0.1 .. end 0.7
      return lerp(start + range, end, fraction) % range; // 1.1, 0.7, 0.6 -> 1.1 + (0.7 - 1.1) * 0.8 = 1.1 - 0.32 = 0.78 % 1 + 0.78
    } else {
      // eg. start 0.7 .. end 0.1
      return lerp(start, end + range, fraction) % range; // 0.7, 1.1, 0.8 -> 0.7 + (1.1 - 0.7) * 0.8 = 0.7 + 0.32 = 1.02 % 1 = 0.02
    }
  }
  return lerp(start, end, fraction) % range;
}