Add MultiLerp function

image/path/path.go

@@ -14,7 +14,8 @@ func P (data ...int) []image.Point {
 }
 
 // Lerp linearally interpolates between two paths. If the paths differ in
-// length, the returned path will only be as long as the smaller one.
+// length, the returned path will only be as long as the smaller one. This
+// function will always return a new slice.
 func Lerp (fac float64, start, end []image.Point) []image.Point {
 	result := make([]image.Point, len(start))
 	for index, startPt := range start {
@@ -27,6 +28,31 @@ func Lerp (fac float64, start, end []image.Point) []image.Point {
 	return result
 }
 
+// MultiLerp is like Lerp, but interpolates through several "stages". This
+// function will always return a new slice. If no stages are provided, it will
+// return nil.
+func MultiLerp (fac float64, stages ...[]image.Point) []image.Point {
+	if len(stages) == 0 { return nil }
+	if len(stages) == 1 {
+		result := make([]image.Point, len(stages[0]))
+		copy(result, stages[0])
+		return result
+	}
+	
+	expandedFac := fac * float64(len(stages))
+	
+	startIndex  := int(expandedFac)
+	if startIndex >= len(stages) { startIndex = len(stages) - 1 }
+	endIndex    := startIndex + 1
+	if endIndex >= len(stages) { endIndex = len(stages) - 1 }
+	
+	start       := stages[startIndex]
+	end         := stages[endIndex]
+	partialFac  := expandedFac - float64(startIndex)
+	println(startIndex, endIndex, int(partialFac * 100))
+	return Lerp(partialFac, start, end)
+}
+
 // Distance returns the distance between two points.
 func Distance (start, end image.Point) float64 {
 	delta := start.Sub(end)