Add AtPosition to TypeSetter

typesetter.go

@@ -89,6 +89,34 @@ type TypeSetter struct {
 	layoutBoundsSpace fixed.Rectangle26_6
 }
 
+// AtPosition returns the index of the rune at the specified position.
+func (this *TypeSetter) AtPosition (position fixed.Point26_6) int {
+	metrics := this.face.Metrics()
+	lastValidIndex := 0
+	index          := 0
+	for _, tok := range this.tokens {
+		pos := tok.position
+		yValid :=
+			position.Y >= pos.Y - metrics.Ascent &&
+			position.Y <= pos.Y + metrics.Descent
+		if !yValid { index += len(tok.runes); continue }
+		
+		for _, runl := range tok.runes {
+			x := pos.X + runl.x
+			xValid := position.X >= pos.X + runl.x
+			if xValid {
+				lastValidIndex = index
+			} else if x > position.X {
+				return lastValidIndex
+			}
+
+			index ++
+		}
+	}
+	index ++
+	return lastValidIndex
+}
+
 // Runes returns an iterator for all runes in the TypeSetter, and their positions.
 func (this *TypeSetter) Runes () RuneIter {
 	this.needFlow()