floats: Implement ScaleTo (#765)

Fixes #737

floats/floats.go

@@ -809,6 +809,17 @@ func Scale(c float64, dst []float64) {
 	}
 }
 
+// ScaleTo multiplies the elements in s by c and stores the result in dst.
+func ScaleTo(dst []float64, c float64, s []float64) []float64 {
+	if len(dst) != len(s) {
+		panic("floats: lengths of slices do not match")
+	}
+	if len(dst) > 0 {
+		f64.ScalUnitaryTo(dst, c, s)
+	}
+	return dst
+}
+
 // Span returns a set of N equally spaced points between l and u, where N
 // is equal to the length of the destination. The first element of the destination
 // is l, the final element of the destination is u.

floats/floats_test.go

@@ -1341,6 +1341,22 @@ func TestScale(t *testing.T) {
 	areSlicesEqual(t, truth, s, "Bad scaling")
 }
 
+func TestScaleTo(t *testing.T) {
+	s := []float64{3, 4, 1, 7, 5}
+	sCopy := make([]float64, len(s))
+	copy(sCopy, s)
+	c := 5.0
+	truth := []float64{15, 20, 5, 35, 25}
+	dst := make([]float64, len(s))
+	ScaleTo(dst, c, s)
+	if !Same(dst, truth) {
+		t.Errorf("Scale to does not match. Got %v, want %v", dst, truth)
+	}
+	if !Same(s, sCopy) {
+		t.Errorf("Source modified during call. Got %v, want %v", s, sCopy)
+	}
+}
+
 func TestSpan(t *testing.T) {
 	receiver1 := make([]float64, 5)
 	truth := []float64{1, 2, 3, 4, 5}