Merge 14b7ca8 into 7c7cb33

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## Unreleased
+
+- Eliminate redundant calls to the optional user supplied transform func during
+  area calculations.
+
 ## v0.35.0
 
 2021-11-23

geom/attr_test.go

@@ -642,6 +642,18 @@ func TestTransformedArea(t *testing.T) {
 	}
 }
 
+func TestTransformedAreaInvocationCount(t *testing.T) {
+	g := geomFromWKT(t, "POLYGON((0 0,0 1,1 0,0 0))")
+	var count int
+	g.Area(WithTransform(func(xy XY) XY {
+		count++
+		return xy
+	}))
+
+	// Each of the 4 points making up the polygon get transformed once each.
+	expectIntEq(t, count, 4)
+}
+
 func TestCentroid(t *testing.T) {
 	for i, tt := range []struct {
 		input  string

geom/type_polygon.go

@@ -398,13 +398,22 @@ func signedAreaOfLinearRing(lr LineString, transform func(XY) XY) float64 {
 	var sum float64
 	seq := lr.Coordinates()
 	n := seq.Length()
-	for i := 0; i < n; i++ {
-		pt0 := seq.GetXY(i)
-		pt1 := seq.GetXY((i + 1) % n)
+	if n == 0 {
+		return 0
+	}
+
+	nthPt := func(i int) XY {
+		pt := seq.GetXY(i)
 		if transform != nil {
-			pt0 = transform(pt0)
-			pt1 = transform(pt1)
+			pt = transform(pt)
 		}
+		return pt
+	}
+
+	pt1 := nthPt(0)
+	for i := 0; i < n-1; i++ {
+		pt0 := pt1
+		pt1 = nthPt(i + 1)
 		sum += (pt1.X + pt0.X) * (pt1.Y - pt0.Y)
 	}
 	return sum / 2