From: Nigel Tao <nigeltao@golang.org>
Date: Thu, 5 Jan 2017 06:37:54 +0000 (+1100)
Subject: image: fix the overlap check in Rectangle.Intersect.
X-Git-Tag: go1.9beta1~1711
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=a855da29dbd7a80c4d87a421c1f88a8603c020fa;p=gostls13.git

image: fix the overlap check in Rectangle.Intersect.

The doc comment for Rectangle.Intersect clearly states, "If the two
rectangles do not overlap then the zero rectangle will be returned."
Prior to this fix, calling Intersect on adjacent but non-overlapping
rectangles would return an empty but non-zero rectangle.

The fix essentially changes
if r.Min.X > r.Max.X || r.Min.Y > r.Max.Y { etc }
to
if r.Min.X >= r.Max.X || r.Min.Y >= r.Max.Y { etc }
(note that the > signs have become >= signs), but changing that line to:
if r.Empty() { etc }
seems clearer (and equivalent).

Change-Id: Ia654e4b9dc805978db3e94d7a9718b6366005360
Reviewed-on: https://go-review.googlesource.com/34853
Reviewed-by: David Crawshaw <crawshaw@golang.org>
---

diff --git a/src/image/geom.go b/src/image/geom.go
index e1cd4dc1e3..ed7dde2c84 100644
--- a/src/image/geom.go
+++ b/src/image/geom.go
@@ -161,7 +161,11 @@ func (r Rectangle) Intersect(s Rectangle) Rectangle {
 	if r.Max.Y > s.Max.Y {
 		r.Max.Y = s.Max.Y
 	}
-	if r.Min.X > r.Max.X || r.Min.Y > r.Max.Y {
+	// Letting r0 and s0 be the values of r and s at the time that the method
+	// is called, this next line is equivalent to:
+	//
+	// if max(r0.Min.X, s0.Min.X) >= min(r0.Max.X, s0.Max.X) || likewiseForY { etc }
+	if r.Empty() {
 		return ZR
 	}
 	return r
diff --git a/src/image/geom_test.go b/src/image/geom_test.go
index 6e9c6a13c2..9fede02721 100644
--- a/src/image/geom_test.go
+++ b/src/image/geom_test.go
@@ -28,6 +28,7 @@ func TestRectangle(t *testing.T) {
 
 	rects := []Rectangle{
 		Rect(0, 0, 10, 10),
+		Rect(10, 0, 20, 10),
 		Rect(1, 2, 3, 4),
 		Rect(4, 6, 10, 10),
 		Rect(2, 3, 12, 5),
@@ -62,9 +63,9 @@ func TestRectangle(t *testing.T) {
 			if err := in(a, s); err != nil {
 				t.Errorf("Intersect: r=%s, s=%s, a=%s, a not in s: %v", r, s, a, err)
 			}
-			if a.Empty() == r.Overlaps(s) {
-				t.Errorf("Intersect: r=%s, s=%s, a=%s: empty=%t same as overlaps=%t",
-					r, s, a, a.Empty(), r.Overlaps(s))
+			if isZero, overlaps := a == (Rectangle{}), r.Overlaps(s); isZero == overlaps {
+				t.Errorf("Intersect: r=%s, s=%s, a=%s: isZero=%t same as overlaps=%t",
+					r, s, a, isZero, overlaps)
 			}
 			largerThanA := [4]Rectangle{a, a, a, a}
 			largerThanA[0].Min.X--