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 e1cd4dc1e3e0980cf7ca152ac13caba83095399a..ed7dde2c84d05cdd00f85fb99d01547aa73af258 100644 (file)
--- 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 6e9c6a13c2cac87b4fdb86597777f163e2a7c1dc..9fede027218c31e54a478cd6a40bd4e34fac577b 100644 (file)
--- 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--