Add more Sum() tests, and further correct bugs found from it

The implementation should be bug-free now. Of course, I thought
that previously, then found more, so we shall see. There are
more tests, though, and they definitely pass, so that's a good
start.

2 files changed, 37 insertions, 18 deletions

diff --git a/integralimg.go b/integralimg.go
index 922b1ce..a197920 100644
--- a/integralimg.go
+++ b/integralimg.go
@@ -140,38 +140,43 @@ func highest(a, b int) int {
 }
 
 func (i Image) topLeft(r image.Rectangle) uint64 {
-	x := highest(r.Min.X, 0)
-	y := highest(r.Min.Y, 0)
-	if x == 0 || y == 0 {
+	b := i.Bounds()
+	x := r.Min.X - 1
+	y := r.Min.Y - 1
+	x = lowest(x, b.Max.X - 1)
+	y = lowest(y, b.Max.Y - 1)
+	if x < 0 || y < 0 {
 		return 0
 	}
 	return i[y][x]
 }
 
 func (i Image) topRight(r image.Rectangle) uint64 {
-	x := lowest(r.Max.X - 1, i.Bounds().Dx() - 1)
-	y := highest(r.Min.Y, 0)
-	if x == 0 || y == 0 {
+	b := i.Bounds()
+	x := lowest(r.Max.X - 1, b.Max.X - 1)
+	y := r.Min.Y - 1
+	y = lowest(y, b.Max.Y - 1)
+	if x < 0 || y < 0 {
 		return 0
 	}
 	return i[y][x]
 }
 
 func (i Image) bottomLeft(r image.Rectangle) uint64 {
-	x := highest(r.Min.X, 0)
-	y := lowest(r.Max.Y - 1, i.Bounds().Dy() - 1)
-	if x == 0 || y == 0 {
+	b := i.Bounds()
+	x := r.Min.X - 1
+	x = lowest(x, b.Max.X - 1)
+	y := lowest(r.Max.Y - 1, b.Max.Y - 1)
+	if x < 0 || y < 0 {
 		return 0
 	}
 	return i[y][x]
 }
 
 func (i Image) bottomRight(r image.Rectangle) uint64 {
-	x := lowest(r.Max.X - 1, i.Bounds().Dx() - 1)
-	y := lowest(r.Max.Y - 1, i.Bounds().Dy() - 1)
-	if x == 0 || y == 0 {
-		return 0
-	}
+	b := i.Bounds()
+	x := lowest(r.Max.X - 1, b.Max.X - 1)
+	y := lowest(r.Max.Y - 1, b.Max.Y - 1)
 	return i[y][x]
 }
 
diff --git a/integralimg_test.go b/integralimg_test.go
index bd8a983..35b353d 100644
--- a/integralimg_test.go
+++ b/integralimg_test.go
@@ -70,11 +70,25 @@ func TestSum(t *testing.T) {
 	draw.Draw(imgplus, b, img, b.Min, draw.Src)
 	draw.Draw(integral, b, img, b.Min, draw.Src)
 
-	sumimg := imgplus.sum(b)
-	sumint := integral.Sum(b)
+	cases := []struct {
+		name string
+		r    image.Rectangle
+	}{
+		{"fullimage", b},
+		{"small", image.Rect(1, 1, 5, 5)},
+		{"toobig", image.Rect(0, 0, 2000, b.Dy())},
+		{"toosmall", image.Rect(-1, -1, 4, 5)},
+		{"small2", image.Rect(0, 0, 4, 4)},
+	}
 
-	if sumimg != sumint {
-		t.Errorf("Sum of integral image differs to regular image: regular: %d, integral: %d\n", sumimg, sumint)
+	for _, c := range cases{
+		t.Run(c.name, func(t *testing.T) {
+			sumimg := imgplus.sum(c.r)
+			sumint := integral.Sum(c.r)
+			if sumimg != sumint {
+				t.Errorf("Sum of integral image differs to regular image: regular: %d, integral: %d\n", sumimg, sumint)
+			}
+		})
 	}
 }