fix operation order for rectangle resize function, add tests

geometry.go

@@ -236,8 +236,8 @@ func (r Rect) Resized(anchor, size Vec) Rect {
 	}
 	fraction := Vec{size.X / r.W(), size.Y / r.H()}
 	return Rect{
-		Min: anchor.Add(r.Min.Sub(anchor)).ScaledXY(fraction),
+		Min: anchor.Add(r.Min.Sub(anchor).ScaledXY(fraction)),
-		Max: anchor.Add(r.Max.Sub(anchor)).ScaledXY(fraction),
+		Max: anchor.Add(r.Max.Sub(anchor).ScaledXY(fraction)),
 	}
 }
 

geometry_test.go

@@ -0,0 +1,31 @@
+package pixel_test
+
+import (
+	"testing"
+
+	"github.com/faiface/pixel"
+)
+
+func TestResizeRect(t *testing.T) {
+	testCases := []pixel.Rect{
+		pixel.R(-10, -10, 10, 10),
+		pixel.R(10, 10, 30, 30),
+	}
+
+	answers := []pixel.Rect{
+		pixel.R(-5, -5, 5, 5),
+		pixel.R(15, 15, 25, 25),
+	}
+
+	for i, rect := range testCases {
+		answer := answers[i]
+
+		// resize rectangle by 50% anchored at it's current center point
+		resizedRect := rect.Resized(rect.Center(), rect.Size().Scaled(0.5))
+
+		if resizedRect != answer {
+			t.Errorf("Rectangle resize was incorrect, got %v, want: %v.", resizedRect, answer)
+		}
+	}
+
+}