Created a Tiled pattern

artist/checkered.go

@@ -26,8 +26,26 @@ func (pattern Checkered) AtWhen (x, y, width, height int) (c color.RGBA) {
 	y %= pattern.CellHeight
 
 	if n % 2 == 0 {
-		return pattern.First.AtWhen(x, y, pattern.CellWidth, pattern.CellHeight)
+		return pattern.First.AtWhen (
+			x, y, pattern.CellWidth, pattern.CellHeight)
 	} else {
-		return pattern.Second.AtWhen(x, y, pattern.CellWidth, pattern.CellHeight)
+		return pattern.Second.AtWhen (
+			x, y, pattern.CellWidth, pattern.CellHeight)
 	}
 }
+
+// Tiled is a pattern that tiles another pattern accross a grid.
+type Tiled struct {
+	Pattern
+	CellWidth, CellHeight int
+}
+
+// AtWhen satisfies the Pattern interface.
+func (pattern Tiled) AtWhen (x, y, width, height int) (c color.RGBA) {
+	x %= pattern.CellWidth
+	y %= pattern.CellHeight
+	if x < 0 { x += pattern.CellWidth  }
+	if y < 0 { y += pattern.CellHeight }
+	return pattern.Pattern.AtWhen (
+		x, y, pattern.CellWidth, pattern.CellHeight)
+}