Added function to calculate a next pointer step on the field

pkg/field/step.go

@@ -0,0 +1,20 @@
+package field
+
+func (f Field) Step(x, y, dx, dy int) (int, int) {
+	x2, y2 := x+dx, y+dy
+	if f.IsIn(x2, y2) {
+		return x2, y2
+	}
+	// We are stepping outside, we need to wrap the Lahey-space
+	for {
+		x2, y2 := x-dx, y-dy
+		if !f.IsIn(x2, y2) {
+			return x, y
+		}
+		x, y = x2, y2
+	}
+}
+
+func (f Field) IsIn(x, y int) bool {
+	return x >= f.x && x < f.x+f.lx && y >= f.y && y < f.y+f.ly
+}