Use a direction value instead of dx, dy pair for movement

mato8.asm

@@ -11,11 +11,15 @@ define down_key 8
 define left_key 4
 define right_key 6
 
-define game_over_reg v8
-define fruit_x_reg v9
-define fruit_y_reg va
-define head_dx_reg vb
-define head_dy_reg vc
+define direction_up 0
+define direction_down 6
+define direction_left 12
+define direction_right 18
+
+define game_over_reg v9
+define fruit_x_reg va
+define fruit_y_reg vb
+define head_direction_reg vc
 define head_x_reg vd
 define head_y_reg ve
 
@@ -26,8 +30,7 @@ start:
 	rnd head_x_reg, 63
 	rnd head_y_reg, 31
 
-	ld head_dx_reg, 1
-	ld head_dy_reg, 0
+	ld head_direction_reg, direction_right
 
 	ld i, single_pixel
 	drw head_x_reg, head_y_reg, 1
@@ -40,19 +43,19 @@ mainloop:
 
 	ld v0, up_key
 	sknp v0
-	call dir_up
+	call turn_up
 
 	ld v0, down_key
 	sknp v0
-	call dir_down
+	call turn_down
 
 	ld v0, left_key
 	sknp v0
-	call dir_left
+	call turn_left
 
 	ld v0, right_key
 	sknp v0
-	call dir_right
+	call turn_right
 
 	call move_snake
 
@@ -106,54 +109,72 @@ spawn_fruit:
 
 	ret
 
-dir_up:
-	; Allow only 90° turns
-	se head_dy_reg, 0
-	ret
-	ld head_dx_reg, 0
-	ld head_dy_reg, #ff
+turn_up:
+	; Don't allow 180° turns (which would kill the snake instantly)
+	se head_direction_reg, direction_down
+	ld head_direction_reg, direction_up
 	ret
 
-dir_down:
-	se head_dy_reg, 0
-	ret
-	ld head_dx_reg, 0
-	ld head_dy_reg, 1
+turn_down:
+	se head_direction_reg, direction_up
+	ld head_direction_reg, direction_down
 	ret
 
-dir_left:
-	se head_dx_reg, 0
-	ret
-	ld head_dx_reg, #ff
-	ld head_dy_reg, 0
+turn_left:
+	se head_direction_reg, direction_right
+	ld head_direction_reg, direction_left
 	ret
 
-dir_right:
-	se head_dx_reg, 0
-	ret
-	ld head_dx_reg, 1
-	ld head_dy_reg, 0
+turn_right:
+	se head_direction_reg, direction_left
+	ld head_direction_reg, direction_right
 	ret
 
 move_snake:
-	ld v0, head_x_reg
-	add head_x_reg, head_dx_reg
-	ld v2, 63
-	and head_x_reg, v2
+	ld v0, head_direction_reg
+	call unpack_direction
 
-	ld v1, head_y_reg
-	add head_y_reg, head_dy_reg
-	ld v2, 31
-	and head_y_reg, v2
+	ld v2, head_x_reg
+	add head_x_reg, v0
+	ld v0, 63
+	and head_x_reg, v0
+
+	ld v3, head_y_reg
+	add head_y_reg, v1
+	ld v0, 31
+	and head_y_reg, v0
 
 	ld i, single_pixel
 	drw head_x_reg, head_y_reg, 1
 	se vf, 0
 	call collision
-	drw v0, v1, 1
+	drw v2, v3, 1
 
 	ret
 
+unpack_direction:
+	jp v0, unpack_direction_table
+	unpack_direction_table:
+		; Up
+		ld v0, 0
+		ld v1, #ff
+		ret
+
+		; Down
+		ld v0, 0
+		ld v1, 1
+		ret
+
+		; Left
+		ld v0, #ff
+		ld v1, 0
+		ret
+
+		; Right
+		ld v0, 1
+		ld v1, 0
+		ret
+
 collision:
 	se head_x_reg, fruit_x_reg
 	jp tail_collision