Store snake's movement to allow longer snake

mato8.asm

@@ -16,9 +16,11 @@ define direction_down 6
 define direction_left 12
 define direction_right 18
 
-define game_over_reg v9
+define game_over_reg v7
-define fruit_x_reg va
+define fruit_x_reg v8
-define fruit_y_reg vb
+define fruit_y_reg v9
+define tail_x_reg va
+define tail_y_reg vb
 define head_direction_reg vc
 define head_x_reg vd
 define head_y_reg ve
@@ -30,6 +32,9 @@ start:
 	rnd head_x_reg, 63
 	rnd head_y_reg, 31
 
+	ld tail_x_reg, head_x_reg
+	ld tail_y_reg, head_y_reg
+
 	ld head_direction_reg, direction_right
 
 	ld i, single_pixel
@@ -131,25 +136,95 @@ turn_right:
 	ret
 
 move_snake:
+	; Save the direction the snake is moving at head's location
+	ld v0, head_x_reg
+	ld v1, head_y_reg
+	call direction_table_index
 	ld v0, head_direction_reg
-	call unpack_direction
+	ld [i], v0
 
-	ld v2, head_x_reg
+	; Move the snake's head
+	call unpack_direction
 	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
 
+	; Draw head location and erase tail location
 	ld i, single_pixel
 	drw head_x_reg, head_y_reg, 1
 	se vf, 0
 	call collision
-	drw v2, v3, 1
+	drw tail_x_reg, tail_y_reg, 1
 
+	; Load the direction the snake was moving at tail's location
+	ld v0, tail_x_reg
+	ld v1, tail_y_reg
+	call direction_table_index
+	ld v0, [i]
+
+	; Move the snake's tail
+	call unpack_direction
+	add tail_x_reg, v0
+	ld v0, 63
+	and tail_x_reg, v0
+	add tail_y_reg, v1
+	ld v0, 31
+	and tail_y_reg, v0
+
+	ret
+
+direction_table_index:
+	; v0 is the X coördinate, from 0 to 63, 00xxxxxx
+	; v1 is the Y coördinate, from 0 to 31, 000yyyyy
+	shl v0, v0
+	shl v0, v0
+	; v0 is the X coördinate shifted left by two, xxxxxx00
+	ld v2, %11100000
+	and v2, v0
+	or v1, v2
+	; v1 is combination of high 3 bits of X coördinate and the Y coördinate, xxxyyyyy
+	ld v2, %00011100
+	and v0, v2
+	; v0 is the low 3 bits of the X coördinate shifted left by two, 000xxx00
+
+	jp v0, direction_table_index_table
+	direction_table_index_table:
+		; 0
+		ld i, #400
+		jp direction_table_index_end
+
+		; 1
+		ld i, #500
+		jp direction_table_index_end
+
+		; 2
+		ld i, #600
+		jp direction_table_index_end
+
+		; 3
+		ld i, #700
+		jp direction_table_index_end
+
+		; 4
+		ld i, #800
+		jp direction_table_index_end
+
+		; 5
+		ld i, #900
+		jp direction_table_index_end
+
+		; 6
+		ld i, #a00
+		jp direction_table_index_end
+
+		; 7
+		ld i, #b00
+
+direction_table_index_end:
+	add i, v1
 	ret
 
 unpack_direction: