Atic Atac: Routine at 9C2F

The edge list is a run of vertex numbers. The first begins a group, each one after it draws a line from that vertex to this one, and $FF ends the group; a second $FF ends the list. Writing it as groups rather than as pairs means a corner shared by three lines is only named once.

The vertex number cannot be known in advance, so as with MARK_ROOM_VISITED the routine writes the instruction that will use it: the displacement in each LD r,(IX+$00) below is overwritten just before it runs.

For room shape $00 the vertices are two nested squares -- (4,187) (4,4) (187,4) (187,187) and (31,160) (31,31) (160,31) (160,160) -- and the groups are 0 to 1,3,4; 2 to 1,3,6; 5 to 1,4,6; 7 to 3,4,6. That is the four outer walls, the four inner ones and the four corner diagonals, each drawn exactly once, which is the "looking into a box" outline every room is built from.

The edge list is a run of vertex numbers. The first begins a group, each one after it draws a line from that vertex to this one, and $FF ends the group; a second $FF ends the list. Writing it as groups rather than as pairs means a corner shared by three lines is only named once. The vertex number cannot be known in advance, so as with MARK_ROOM_VISITED the routine writes the instruction that will use it: the displacement in each LD r,(IX+$00) below is overwritten just before it runs. For room shape $00 the vertices are two nested squares -- (4,187) (4,4) (187,4) (187,187) and (31,160) (31,31) (160,31) (160,160) -- and the groups are 0 to 1,3,4; 2 to 1,3,6; 5 to 1,4,6; 7 to 3,4,6. That is the four outer walls, the four inner ones and the four corner diagonals, each drawn exactly once, which is the "looking into a box" outline every room is built from.
DRAW_OUTLINE	9C2F	LD A,(HL)	$FF ends the whole list.
	9C30	INC HL
	9C31	CP $FF
	9C33	RET Z
	9C34	SLA A	Double the vertex number and patch it into both halves of the 16-bit fetch below.
	9C36	LD ($9C3F),A
	9C39	INC A
	9C3A	LD ($9C42),A
FROM_VERTEX_X	9C3D	LD C,(IX+$00)	Reads as (IX+$00) but the displacements were just rewritten.
FROM_VERTEX_Y	9C40	LD B,(IX+$00)
DRAW_OUTLINE_0	9C43	PUSH BC
	9C44	LD A,(HL)
	9C45	INC HL
	9C46	CP $FF
	9C48	JR Z,OUTLINE_DONE
	9C4A	SLA A
	9C4C	LD ($9C55),A	TO_VERTEX_X+2: the displacement in the fetch below
	9C4F	INC A
	9C50	LD ($9C58),A	TO_VERTEX_Y+2: likewise
TO_VERTEX_X	9C53	LD E,(IX+$00)
TO_VERTEX_Y	9C56	LD D,(IX+$00)
	9C59	PUSH HL
	9C5A	CALL DRAW_LINE	Draw one line, corner to corner.
	9C5D	POP HL
	9C5E	POP BC
	9C5F	JR DRAW_OUTLINE_0