From 07561f4824a7a4af3cf9e99fd8e23e9cdd6aae42 Mon Sep 17 00:00:00 2001
From: nmlgc <nmlgc@nmlgc.net>
Date: Mon, 7 Feb 2022 17:21:35 +0100
Subject: [PATCH] [Reverse-engineering] [th03] Collision bitmap: Drawing
 vertical lines
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Ironically only used for pellets, which are now confirmed to have a 2×2
(screen) / 2×1 (VRAM) hitbox. Might have been "decompilable", but it's
sandwiched between undecompilable functions.

Part of P0182, funded by Lmocinemod and [Anonymous].
---
 th03/main/collmap.asm | 60 +++++++++++++++++++++++++++++++++++++++++++
 th03/main/collmap.hpp |  7 +++++
 th03_main.asm         | 55 ++-------------------------------------
 3 files changed, 69 insertions(+), 53 deletions(-)

diff --git a/th03/main/collmap.asm b/th03/main/collmap.asm
index a4414d38..94e1abaa 100644
--- a/th03/main/collmap.asm
+++ b/th03/main/collmap.asm
@@ -10,6 +10,7 @@ include th03/main/collmap.inc
 
 extrn _collmap_topleft:Point
 extrn _collmap_stripe_tile_w:word
+extrn _collmap_tile_h:word
 extrn _collmap_bottomright:Point
 extrn _collmap_pid:byte
 extrn _collmap:byte:(PLAYER_COUNT * COLLMAP_SIZE)
@@ -31,6 +32,65 @@ MAIN_04 group COLLMAP_TEXT
 	.code
 	assume cs:MAIN_04
 
+public _collmap_set_vline
+_collmap_set_vline proc far
+	@@pattern         	equ <ah>
+	@@first_bit       	equ <cl>
+	@@first_bit_wide  	equ <cx>	; Just remove this
+	@@rows_from_bottom	equ <dx>	; Ensures clipping at the bottom edge
+	@@collmap_p       	equ <bx>
+
+	mov	ax, _collmap_topleft.x
+	mov	dx, _collmap_topleft.y
+
+	; Clip
+	or	ax, ax
+	js	short @@ret
+	cmp	ax, (PLAYFIELD_W shl 4)
+	jge	short @@ret
+	or	dx, dx
+	js	short @@ret
+	cmp	dx, (PLAYFIELD_H shl 4)
+	jge	short @@ret
+
+	xor	@@collmap_p, @@collmap_p
+	cmp	_collmap_pid, 0
+	jz	short @@set_collmap_p
+	mov	@@collmap_p, COLLMAP_SIZE
+
+@@set_collmap_p:
+	add	@@collmap_p, offset _collmap
+
+	sar	ax, (SUBPIXEL_BITS + COLLMAP_TILE_W_BITS)
+	sar	dx, (SUBPIXEL_BITS + COLLMAP_TILE_H_BITS)
+
+	mov     	@@first_bit_wide, ax     	; first_bit = AX;
+	sar     	ax, 3                    	; AX /= 8; (bits per byte)
+	and     	@@first_bit_wide, (8 - 1)	; first_bit &= (8 - 1);
+	add     	@@collmap_p, dx          	; collmap_p += (
+	offset_x	ax, al                   	; 	DX + (AX * COLLMAP_H)
+	add     	@@collmap_p, ax          	; );
+
+	mov	ax, COLLMAP_H
+	sub	ax, dx
+	mov	@@rows_from_bottom, ax
+	mov	@@pattern, 80h
+	shr	@@pattern, @@first_bit
+	mov	cx, _collmap_tile_h
+	even
+
+@@row_loop:
+	or	[@@collmap_p], @@pattern
+	inc	@@collmap_p
+	dec	@@rows_from_bottom
+	loopne	@@row_loop
+
+@@ret:
+	ret
+_collmap_set_vline endp
+	even
+
+
 public _collmap_set_slope_striped
 _collmap_set_slope_striped proc c far
 	uses si, di
diff --git a/th03/main/collmap.hpp b/th03/main/collmap.hpp
index 9e2c1586..1b5a6252 100644
--- a/th03/main/collmap.hpp
+++ b/th03/main/collmap.hpp
@@ -41,6 +41,13 @@ extern unsigned char collmap_pid;
 extern uint8_t collmap[PLAYER_COUNT][COLLMAP_MEMORY_W][COLLMAP_H];
 // -------------------
 
+// Sets all bits on a straight 1-tile wide vertical line, clipped to the
+// height of the collision bitmap. Takes the following parameters:
+// • [collmap_topleft] (X and Y)
+// • [collmap_tile_h] (height of the line)
+// • [collmap_pid]
+void collmap_set_vline();
+
 // Sets all bits on every [COLLMAP_SLOPE_VSTRIPE_DISTANCE]th row of a
 // potentially sloped vertical line across one whole playfield.
 // Takes the following parameters:
diff --git a/th03_main.asm b/th03_main.asm
index afe01765..e490439b 100644
--- a/th03_main.asm
+++ b/th03_main.asm
@@ -17080,61 +17080,10 @@ loc_13B2C:
 locret_13B41:
 		retf
 sub_13A6E	endp
-
-
-; =============== S U B	R O U T	I N E =======================================
-
-
-sub_13B42	proc far
-		mov	ax, _collmap_topleft.x
-		mov	dx, _collmap_topleft.y
-		or	ax, ax
-		js	short locret_13B98
-		cmp	ax, (PLAYFIELD_W shl 4)
-		jge	short locret_13B98
-		or	dx, dx
-		js	short locret_13B98
-		cmp	dx, (PLAYFIELD_H shl 4)
-		jge	short locret_13B98
-		xor	bx, bx
-		cmp	_collmap_pid, 0
-		jz	short loc_13B68
-		mov	bx, COLLMAP_SIZE
-
-loc_13B68:
-		add	bx, offset _collmap
-		sar	ax, 5
-		sar	dx, 5
-		mov	cx, ax
-		sar	ax, 3
-		and	cx, 7
-		add	bx, dx
-		mov	ah, COLLMAP_H
-		mul	ah
-		add	bx, ax
-		mov	ax, COLLMAP_H
-		sub	ax, dx
-		mov	dx, ax
-		mov	ah, 80h
-		shr	ah, cl
-		mov	cx, _collmap_tile_h
-		nop
-
-loc_13B92:
-		or	[bx], ah
-		inc	bx
-		dec	dx
-		loopne	loc_13B92
-
-locret_13B98:
-		retf
-sub_13B42	endp
-
-; ---------------------------------------------------------------------------
-		nop
 main_04_TEXT	ends
 
 COLLMAP_TEXT	segment byte public 'CODE' use16
+	extern _collmap_set_vline:proc
 	extern _collmap_set_slope_striped:proc
 COLLMAP_TEXT	ends
 
@@ -24959,7 +24908,7 @@ loc_17BA9:
 		mov	_collmap_topleft.y, ax
 		mov	al, [si+10h]
 		mov	_collmap_pid, al
-		call	sub_13B42
+		call	_collmap_set_vline
 
 loc_17BCA:
 		jmp	loc_17A3D