From 5e4f1f0f1d3be03c31ae0e3bee739c9645879e20 Mon Sep 17 00:00:00 2001
From: nmlgc <nmlgc@nmlgc.net>
Date: Mon, 14 Mar 2022 16:45:05 +0100
Subject: [PATCH] [Reverse-engineering] [th04] Gengetsu: Wave teleport
 amplitude

Given how close the string literals of TH04's boss defeat sequence
function are to the end of its data segment, it makes sense to figure
out the three values after it right now.

Part of P0187, funded by [Anonymous] and Blue Bolt.
---
 Makefile.mak                         |  2 +-
 th04/boss_x2.cpp                     |  1 +
 th04/main/boss/{bx_1.asm => bx1.asm} |  0
 th04/main/boss/{bx_1.cpp => bx1.cpp} |  0
 th04/main/boss/bx2.cpp               |  7 ++++++
 th04_main.asm                        | 35 ++++++++++++++--------------
 6 files changed, 26 insertions(+), 19 deletions(-)
 create mode 100644 th04/boss_x2.cpp
 rename th04/main/boss/{bx_1.asm => bx1.asm} (100%)
 rename th04/main/boss/{bx_1.cpp => bx1.cpp} (100%)
 create mode 100644 th04/main/boss/bx2.cpp

diff --git a/Makefile.mak b/Makefile.mak
index 946f6ee3..c3fbe250 100644
--- a/Makefile.mak
+++ b/Makefile.mak
@@ -126,7 +126,7 @@ bin\th04\op.exe: bin\th04\op.obj th04\m_char.cpp bin\th01\vplanset.obj bin\th02\
 $**
 |
 
-bin\th04\main.exe: bin\th04\main.obj bin\th04\slowdown.obj th04\ems.cpp th04\playfld.cpp th04\f_dialog.cpp th04\dialog.cpp bin\th04\player_p.obj bin\th04\scoreupd.obj th04\hud_ovrl.cpp bin\th04\cfg_lres.obj bin\th01\vplanset.obj bin\th03\vector2.obj bin\th02\frmdely1.obj bin\th03\hfliplut.obj th04\mpn_free.cpp bin\th04\input_w.obj th04\mpn_l_i.cpp bin\th04\vector.obj bin\th04\snd_pmdr.obj bin\th04\snd_mmdr.obj bin\th04\snd_kaja.obj bin\th04\snd_mode.obj bin\th04\snd_load.obj bin\th04\cdg_put.obj bin\th04\exit.obj bin\th04\initmain.obj bin\th04\cdg_p_na.obj bin\th04\cdg_p_pr.obj bin\th04\input_s.obj bin\th04\snd_se_r.obj bin\th04\snd_se.obj bin\th04\cdg_load.obj th04\gather.cpp bin\th04\scrolly3.obj bin\th04\motion_3.obj th04\mb_dft.cpp bin\th04\it_spl_u.obj th04\bullet_u.cpp th04\bullet_a.cpp
+bin\th04\main.exe: bin\th04\main.obj bin\th04\slowdown.obj th04\ems.cpp th04\playfld.cpp th04\f_dialog.cpp th04\dialog.cpp bin\th04\player_p.obj bin\th04\scoreupd.obj th04\hud_ovrl.cpp bin\th04\cfg_lres.obj bin\th01\vplanset.obj bin\th03\vector2.obj bin\th02\frmdely1.obj bin\th03\hfliplut.obj th04\mpn_free.cpp bin\th04\input_w.obj th04\mpn_l_i.cpp bin\th04\vector.obj bin\th04\snd_pmdr.obj bin\th04\snd_mmdr.obj bin\th04\snd_kaja.obj bin\th04\snd_mode.obj bin\th04\snd_load.obj bin\th04\cdg_put.obj bin\th04\exit.obj bin\th04\initmain.obj bin\th04\cdg_p_na.obj bin\th04\cdg_p_pr.obj bin\th04\input_s.obj bin\th04\snd_se_r.obj bin\th04\snd_se.obj bin\th04\cdg_load.obj th04\gather.cpp bin\th04\scrolly3.obj bin\th04\motion_3.obj th04\mb_dft.cpp bin\th04\it_spl_u.obj th04\bullet_u.cpp th04\bullet_a.cpp th04\boss_x2.cpp
 	$(CC) $(CFLAGS) $(LARGE_LFLAGS) -DGAME=4 -DBINARY='M' -3 -Z -nbin\th04\ -eMAIN.EXE @&&|
 $**
 |
diff --git a/th04/boss_x2.cpp b/th04/boss_x2.cpp
new file mode 100644
index 00000000..c0d4ca84
--- /dev/null
+++ b/th04/boss_x2.cpp
@@ -0,0 +1 @@
+#include "th04/main/boss/bx2.cpp"
diff --git a/th04/main/boss/bx_1.asm b/th04/main/boss/bx1.asm
similarity index 100%
rename from th04/main/boss/bx_1.asm
rename to th04/main/boss/bx1.asm
diff --git a/th04/main/boss/bx_1.cpp b/th04/main/boss/bx1.cpp
similarity index 100%
rename from th04/main/boss/bx_1.cpp
rename to th04/main/boss/bx1.cpp
diff --git a/th04/main/boss/bx2.cpp b/th04/main/boss/bx2.cpp
new file mode 100644
index 00000000..eb548015
--- /dev/null
+++ b/th04/main/boss/bx2.cpp
@@ -0,0 +1,7 @@
+/// Extra Stage Boss #2 - Gengetsu
+/// ------------------------------
+
+#include "platform.h"
+
+#define wave_amp	gengetsu_wave_amp
+uint8_t wave_amp = 0x00; // Should really have been signed.
diff --git a/th04_main.asm b/th04_main.asm
index 25f12b1b..7f82a8b3 100644
--- a/th04_main.asm
+++ b/th04_main.asm
@@ -11891,19 +11891,19 @@ var_2		= word ptr -2
 		mov	si, ax
 		cmp	_boss_phase, PHASE_EXPLODE_BIG
 		jnb	loc_1306D
-		cmp	amp_237F8, 0
+		cmp	_gengetsu_wave_amp, 0
 		jz	short loc_12FE7
 		push	di
 		push	ax
 		mov	al, _boss_sprite
 		mov	ah, 0
 		push	ax
-		mov	al, amp_237F8
+		mov	al, _gengetsu_wave_amp
 		mov	ah, 0
 		mov	dx, 80
 		sub	dx, ax
 		push	dx
-		push	word ptr amp_237F8
+		push	word ptr _gengetsu_wave_amp
 		mov	al, _boss_angle
 		mov	ah, 0
 		push	ax
@@ -11915,12 +11915,12 @@ var_2		= word ptr -2
 		mov	ah, 0
 		inc	ax
 		push	ax
-		mov	al, amp_237F8
+		mov	al, _gengetsu_wave_amp
 		mov	ah, 0
 		mov	dx, 80
 		sub	dx, ax
 		push	dx
-		push	word ptr amp_237F8
+		push	word ptr _gengetsu_wave_amp
 		mov	al, _boss_angle
 		mov	ah, 0
 		push	ax
@@ -20092,7 +20092,7 @@ loc_18682:
 		retn
 mugetsu_18655	endp
 
-include th04/main/boss/bx_1.asm
+include th04/main/boss/bx1.asm
 
 ; =============== S U B	R O U T	I N E =======================================
 
@@ -30367,20 +30367,20 @@ loc_1F994:
 		add	_boss_pos.cur.x, ax
 		cmp	_boss_phase_frame, 32
 		jg	short loc_1F9A9
-		mov	al, amp_237F8
+		mov	al, _gengetsu_wave_amp
 		add	al, 2
 		jmp	short loc_1F9AE
 ; ---------------------------------------------------------------------------
 
 loc_1F9A9:
-		mov	al, amp_237F8
+		mov	al, _gengetsu_wave_amp
 		add	al, -2
 
 loc_1F9AE:
-		mov	amp_237F8, al
+		mov	_gengetsu_wave_amp, al
 		cmp	_boss_phase_frame, 64
 		jnz	short loc_1F9C1
-		mov	amp_237F8, 0
+		mov	_gengetsu_wave_amp, 0
 		mov	al, 1
 		pop	bp
 		retn
@@ -30433,18 +30433,18 @@ loc_1FA00:
 loc_1FA07:
 		cmp	_boss_phase_frame, 64
 		jg	short loc_1FA14
-		inc	amp_237F8
+		inc	_gengetsu_wave_amp
 		jmp	short loc_1FA18
 ; ---------------------------------------------------------------------------
 
 loc_1FA14:
-		dec	amp_237F8
+		dec	_gengetsu_wave_amp
 
 loc_1FA18:
 		cmp	_boss_phase_frame, 128
 		jnz	short loc_1FA2F
 		mov	_boss_pos.cur.x, (192 shl 4)
-		mov	amp_237F8, 0
+		mov	_gengetsu_wave_amp, 0
 		mov	al, 1
 		pop	bp
 		retn
@@ -31335,7 +31335,7 @@ gengetsu_20202	proc near
 		mov	bp, sp
 		cmp	byte_259EF, 0
 		jz	short loc_20220
-		cmp	amp_237F8, 0
+		cmp	_gengetsu_wave_amp, 0
 		jnz	short loc_20220
 		call	sub_1E5D8 pascal, (48 shl 4) or ((48 shl 4) shl 16), 10
 		jmp	short loc_20233
@@ -31344,7 +31344,7 @@ gengetsu_20202	proc near
 loc_20220:
 		cmp	_boss_sprite, 0
 		jz	short loc_20233
-		cmp	amp_237F8, 0
+		cmp	_gengetsu_wave_amp, 0
 		jnz	short loc_20233
 		call	sub_1E64E
 		pop	bp
@@ -31855,7 +31855,7 @@ loc_206B1:
 ; ---------------------------------------------------------------------------
 
 loc_206B6:
-		cmp	amp_237F8, 0
+		cmp	_gengetsu_wave_amp, 0
 		jnz	short loc_206C9
 		mov	ax, _boss_pos.cur.x
 		mov	_homing_target.x, ax
@@ -32536,8 +32536,7 @@ aSt06bk2_cdg	db 'st06bk2.cdg',0
 aSt06b_bb	db 'st06b.bb',0
 byte_237F6	db 0
 byte_237F7	db 0
-amp_237F8	db 0
-	even
+	extern _gengetsu_wave_amp:byte
 
 	.data?