1 files changed, 69 insertions, 0 deletions
diff --git a/engine/battle/core.asm b/engine/battle/core.asm
new file mode 100644
index 0000000..c79fb22
--- /dev/null
+++ b/engine/battle/core.asm
@@ -0,0 +1,69 @@
+include "constants.asm"
+
+SECTION "Battle Random", ROMX [$63DA], BANK [$0F]
+_BattleRandom: ; 3e3da (f:63da)
+; If the normal RNG is used in a link battle it'll desync.
+; To circumvent this a shared PRNG is used instead.
+
+; But if we're in a non-link battle we're safe to use it
+	ld a, [wLinkMode]
+	and a
+	jp z, Random
+
+; The PRNG operates in streams of 10 values.
+
+; Which value are we trying to pull?
+	push hl
+	push bc
+	ld a, [wLinkBattleRNCount]
+	ld c, a
+	ld b, $0
+	ld hl, wLinkBattleRNs
+	add hl, bc
+	inc a
+	ld [wLinkBattleRNCount], a
+
+; If we haven't hit the end yet, we're good
+	cp 9 ; number of seeds, including the last one. see comment in pokecrystal
+	ld a, [hl]
+	pop bc
+	pop hl
+	ret c
+
+; If we have, we have to generate new pseudorandom data
+; Instead of having multiple PRNGs, ten seeds are used
+	push hl
+	push bc
+	push af
+
+; Reset count to 0
+	xor a
+	ld [wLinkBattleRNCount], a
+	ld hl, wLinkBattleRNs
+	ld b, 9 ; number of seeds; in release, this was increased to 10
+
+; Generate next number in the sequence for each seed
+; a[n+1] = (a[n] * 5 + 1) % 256
+.loop
+	; get last #
+	ld a, [hl]
+
+	; a * 5 + 1
+	ld c, a
+	add a
+	add a
+	add c
+	inc a
+
+	; update #
+	ld [hli], a
+	dec b
+	jr nz, .loop
+
+; This has the side effect of pulling the last value first,
+; then wrapping around. As a result
+
+	pop af
+	pop bc
+	pop hl
+	ret