INCLUDE "constants.asm"

SECTION "engine/battle/core.asm", ROMX
_BattleRandom:
; 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