Import stuff from pokecrystal; diff gold and silver

1 files changed, 84 insertions, 0 deletions

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+Random:: ; 2f8c
+; A simple hardware-based random number generator (RNG).
+
+; Two random numbers are generated by adding and subtracting
+; the divider to the respective values every time it's called.
+
+; The divider is a register that increments at a rate of 16384Hz.
+; For comparison, the Game Boy operates at a clock speed of 4.2MHz.
+
+; Additionally, an equivalent function is executed in VBlank.
+
+; This leaves a with the value in hRandomSub.
+
+	push bc
+
+	ld a, [rDIV]
+	ld b, a
+	ld a, [hRandomAdd]
+	adc b
+	ld [hRandomAdd], a
+
+	ld a, [rDIV]
+	ld b, a
+	ld a, [hRandomSub]
+	sbc b
+	ld [hRandomSub], a
+
+	pop bc
+	ret
+; 2f9f
+
+BattleRandom:: ; 2f9f
+; _BattleRandom lives in another bank.
+
+; It handles all RNG calls in the battle engine, allowing
+; link battles to remain in sync using a shared PRNG.
+
+	ld a, [hROMBank]
+	push af
+	ld a, BANK(_BattleRandom)
+	rst Bankswitch
+
+	call _BattleRandom
+
+	ld [PredefTemp + 1], a
+	pop af
+	rst Bankswitch
+	ld a, [PredefTemp + 1]
+	ret
+; 2fb1
+
+
+RandomRange:: ; 2fb1
+; Return a random number between 0 and a (non-inclusive).
+
+	push bc
+	ld c, a
+
+	; b = $100 % c
+	xor a
+	sub c
+.mod
+	sub c
+	jr nc, .mod
+	add c
+	ld b, a
+
+	; Get a random number
+	; from 0 to $ff - b.
+	push bc
+.loop
+	call Random
+	ld a, [hRandomAdd]
+	ld c, a
+	add b
+	jr c, .loop
+	ld a, c
+	pop bc
+
+	call SimpleDivide
+
+	pop bc
+	ret
+; 2fcb