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Random::
; 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
ldh a, [rDIV]
ld b, a
ldh a, [hRandomAdd]
adc b
ldh [hRandomAdd], a
ldh a, [rDIV]
ld b, a
ldh a, [hRandomSub]
sbc b
ldh [hRandomSub], a
pop bc
ret
BattleRandom::
; _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.
ldh a, [hROMBank]
push af
ld a, BANK(_BattleRandom)
rst Bankswitch
call _BattleRandom
ld [wPredefHL + 1], a
pop af
rst Bankswitch
ld a, [wPredefHL + 1]
ret
RandomRange::
; 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
ldh a, [hRandomAdd]
ld c, a
add b
jr c, .loop
ld a, c
pop bc
call SimpleDivide
pop bc
ret
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