pokecrystal-board/home/random.asm

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NASM
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Random::
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; 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]
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ld b, a
ldh a, [hRandomAdd]
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adc b
ldh [hRandomAdd], a
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ldh a, [rDIV]
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ld b, a
ldh a, [hRandomSub]
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sbc b
ldh [hRandomSub], a
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pop bc
ret
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BattleRandom::
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; _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]
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push af
ld a, BANK(_BattleRandom)
rst Bankswitch
call _BattleRandom
ld [wPredefHL + 1], a
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pop af
rst Bankswitch
ld a, [wPredefHL + 1]
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ret
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RandomRange::
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; Return a random number between 0 and a (non-inclusive).
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push bc
ld c, a
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; b = $100 % c
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xor a
sub c
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.mod
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sub c
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jr nc, .mod
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add c
ld b, a
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; Get a random number
; from 0 to $ff - b.
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push bc
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.loop
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call Random
ldh a, [hRandomAdd]
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ld c, a
add b
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jr c, .loop
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ld a, c
pop bc
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call SimpleDivide
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pop bc
ret