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GenRandom: ; 0x959
push bc
push de
push hl
ld a, [wRNGPointer]
ld c, a
ld b, $0 ;load ??? into c
inc a
cp 54 + 1 ;inc ???, if ??? is 55 do alot of subtraction and make ??? 0
jr nz, .asm_96e
call UpdateRNG
xor a
ld bc, $0000
.asm_96e
ld [wRNGPointer], a ;place wRNGPointer + 1 back in
ld hl, wRNGValues ;choose number generated based on wRNGPointer and all the subtraction
add hl, bc
ld a, [hl]
pop hl
pop de
pop bc
ret
ResetRNG: ; 0x97a
ld a, [wRNGModulus]
ld d, a
ld a, $0 ; wasted instruction (debug that was never commented out?)
; [wRNGSub] = [sRNGMod] % [wRNGModulus]
ld a, [sRNGMod]
.modulo
cp d
jr c, .okay
sub d
jr .modulo
.okay
ld [wRNGSub], a
ld [wRNGSub2], a
ld e, $1
ld hl, .Data
ld a, 54
.copy_prng
push af
ld c, [hl]
inc hl
ld b, $0
push hl
ld hl, wRNGValues
add hl, bc
ld [hl], e
ld a, [wRNGSub]
sub e
jr nc, .next
add d
.next
ld e, a
ld a, [hl]
ld [wRNGSub], a
pop hl
pop af
dec a
jr nz, .copy_prng
call UpdateRNG
call UpdateRNG
call UpdateRNG
ld a, $0 ; wasted instruction (debug that was never commented out?)
call GenRandom
ld [sRNGMod], a
ret
.Data
; offsets from wRNGValues
db $14, $29, $07, $1c, $31, $0f, $24, $02, $17
db $2c, $0a, $1f, $34, $12, $27, $05, $1a, $2f
db $0d, $22, $00, $15, $2a, $08, $1d, $32, $10
db $25, $03, $18, $2d, $0b, $20, $35, $13, $28
db $06, $1b, $30, $0e, $23, $01, $16, $2b, $09
db $1e, $33, $11, $26, $04, $19, $2e, $0c, $21
UpdateRNG: ; 0x9fa
; Adjusts two RNG values using wRNGModulus
ld a, [wRNGModulus]
ld d, a
; [d812] = ([d812] - 24 * [d831]) % [d810]
ld bc, wRNGValues
ld hl, wRNGValues + $1f
ld e, $18
.loop
ld a, [bc]
sub [hl]
jr nc, .no_carry
add d
.no_carry
ld [bc], a
dec e
jr nz, .loop
; [d82a] = ([d82a] - 31 * [d812]) % [d810]
ld bc, wRNGValues + $18 ; d82a
ld hl, wRNGValues
ld e, $1f
.loop2
ld a, [bc]
sub [hl]
jr nc, .no_carry2
add d
.no_carry2
ld [bc], a
dec e
jr nz, .loop2
ret
RandomRange: ; 0xa21
; Random value 0 <= x < a
push bc
push hl
ld c, a
ld b, $0
ld hl, EvensAndOdds
add hl, bc
ld l, [hl]
call GenRandom
call MultiplyAbyL_AncientEgyptian
inc h
srl h
ld a, h
pop hl
pop bc
ret
EvensAndOdds:
; The first 128 bytes are the first 128 even numbers starting at 0.
; The next 128 bytes are the first 128 odd numbers starting at 1.
; The (a)th element is essentially what you'd get from rlca.
x = 0
REPT 128
db x | ((x >> 7) & 1)
x = x + 2
ENDR
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