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;;;
;; Pseudo-random number implementation by using a pre-calculated table.
;;
;; I know, not too flashy, but it's good enough for this game :)
.segment "CODE"
.scope Prng
;; Current random value. Initialized on the title to game transition.
zp_rand = $0A
;; Last random value as stored by
;; Prng::random_non_repeated_valid_y_coordinate().
zp_last_rand = $12
;; How many attempts should Prng::random_non_repeated_valid_y_coordinate()
;; take to find a unique value.
RAND_ATTEMPTS = 3
;; Updates the 'a' register with the next random number set after the
;; current value of `zp_rand`, while also making sure that it is a valid
;; screen Y coordinate.
;;
;; NOTE: the 'y' register is preserved.
.proc random_valid_y_coordinate
;; Get the new random number and store it right away.
ldx zp_rand
lda valid_y_rand_table, x
sta zp_rand
rts
.endproc
;; Calls Prng::random_valid_y_coordinate() multiple times until we get a
;; different value than what we got the last time we called this
;; function. Realistically this should just take 1 attempt for most cases,
;; and in the worst case just 2. Nevertheless, we have set a generous value
;; to 'RAND_ATTEMPTS' just in case.
;;
;; NOTE: the 'y' register is preserved.
.proc random_non_repeated_valid_y_coordinate
tya
pha
ldy #RAND_ATTEMPTS
@again:
jsr Prng::random_valid_y_coordinate
cmp Prng::zp_last_rand
bne @end
dey
bne @again
@end:
sta Prng::zp_last_rand
pla
tay
lda Prng::zp_last_rand
rts
.endproc
.endscope
;; The pre-computed table.
;;
;; NOTE: generated via bin/rand.rb; read more on this there.
valid_y_rand_table:
.byte $84, $76, $85, $80, $55, $7E, $7C, $73, $7F, $52, $79, $81, $53, $75, $3D, $51
.byte $54, $6C, $88, $3C, $87, $6E, $57, $82, $77, $3E, $6B, $86, $6A, $3A, $3F, $83
.byte $78, $7A, $72, $70, $7D, $6F, $6D, $71, $7B, $74, $3B, $56, $84, $76, $85, $80
.byte $55, $7E, $7C, $73, $7F, $52, $79, $81, $53, $75, $3D, $51, $54, $6C, $88, $3C
.byte $87, $6E, $57, $82, $77, $3E, $6B, $86, $6A, $3A, $3F, $83, $78, $7A, $72, $70
.byte $7D, $6F, $6D, $71, $7B, $74, $3B, $56, $84, $76, $85, $80, $55, $7E, $7C, $73
.byte $7F, $52, $79, $81, $53, $75, $3D, $51, $54, $6C, $88, $3C, $87, $6E, $57, $82
.byte $77, $3E, $6B, $86, $6A, $3A, $3F, $83, $78, $7A, $72, $70, $7D, $6F, $6D, $71
.byte $7B, $74, $3B, $56, $84, $76, $85, $80, $55, $7E, $7C, $73, $7F, $52, $79, $81
.byte $53, $75, $3D, $51, $54, $6C, $88, $3C, $87, $6E, $57, $82, $77, $3E, $6B, $86
.byte $6A, $3A, $3F, $83, $78, $7A, $72, $70, $7D, $6F, $6D, $71, $7B, $74, $3B, $56
.byte $84, $76, $85, $80, $55, $7E, $7C, $73, $7F, $52, $79, $81, $53, $75, $3D, $51
.byte $54, $6C, $88, $3C, $87, $6E, $57, $82, $77, $3E, $6B, $86, $6A, $3A, $3F, $83
.byte $78, $7A, $72, $70, $7D, $6F, $6D, $71, $7B, $74, $3B, $56, $84, $76, $85, $80
.byte $55, $7E, $7C, $73, $7F, $52, $79, $81, $53, $75, $3D, $51, $54, $6C, $88, $3C
.byte $87, $6E, $57, $82, $77, $3E, $6B, $86, $6A, $3A, $3F, $83, $78, $7A, $72, $70
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