135 lines
3.5 KiB
Plaintext
135 lines
3.5 KiB
Plaintext
;license:MIT
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;(c) 2019 by qkumba
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;(c) 2020 by 4am
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;
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; graphic text animation routine
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; used by credits page and interstitials during play
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;
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; Public functions:
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; - HeavySilkReveal
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;
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targetVTAB = $F0
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minHTAB = $F1
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maxHTAB = $F2
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randomchar = $F3
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revealchar = $F4
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revealtext = $FE
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;------------------------------------------------------------------------------
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; HeavySilkReveal
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; display animation with small letters to reveal hidden text
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;
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; in: A/Y points to length-prefixed string to reveal
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; targetVTAB contains 0-based row for reveal string (0..39 but no wrapping so be careful)
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; minHTAB contains starting 0-based column for reveal string (0..23)
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; out: VTAB clobbered
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; HTAB clobbered
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; maxHTAB clobbered
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; randomchar clobbered
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; revealchar clobbered
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; revealtext clobbered [word]
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;------------------------------------------------------------------------------
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HeavySilkReveal
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+ST16 revealtext
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ldy #0
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lda (revealtext), y
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clc
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adc minHTAB
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sta maxHTAB
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RevealStartingChar=*+1
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lda #$41 ; 'A'
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@outerloop
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sta randomchar
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; init RNG
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ldy #1
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sty @rnd1+1
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dey
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sty @rnd2+1
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@loop
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; generate next value in linear feedback shift register (LFSR)
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ldy @rnd1+1
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ldx @rnd2+1
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lsr @rnd2+1
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ror @rnd1+1
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bcc +
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; feedback polynomial form #$03FC for period of 1023
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; https://en.wikipedia.org/wiki/Linear-feedback_shift_register
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; https://users.ece.cmu.edu/~koopman/lfsr/index.html
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lda @rnd1+1
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eor #$FC
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sta @rnd1+1
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lda @rnd2+1
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eor #$03
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sta @rnd2+1
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; calculate VTAB/HTAB from 2-byte LFSR value
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+ lda #24
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sta VTAB
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dey
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cpy #$FF
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bne +
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dex
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+ tya
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- dec VTAB
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bmi @loop
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sec
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sbc #40
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bcs -
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dex
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bpl -
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eor #$FF
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sta HTAB
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jsr @draw
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lda randomchar
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cmp #$20
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beq +
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inc randomchar
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cmp #$5A
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bcc +
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sbc #$19
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sta randomchar
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; wait after drawing each letter, while checking for keypress
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+ lda #$0A
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jsr WaitForKeyWithTimeout
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bmi @exit
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; exit conditions for feedback polynomial
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@rnd2 lda #0 ; SMC
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bne @loop
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@rnd1 lda #0 ; SMC
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cmp #1
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bne @loop
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; loop back to do the reveal
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lda randomchar
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cmp #$20
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beq @exit
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sta RevealStartingChar
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lda #$20
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bne @outerloop ; always branches
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@exit rts
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@draw
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; check if this position is part of the reveal text
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lda VTAB
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cmp targetVTAB
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bne @drawRandom
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lda HTAB
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cmp minHTAB
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bcc @drawRandom
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cmp maxHTAB
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bcc @drawReveal
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@drawRandom
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; draw 'random' character
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lda #randomchar
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- ldy #$00
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ldx #$01
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jmp DrawHeavySilkBuffer
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@drawReveal
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; draw character from reveal text
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sec
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sbc minHTAB
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tay
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iny
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lda (revealtext), y
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sta revealchar
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lda #revealchar
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bne - ; always branches
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