;===================================================
; Airship-like Camera (Up and Down Swaying Camera)
; by KevinM
;===================================================

; Max speed to move the camera in each phase (going up, going down)
; measured in 1/$100 pixels/frame
; The movement will switch direction when reaching this value
; The two values should be equal (but opposite) or the camera will
; drift away after enough cycles.
speed:
    dw -$0060,$0060

; How fast the speed increases in each phase (going up, going down)
; The two values should be equal (but opposite) or the camera will
; drift away after enough cycles.
acceleration:
    dw -$0001,$0001

; Addresses defines
!phase_addr    = $1443|!addr
!position_addr = $1464|!addr
!speed_addr    = $1448|!addr
!fraction_addr = $1450|!addr

init:
    stz $1412|!addr
    rep #$20
    stz !phase_addr
    stz !speed_addr
    stz !fraction_addr
    sep #$20
    rtl

main:
LDA #$00
STA $1412
    lda $9D : ora $13D4|!addr : bne .return
    lda $71 : cmp #$0A : beq .return
.run:
    lda !phase_addr : and #$01 : asl : tax
    rep #$20
    lda !speed_addr : clc : adc acceleration,x : sta !speed_addr
    sec : sbc speed,x : eor speed,x : bmi +
    lda speed,x : sta !speed_addr
    inc !phase_addr
    stz !fraction_addr
+   lda !fraction_addr : and #$00FF
    clc : adc !speed_addr : sta !fraction_addr
    and #$FF00
    bpl +
    ora #$00FF
+   xba
    clc : adc !position_addr : sta !position_addr
    sep #$20
.return:
    rtl

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