;global variables & constants

speed = 1           ;speed
intX = 255 - 24         ;x is leftSprite
intY = 30+20            ;Y is bottomSprite
x0 = $D000
y0 = $D001
xMSB = $D010
dRight = 88-23          ;343-255 = 88 - 23 x collision threshold with msb set - sprite width
cRight = dRight-speed       ;final x collision threshold one frame before
cLeft = 24+speed        ; same
cTop = 30           ;y collision
cBottom = 229           ; same
dx .byte speed
dy .byte speed
temp .byte 0
inter .word ?


    *       = $1000

 start

                ;init sprite & isr vector

    lda #1
    sta $d015
    lda #intX
    sta x0
    lda #intY
    sta y0

    lda $314
    sta inter
    lda $315
    sta inter+1

    sei
    lda #<main_loop
    sta $314
    lda #>main_loop
    sta $315
    cli
    rts


                            ;update sprites
main_loop

    lda x0
    ldx dx
    ldy xMSB
    cpx #128        ;direction check
    bcs goingL
goingR
    cpy #1
    beq msbSetR
    cmp #256 - speed    ;256 is for overflow - speed is for a frame before that speed is  intended always > 0
    bcc endC        ;just update
    lda #1
    sta xMSB
endC    clc
    adc dx
    sta x0
        ;jmp (inter)
yUpdate
    lda y0
    ldy dy
    cpy #128        ;direction check
    bcs goingU
goingD

    cmp #cBottom - speed
    bcc endCY       ;just update
    sec
    sbc #cBottom
    sta temp
    lda #cBottom
    sec
    sbc temp
    sta y0
    ldx #-speed
    stx dy

endCY   clc
    adc dy
    sta y0
    jmp (inter)         ;back to isr

goingL
    cpy #1
    beq msbSetL
    ; if MSB is not set
    cmp #cLeft
    bcs endC        ;just update
    sta temp
    lda #cLeft
    sec
    sbc temp
    sta temp
    lda #cLeft
    clc
    adc temp
    ldx #speed      ;invert direction
    stx dx
    jmp (inter)

msbSetR
    cmp #cRight + speed
    bcc endC
    sec
    sbc #cRight
    sta temp
    lda #dRight
    sec
    sbc temp
    sta x0
    lda #-speed
    sta dx
    lda x0
jEndC   jmp endC

wrapR
    sec
    sbc #cRight
    sta temp
    lda #speed
    sec
    sbc temp
    clc
    adc #cRight
    sta x0
    lda #-speed
    sta dx
    jmp (inter)

goingU
    cmp #cTop + 20 + speed + 1 ; +1 for bcs
    bcs endCY       ;just update
    sta temp
    lda #cTop + 20
    sec
    sbc temp
    sta temp
    lda #cTop+20
    clc
    adc temp
    sta y0
    ldx #speed
    stx dy
    jmp endCY

msbSetL
    cmp #speed + 1      ;check if we need to clear MSB, + 1 for bcs
    bcs jEndC       ;trampoline to endC, just update
    sta temp
    lda speed
    sec
    sbc temp
    lda #0
    sta xMSB
    jmp endC