Custom scrolling

init:
    STZ $1411
RTL

main:
    STZ $04 : STZ $05 : STZ $06 ; Clear some scratch RAM
    LDX #$0C ; > Set-up for a loop through the sprites
    -
        DEX   ; \ Leave the loop if we ran out of sprites
        BMI + ; /
        LDA $9E,X ; \ Keep looping if the sprite isn't a
        CMP #$63  ; |   checkerboarded/brown line-guided platform
        BNE -     ; /
        LDA $14C8,X ; \ O and it has to be alive
        CMP #$08    ; |
        BMI -       ; /
        LDA $04         ; \
        CLC : ADC $E4,X ; | Add the position of the sprite to $04
        STA $04         ; |
        LDA $05         ; |
        ADC $14E0,X     ; |
        STA $05         ; /
        INC $06 ; > Hey, we found one!
        LDA $06 : CMP #$02 ; \ Stop looking after we found 2
    BNE -                  ; /
        LSR $05 ; \ Divide $04 by 2
        ROR $04 ; /
    -
        LDA $04        ; \
        SEC : SBC #$80 ; | Subtract 0x80 from the average to find the camera
        STA $04        ; |   position
        LDA $05        ; |
        SBC #$00       ; /
        BMI ++         ; \ Set $04 if its value is position
            STA $05    ; /
            REP #$30   ; \ And update all the relevant RAM addresses
        BRA .moveFix   ; /
        ++       ; \ Otherwise, set the camera position to 0
        REP #$30 ; |
        STZ $04  ; /
    BRA .moveFix ; > Update all the relevant RAM addresses
    +       ; > If we ran out of sprites to search
    LDA $06 ; > And we found a sprite
    BNE -   ; > Don't divide $04 by 2
    LDA #$01  ; \ If we found no sprites, then center the camera around mario
    STA $1411 ; /
RTL
.moveFix ; > This sets all the relevant RAM addresses
    LDA $04
    STA $1462
    LDA $1A
    SEC : SBC $04
    SEP #$30
    LDA $17BD
    LDA $04 : STA $1A
    LDA $05 : STA $1B
RTL