CHECK DIGIT

; CHECK DIGIT: a PATCH job. A position-weighted fold runs over the whole key
; (multiply the running value, add the next byte, repeat) and the result is
; compared against a stored total. The fold itself is correct: the seed is clean,
; the weight is right, every byte is walked. The STORED TOTAL is what drifted.
; Only the marked total (label "total") is yours to edit; the fold and the verdict
; are frozen, so there is no shortcut: you have to run the fold on the real key and
; set the total to what it actually produces.
;
; The real key is CRC16. The fold is r3 = (r3 * 3 + byte) per byte, starting from
; zero, which lands on a specific final value. Compute that value and store it.
;
; r0 = 1 on accept; rule: nonzero. SHIPPED total is 0xC0, a value the real fold
; never reaches, so the key is bounced.

        len   r2
        cmp   r2, 5
        jnz   bad

        mov   r3, 0         ; fold accumulator (correctly cleared)
        mov   r7, 0
loop:   cmp   r7, 5
        jge   total
        mul   r3, 3         ; position weight
        ldb   r0, [r7]
        add   r3, r0
        inc   r7
        jmp   loop

total:  cmp   r3, 0xC0      ; SHIPPED BROKEN: wrong stored total -> compute the real fold
        jnz   bad
        mov   r0, 1
        ret
bad:    mov   r0, 0
        ret