VALID SERIAL

; t05_checkdigit: keygen-style serial check (a Luhn-flavored check digit done
; with 8-bit hand-math). The serial is 7 ASCII digits: 6 body digits then a
; trailing CHECK digit. With alternating weights 1,3,1,3,1,3 the weighted sum of
; the body, taken mod 10, must equal the check digit.
;
; The 8-bit ceiling forces the design choice: a raw weighted sum can exceed 255,
; so we keep the running total REDUCED mod 10 as we go (an inner subtract-10 loop)
; rather than summing wide then dividing. That is the explicit-carry hand-math the
; core is built around. r0 = 1 on accept. Accept rule: nonzero.
;
; sample "1234565": body 123456, weighted sum 45, 45 mod 10 = 5 = the check digit.

        len   r2
        cmp   r2, 7
        jnz   bad           ; exactly 7 digits

        ; --- validate every char is an ASCII digit 0x30..0x39 -------------
        mov   r7, 0
dchk:   cmp   r7, r2
        jge   sum0
        ldb   r0, [r7]
        cmp   r0, 0x30
        jl    bad
        cmp   r0, 0x39
        jg    bad
        inc   r7
        jmp   dchk

        ; --- weighted sum of the 6 body digits, kept mod 10 ---------------
sum0:   mov   r3, 0         ; r3 = running sum (always 0..9)
        mov   r7, 0         ; index 0..5
        mov   r4, 1         ; r4 = current weight (toggles 1 <-> 3)
wloop:  cmp   r7, 6
        jge   final
        ldb   r0, [r7]
        sub   r0, 0x30      ; ASCII -> digit value 0..9
        mov   r5, r0        ; r5 = digit
        ; r6 = weight * digit  (max 3*9 = 27, fits a byte)
        mov   r6, 0
        mov   r1, r4        ; loop weight times, adding the digit
mulw:   cmp   r1, 0
        jz    addt
        add   r6, r5
        dec   r1
        jmp   mulw
addt:   add   r3, r6        ; running sum += term  (max 9 + 27 = 36)
red:    cmp   r3, 10        ; reduce mod 10 by repeated subtraction
        jl    nextw
        sub   r3, 10
        jmp   red
nextw:  ; toggle weight 1 <-> 3 (3: (w: 1) maps 1->3, 3->1)
        cmp   r4, 1
        jz    w3
        mov   r4, 1
        jmp   adv
w3:     mov   r4, 3
adv:    inc   r7
        jmp   wloop

        ; --- compare the computed check digit to the 7th input digit ------
final:  mov   r7, 6
        ldb   r0, [r7]
        sub   r0, 0x30      ; the supplied check digit value
        cmp   r0, r3
        jnz   bad

        mov   r0, 1
        ret
bad:    mov   r0, 0
        ret