Position Matters

; z3_weighted_sum: "Position Matters". Not a plain byte sum: each byte is
; weighted by its 1-based position before summing. sum = 1*c0 + 2*c1 + ... + N*cN-1.
; The multiply-by-running-index in the loop is the tell. Technique: weighted
; checksum. r0 = 1 on accept. Accept rule: nonzero.
;
; 8-BIT NOTE: the ARM original sums to 2044 (0x7FC), which overflows a byte.
; The byte-wide engine keeps the weighted total mod 256, so the gate is the LOW
; BYTE of that sum: 2044 & 0xFF = 0xFC. Same technique (position*value fold,
; multiply seen in the loop), same difficulty, reduced to the 8-bit ceiling.
;
; sample "SECTOR9": 1*S + 2*E + 3*C + 4*T + 5*O + 6*R + 7*'9' = 2044; low byte 0xFC.

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

        mov   r3, 0         ; r3 = running weighted sum (mod 256)
        mov   r7, 0         ; r7 = index 0..6
        mov   r4, 1         ; r4 = weight = index + 1

wloop:  cmp   r7, 7
        jge   final
        ldb   r0, [r7]      ; r0 = input[r7]
        ; r5 = weight * byte, by repeated add (weight is small: 1..7)
        mov   r5, 0
        mov   r1, r4        ; loop r4 times
mulw:   cmp   r1, 0
        jz    addt
        add   r5, r0
        dec   r1
        jmp   mulw
addt:   add   r3, r5        ; running sum += weight*byte  (wraps mod 256)
        inc   r4            ; next weight
        inc   r7
        jmp   wloop

final:  cmp   r3, 0xFC      ; low byte of 2044
        jnz   bad
        mov   r0, 1
        ret
bad:    mov   r0, 0
        ret