Push And Pull

; llama_sub3: alternating add/subtract cipher. The old version folded a fixed
; subtract into the compare. Here the transform runs and FLIPS sign by position:
; even offsets get +3, odd offsets get -3 before the compare, so neighbouring
; letters move opposite ways and the single "minus a constant" reflex is wrong on
; half the bytes. Technique: alternating add/subtract cipher. r0 = 1 on accept.
; Accept rule: nonzero.
;
; sample "GRID": even idx +3, odd idx -3, then compare.

        len   r1
        cmp   r1, 4
        jnz   bad           ; exactly 4 bytes

        mov   r7, 0
        ldb   r0, [r7]
        add   r0, 3         ; index 0 (even) -> +3
        cmp   r0, 0x4a
        jnz   bad
        inc   r7
        ldb   r0, [r7]
        sub   r0, 3         ; index 1 (odd) -> -3
        cmp   r0, 0x4f
        jnz   bad
        inc   r7
        ldb   r0, [r7]
        add   r0, 3         ; index 2 (even) -> +3
        cmp   r0, 0x4c
        jnz   bad
        inc   r7
        ldb   r0, [r7]
        sub   r0, 3         ; index 3 (odd) -> -3
        cmp   r0, 0x41
        jnz   bad

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