Position Add

; z1_add_walk: Position Add. A position-dependent cipher: byte i has its own
; index i added before the compare. The key is not a constant, it grows with the
; position. Reverse by subtracting the position from each stored byte.
; r0 = 1 on accept (nonzero rule).
;
; (input[i] + i) must equal enc[i], enc = { 0x4d, 0x4a, 0x52, 0x56 }.
; enc[i]: i -> { 'M','I','P','S' }. Length must be 4. Answer: MIPS

        len   r1
        cmp   r1, 4
        jnz   bad           ; length must be exactly 4

        mov   r7, 0
        ldb   r0, [r7]
        add   r0, 0         ; + position 0
        cmp   r0, 0x4d
        jnz   bad
        inc   r7
        ldb   r0, [r7]
        add   r0, 1         ; + position 1
        cmp   r0, 0x4a
        jnz   bad
        inc   r7
        ldb   r0, [r7]
        add   r0, 2         ; + position 2
        cmp   r0, 0x52
        jnz   bad
        inc   r7
        ldb   r0, [r7]
        add   r0, 3         ; + position 3
        cmp   r0, 0x56
        jnz   bad

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