THRESHOLD

; e04_threshold: a real threshold gate. The old version accepted any string
; with a single byte at or above 0x40, so almost everything passed. This version
; pins three independent facts that the player must recover together:
;   1. the length is exactly 4 bytes,
;   2. exactly 2 of those bytes are at or above the threshold 0x40 ('@'),
;   3. the 16-bit sum of all 4 bytes equals 263 (0x0107).
; Technique: threshold count plus byte sum. The threshold count is the new idea;
; the sum is reused from CHECKSUM so a one-byte slip always breaks the total.
; r0 = 1 on accept. Accept rule: nonzero.
;
; The 8-bit registers cannot hold a 4-byte sum (up to 1020), so we keep the low
; byte in r3 and count the carries (the high byte) in r4 by hand, exactly as
; CHECKSUM does. target 263 = 0x0107 -> high byte 1, low byte 0x07.
;
; sample "12JZ": bytes 0x31 0x32 0x4a 0x5a.
;   sum  = 49 + 50 + 74 + 90 = 263 = 0x0107.   (high 1, low 0x07)
;   >= 0x40: 0x4a and 0x5a -> exactly 2 over the threshold.

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

        mov   r3, 0         ; r3 = running sum, low byte
        mov   r4, 0         ; r4 = running sum, high byte (carry count)
        mov   r5, 0         ; r5 = count of bytes at or above 0x40
        mov   r7, 0         ; index

loop:   cmp   r7, 4
        jge   final
        ldb   r0, [r7]      ; r0 = next input byte

        ; --- accumulate the 16-bit sum -----------------------------------
        add   r3, r0        ; low += byte  (wraps mod 256)
        cmp   r3, r0        ; if the new low is below the byte we added,
        jge   nocarry       ; the add wrapped -> carry into the high byte
        inc   r4
nocarry:
        ; --- count bytes at or above the threshold 0x40 ------------------
        cmp   r0, 0x40
        jl    under         ; below '@' does not count
        inc   r5
under:
        inc   r7
        jmp   loop

        ; --- all three facts must hold -----------------------------------
final:  cmp   r5, 2         ; exactly two bytes at or above the threshold
        jnz   bad
        cmp   r4, 1         ; sum high byte must be 1
        jnz   bad
        cmp   r3, 0x07      ; sum low byte must be 7
        jnz   bad

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