Ancilla_CheckForAvailableSlot

Ancilla_CheckForAvailableSlot:
{
    SLOT = $0C4A
    ancilla_type = A
    arg_num_ancilla = Y ; num allowed - 1

    ; 1. Make sure number of same-type ancillae in slot 0-4 is not equal the limit
    ; Note, this ignores slot 5-9
    {
        for (y = 0, x = 4; x >= 0; --x) {
            if (SLOT[x] == 0) {
                y += 1;
            }
        }

        if (y == (arg_num_ancilla + 1)) {
            return -1;
        }
    }

    ; 2. Define slot range
    ; Bombs & some wall rock thing can only be put in slot 0-1.
    ; Anything else can be put in 0-4.
    {
        if (ancilla_type in [Bomb, WallExplosionRock]) {
            y = 1
        } else {
            y = 4
        }
    }

    ; 3. Search slot range for open spot
    ; If a slot between 0-1 or 0-4 is empty, return it
    {
        for (; y >= 0; --y) {
            if (SLOT[y] == 0) {
                return y
            }
        }
    }

    ; 4. Wider search.
    ; It's supposed to remove unimportant things (sparks, wall arrows) in place of the new ancilla in the
    ; case where there's no slot open.
    ; Note that 03C4 persists. It's set to 6 after revival from fairy, which means it's possible to get
    ; a bomb, somaria, arrow etc. in slot 5, which is outside of the normal range.
    {
        if (03C4 < 0) {
            03C4 = arg_num_ancilla
        }

        for (; 03C4 >= 0; --03C4) {
            if (SLOT[03C4] in [SwordChargeSpark, IceRodSpark, WallArrow]) {
                return 03C4
            }
        }
    }

    ; We found no slots anywhere.
    return -1
}