#!/usr/bin/env python3

"""
Disassembler for nandgame.

Using my own flavor of assembly language.
I don't like the "C-style" one nandgame introduces.
"""

import sys


def decode_jump(ins: int) -> str:
    if (ins & 0x7) == 0:
        return ""
    if (ins & 0x7) == 0x7:
        return "j"

    jl = (ins & (1 << 2)) != 0
    je = (ins & (1 << 1)) != 0
    jg = (ins & (1 << 0)) != 0

    # implied: and not jg
    if jl and je:
        return "jle"
    # implied: and not je
    if jl and jg:
        return "jne"
    # implied: and not je
    if je and jg:
        return "jge"

    # implied: only one flag is 1
    if jl:
        return "jl"
    if je:
        return "je"
    if jg:
        return "jg"

    return "<unknown>"


# return op, and whether it's a one-op or two-op
def decode_ins(ins: int) -> (str, bool):
    opcode = (ins >> 8) & 0x03
    ar_n_log = (ins & (1 << 10)) != 0
    opcode |= ar_n_log << 2

    if opcode == 0b000:
        return "and", True
    if opcode == 0b001:
        return "or", True
    if opcode == 0b010:
        return "xor", True
    if opcode == 0b011:
        return "neg", False
    if opcode == 0b100:
        return "add", True
    if opcode == 0b101:
        return "inc", False
    if opcode == 0b110:
        return "sub", True
    if opcode == 0b111:
        return "dec", False

    return "<?>"


# normally, X = arg1 = D
def decode_arg1(ins: int) -> str:
    use_mem = (ins & (1 << 12)) != 0
    zx = (ins & (1 << 7)) != 0
    sw = (ins & (1 << 6)) != 0

    if zx:
        return "0"
    if not sw:
        return "D"
    return "M" if use_mem else "A"


# normally, Y = arg2 = A
def decode_arg2(ins: int) -> str:
    use_mem = (ins & (1 << 12)) != 0

    # don't care, only X is zeroed
    # zx = (ins & (1 << 7)) != 0
    sw = (ins & (1 << 6)) != 0

    if sw:
        return "D"
    return "M" if use_mem else "A"


def decode_dest(ins: int) -> str:
    dA = (ins & (1 << 5)) != 0
    dD = (ins & (1 << 4)) != 0
    dM = (ins & (1 << 3)) != 0
    dest = ""
    if dA:
        dest += "A "
    if dD:
        dest += "D "
    if dM:
        dest += "M"

    return dest


def decode_instruction_complete(ins: int) -> list[str]:
    """
    Will return a 5 element list/tuple/whatever
    mnemonic, destination, X, Y, jumpdest
    """
    if ins & 0x8000 == 0:
        # mov? ldr? ldi? aaaaaaaaaaa....
        return ["mov", "A", f"#{ins}", "", ""]
    else:
        codename, two_op = decode_ins(ins)
        dest = decode_dest(ins)
        op1 = decode_arg1(ins)
        op2 = decode_arg2(ins) if two_op else ""
        jumpdest = decode_jump(ins)
        return [codename, dest, op1, op2, jumpdest]


def print_decoded(ins: int) -> str:
    (codename, dest, op1, op2, jumpdest) = decode_instruction_complete(ins)

    dest_str = f"{dest:<5}, " if dest else 7 * " "
    jumpdest_str = f"; {jumpdest}" if jumpdest else ""
    op2_str = f", {op2}" if op2 else ""
    return f"{codename:<5}{dest_str}{op1:<2}{op2_str:<5}{jumpdest_str}"


def main():
    if len(sys.argv) != 2:
        print(f"Usage: {sys.argv[0]} [filename]")
        sys.exit(1)

    try:
        filename = sys.argv[1]
        with open(filename, "rb") as f:
            while True:
                insb = f.read(2)
                if not insb:
                    break
                ins = int.from_bytes(insb)
                print(f"\t{insb[0]:02x} {insb[1]:02x}\t{print_decoded(ins)}")

    except FileNotFoundError:
        print(f"File {filename} not found.")
        sys.exit(1)
    # head, tail...
    except BrokenPipeError:
        sys.exit(0)


if __name__ == "__main__":
    main()