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from sys import stderr
from argparse import ArgumentParser

EXIT_CODES = {
    "file_error": 1,
    "illegal_char": 2,
    "size_exceeded": 3,
    "syntax_error": 4,
    "addr_error": 5,
    "symbol_error": 6,
}

PREDEFINED_SYMBOL_CNT = 23
MAX_INST_LEN = 32768
MAX_ADDR = 32767

PREDEFINED_SYMBOLS = {
    "SP": 0,
    "LCL": 1,
    "ARG": 2,
    "THIS": 3,
    "THAT": 4,
    "R0": 0,
    "R1": 1,
    "R2": 2,
    "R3": 3,
    "R4": 4,
    "R5": 5,
    "R6": 6,
    "R7": 7,
    "R8": 8,
    "R9": 9,
    "R10": 10,
    "R11": 11,
    "R12": 12,
    "R13": 13,
    "R14": 14,
    "R15": 15,
    "SCREEN": 16384,
    "KBD": 24576,
}

DEST = {
    "": 0b000 << 3,
    "M": 0b001 << 3,
    "D": 0b010 << 3,
    "MD": 0b011 << 3,
    "A": 0b100 << 3,
    "AM": 0b101 << 3,
    "AD": 0b110 << 3,
    "AMD": 0b111 << 3,
}

JMP = {
    "": 0b000,
    "JGT": 0b001,
    "JEQ": 0b010,
    "JGE": 0b011,
    "JLT": 0b100,
    "JNE": 0b101,
    "JLE": 0b110,
    "JMP": 0b111,
}


COMP = {
    "0": 0b0101010 << 6,
    "1": 0b0111111 << 6,
    "-1": 0b0111010 << 6,
    "D": 0b0001100 << 6,
    "A": 0b0110000 << 6,
    "M": 0b1110000 << 6,
    "!D": 0b0001101 << 6,
    "!A": 0b0110001 << 6,
    "!M": 0b1110001 << 6,
    "-D": 0b0001111 << 6,
    "-A": 0b0110011 << 6,
    "-M": 0b1110011 << 6,
    "D+1": 0b0011111 << 6,
    "A+1": 0b0110111 << 6,
    "M+1": 0b1110111 << 6,
    "D-1": 0b0001110 << 6,
    "A-1": 0b0110010 << 6,
    "M-1": 0b1110010 << 6,
    "D+A": 0b0000010 << 6,
    "A+D": 0b0000010 << 6,
    "D+M": 0b1000010 << 6,
    "M+D": 0b1000010 << 6,
    "D-A": 0b0010011 << 6,
    "D-M": 0b1010011 << 6,
    "A-D": 0b0000111 << 6,
    "M-D": 0b1000111 << 6,
    "D&A": 0b0000000 << 6,
    "A&D": 0b0000000 << 6,
    "D&M": 0b1000000 << 6,
    "M&D": 0b1000000 << 6,
    "D|A": 0b0010101 << 6,
    "A|D": 0b0010101 << 6,
    "D|M": 0b1010101 << 6,
    "M|D": 0b1010101 << 6,
}


def find_illegal_symbol_char(symbol):
    for c in symbol:
        if (not c.isascii()) or (not (c.isalnum() or c in "_.$:")):
            return c

    return None


def print_symbols(symbols):
    print("====== SYMBOLS =====")
    print("label\taddr")
    for label, addr in symbols.items():
        print(f"{label}\t{addr}")


def print_binary_and_asm(binary, asm_lines):
    # binary and asm_lines are assumed to be of the same length
    print("====== RESULTS =====")
    print("addr\tbinary          \tinst")
    for line_no, bin, asm in zip(range(len(binary)), binary, asm_lines):
        print(f"{line_no}\t{bin:016b}\t{asm}")


def write_binary(file, binary):
    for inst in binary:
        file.write(f"{inst:016b}\n")


def assemble_inst(asm_line):
    if asm_line.startswith("@"):
        # A instruction
        addr_str = asm_line[1:]
        addr = int(addr_str)
        if addr > MAX_ADDR:
            print(f"Address out of range: {addr}", file=stderr)
            exit(EXIT_CODES["addr_error"])

        return addr
    else:
        # dest=comp;jmp
        inst = 0xE000
        dest, _, rhs = asm_line.partition("=")
        if not rhs:
            rhs = dest
            dest = ""

        comp, _, jmp = rhs.partition(";")
        if dest not in DEST:
            print(f"Invalid destination: {dest}", file=stderr)
            exit(EXIT_CODES["syntax_error"])

        inst |= DEST[dest]

        if jmp not in JMP:
            print(f"Invalid jump instruction: {jmp}", file=stderr)
            exit(EXIT_CODES["syntax_error"])

        inst |= JMP[jmp]

        if comp not in COMP:
            print(f"Invalid computation: {comp}", file=stderr)
            exit(EXIT_CODES["syntax_error"])

        inst |= COMP[comp]

        return inst


def assembler(input_fn, verbose):
    try:
        input_file = open(input_fn, "r")
    except FileNotFoundError:
        print(f"Cannot open input file: {input_fn}", file=stderr)
        exit(EXIT_CODES["file_error"])

    asm_lines = []
    line = input_file.readline()
    asm_line_cnt = 0
    user_symbols = {}
    while line:
        line = line.rstrip("\n")
        # throw away comment and whitespace
        asm_line = line.partition("//")[0].replace(" ", "").replace("\t", "")
        if not asm_line:
            # skip blank or comment line
            line = input_file.readline()
            continue

        if asm_line.startswith("(") and asm_line.endswith(")"):
            # asm_line may be a label
            label = asm_line[1:-1]
            illegal_char = find_illegal_symbol_char(label)
            if illegal_char is not None:
                print(f"Illegal character: {illegal_char}", file=stderr)
                exit(EXIT_CODES["illegal_char"])

            if label in user_symbols or label in PREDEFINED_SYMBOLS:
                print(f"Symbol {label} already exists", file=stderr)
                exit(EXIT_CODES["symbol_error"])

            user_symbols[label] = asm_line_cnt
        else:
            # asm_line may be an instruction
            asm_lines.append(asm_line)
            asm_line_cnt += 1
            if asm_line_cnt > MAX_INST_LEN:
                print(
                    f"Max number of instruction ({MAX_INST_LEN}) exceeded", file=stderr
                )
                exit(EXIT_CODE_SIZE_EXCEEDED)

        line = input_file.readline()

    input_file.close()

    # find and assign address to variables on the fly
    addr = 16  # variable addresses start at 16
    for line_no, asm_line in enumerate(asm_lines):
        if not asm_line.startswith("@"):
            continue

        addr_str = asm_line[1:]  # whatever comes after the @
        if not addr_str:
            print("Address cannot be empty", file=stderr)
            exit(EXIT_CODES["addr_error"])

        if addr_str.isascii() and addr_str.isdecimal():
            # address is decimal constant
            continue

        if addr_str in PREDEFINED_SYMBOLS:
            asm_lines[line_no] = f"@{PREDEFINED_SYMBOLS[addr_str]}"
        elif addr_str in user_symbols:
            asm_lines[line_no] = f"@{user_symbols[addr_str]}"
        else:
            user_symbols[addr_str] = addr
            asm_lines[line_no] = f"@{addr}"
            addr += 1

    binary = []
    for asm_line in asm_lines:
        binary.append(assemble_inst(asm_line))

    if verbose:
        print_symbols(user_symbols)
        print_binary_and_asm(binary, asm_lines)

    output_fn = (
        input_fn[:-4] + ".hack" if input_fn.endswith(".asm") else input_fn + ".hack"
    )
    try:
        output_file = open(output_fn, "w")
    except:
        print(f"Cannot open output file: {output_fn}", file=stderr)

    write_binary(output_file, binary)
    output_file.close()
    print(f"Binary written to {output_fn}")


if __name__ == "__main__":
    parser = ArgumentParser()
    parser.add_argument("-v", "--verbose", action="store_true", help="verbose mode")
    parser.add_argument("input_fn", help="input file in assembly")
    args = parser.parse_args()
    assembler(args.input_fn, args.verbose)