Scripts to extract/pack files from/to a BG4 archive. BG4 archives are used in some of the Mario & Luigi games. Make sure to modify the paths at the top of each script as necessary! Files need to be decompressed/recompressed using Backwards LZ77. I found Kuriimu2 to work well for Backwards LZ77.

bg4_extract.py

import struct
import json
import os

# MODIFY THESE
archive_file = "./BAI.dat"
out_dir = "./BAI_CONTENTS/"
entries_json = "./entries.json"
# MODIFY THESE

if not os.path.exists(out_dir):
    os.mkdir(out_dir)

header_magic = b"BG4\0"
header_version_0105_ssdx = 0x0105  # 1.5 (SSDX)
header_version_0104_ssdx = 0x0104  # 1.4 (SSDX)
header_version_0305_bisdx = 0x0305  # 3.5 (BIS)
header_file_entry_count = 0
header_meta_sec_size = 0
header_file_entry_count_derived = 0
header_file_entry_count_multiplier = 1

entry_file_offset = 0
entry_file_size = 0
entry_name_hash = 0
entry_name_offset = 0

with open(archive_file, "rb") as f:
    # Header
    magic = f.read(4)
    if magic != header_magic:
        print("Invalid header magic")
        exit(1)

    version = struct.unpack("<H", f.read(2))[0]
    if version not in [header_version_0105_ssdx, header_version_0104_ssdx, header_version_0305_bisdx]:
        print("Invalid header version")
        exit(1)

    file_entry_count = struct.unpack("<H", f.read(2))[0]
    meta_sec_size = struct.unpack("<I", f.read(4))[0]

    file_entry_count_derived = struct.unpack("<H", f.read(2))[0]
    file_entry_count_multiplier = struct.unpack("<H", f.read(2))[0]

    print("Header:")
    print("  File entry count: %d" % file_entry_count)
    print("  Meta section size: %d" % meta_sec_size)
    print("  Derived file entry count: %d" % file_entry_count_derived)
    print("  File entry count multiplier: %d" % file_entry_count_multiplier)

    # File entries
    entries = []
    for i in range(file_entry_count):
        entry = {}
        entry["offset"] = f.tell()
        entry["file_offset"] = struct.unpack("<I", f.read(4))[0] & 0x7FFFFFFF
        entry["file_size"] = struct.unpack("<I", f.read(4))[0] & 0x7FFFFFFF
        entry["name_hash"] = struct.unpack("<I", f.read(4))[0]
        entry["name_offset"] = struct.unpack("<H", f.read(2))[0]
        
        entries.append(entry)

        if entry["name_offset"] == 0x087C or entry["name_offset"] == 0x0891:
            print(f'{hex(entry["offset"])}: {entry}')

    earliest_name = 100000000
    
    origin = f.tell()
    # File names
    names = []
    for i in range(file_entry_count):
        f.seek(origin + entries[i]["name_offset"])
        earliest_name = min(earliest_name, f.tell())
        name = ""

        while True:
            c = f.read(1)
            if c == b"\0":
                break
            name += c.decode("ascii")
        names.append(name)
        entries[i]["name"] = name
    
    furthest = 0
    earliest_file = 100000000

    # Extract files
    for i in range(file_entry_count):
        f.seek(entries[i]["file_offset"])
        earliest_file = min(earliest_file, entries[i]["file_offset"])
        data = f.read(entries[i]["file_size"])
        furthest = max(furthest, f.tell())
        with open(out_dir + names[i], "wb") as out:
            out.write(data)

with open(entries_json, "w") as out:
    json.dump(entries, out, indent=4)

bg4_pack.py

import struct
import json

# MODIFY THESE
# data for new archive
in_dir = "./BAI_RECOMPRESSED/"
out_archive_file = "./BAI.dat_RECOMPRESS"

# data obtained from the original archive and bg4_extract script
entries_json = "./entries.json"
original_archive_file = "./BAI.dat"
# MODIFY THESE

header_magic = b"BG4\0"
header_version = 0x0305  # 3.5 (BIS)
header_file_entry_count = 0
header_meta_sec_size = 0
header_file_entry_count_derived = 0
header_file_entry_count_multiplier = 1

entry_file_offset = 0
entry_file_size = 0
entry_name_hash = 0
entry_name_offset = 0

with open(entries_json, "r") as f:
    entries = json.load(f)

with open(original_archive_file, "rb") as f:
    f.seek(8)
    header_end = struct.unpack("<H", f.read(2))[0]
    f.seek(0)
    data = f.read(header_end)

with open(out_archive_file, "wb") as f:
    f.write(data)

    current = f.tell()

    f.seek(current)

    for entry in entries:
        file_offset = f.tell()
        
        with open(in_dir + entry["name"], "rb") as g:
            f.write(g.read())

        file_size = f.tell() - file_offset

        current = f.tell()

        f.seek(entry["offset"])
        f.write(struct.pack("<I", file_offset | 0x80000000))
        f.write(struct.pack("<I", file_size))

        f.seek(current)

rename_kuriimu_out.py

import shutil
import os


def move(src, dest):
    if not os.path.exists(dest):
        os.mkdir(dest)
    for file in os.listdir(src):
        if file.endswith(".out"):
            shutil.move(src + file, dest + file[:-4])


# MODIFY THIS (src, dest)
move("./BAI_CONTENTS/", "./BAI_DECOMPRESSED/")