simple script to fix the header of a .nro.lz4 file, so you can decompress it with standard tools

nro-lz4-fixup.py

#!/usr/bin/env python3

# nro-lz4-fixup.py
# simple script to fix the header of a .nro.lz4 file,
# so you can decompress it with standard tools (namely, lz4, the cli utility)

# doesn't work on all .nro.lz4 files. idk why. something stupid i'm sure. ymmv

# License: MIT

import os
import sys
from struct import pack

# argument: .nro.lz4 file
with open(sys.argv[1], 'rb') as file:
    infile_bytes = file.read()

l = len(sys.argv[1])
filename = (sys.argv[1][0:l-8]) + '_' + (sys.argv[1][l-8:l])
with open(filename, 'wb') as out:
	# Legacy format lz4 magic
	out.write(b'\x02\x21\x4C\x18')
	# write size of compressed content
	compressed_size = len(infile_bytes)-4
	out.write(pack('<L', compressed_size))
	# copy compressed content
	out.write(infile_bytes[4:])