#!/usr/bin/env python3
import io
import os
import struct
import subprocess
import sys
import tempfile


def get_dict(fp):
	magic = fp.read(4)
	assert magic == b'\x5D\x2A\x4D\x18', 'not a valid warc.zst with a custom dictionary'
	dictSize = fp.read(4)
	assert len(dictSize) == 4, 'missing dict size'
	dictSize = struct.unpack('<I', dictSize)[0]
	assert dictSize >= 4, 'dict too small'
	assert dictSize < 100 * 1024**2, 'dict too large'
	ds = []
	dlen = 0
	while dlen < dictSize:
		c = fp.read(dictSize - dlen)
		if c is None or c == b'': # EOF
			break
		ds.append(c)
		dlen += len(c)
	d = b''.join(ds)
	assert len(d) == dictSize, f'could not read dict fully: expected {dictSize}, got {len(d)}'
	assert d.startswith(b'\x28\xB5\x2F\xFD') or d.startswith(b'\x37\xA4\x30\xEC'), 'not a valid dict'
	if d.startswith(b'\x28\xB5\x2F\xFD'): # Compressed dict
		# Decompress with unzstd
		p = subprocess.Popen(['unzstd'], stdin = subprocess.PIPE, stdout = subprocess.PIPE, stderr = subprocess.PIPE)
		out, err = p.communicate(d)
		assert p.returncode == 0, f'unzstd exited non-zero: return code {p.returncode}, stderr: {err!r}'
		d = out
	#elif d.startswith(b'\x37\xA4\x30\xEC'): # Uncompressed dict, nothing to do
	return d


if (len(sys.argv) != 2 and sys.stdin.isatty()) or sys.argv[1:2] == ['--help'] or sys.argv[1:2] == ['-h']:
	print('Usage: unzstd-warc [FILE]', file = sys.stderr)
	print('Decompresses FILE or stdin and writes its contents to stdout', file = sys.stderr)
	sys.exit(1)


if len(sys.argv) == 2:
	with open(sys.argv[1], 'rb') as fp:
		d = get_dict(fp)
else:
	d = get_dict(sys.stdin.buffer.raw)
with tempfile.NamedTemporaryFile() as dfp:
	dfp.write(d)
	args = ['zstdcat', '-D', dfp.name]
	if len(sys.argv) == 2:
		args.append(sys.argv[1])
	pzstd = subprocess.Popen(args)
	pzstd.communicate()