Add zstdwarccat

zstdwarccat

@@ -0,0 +1,37 @@
+#!/usr/bin/env python3
+import io
+import os
+import struct
+import subprocess
+import sys
+import tempfile
+
+
+if len(sys.argv) != 2 or sys.argv[1] == '--help' or sys.argv[1] == '-h':
+	print('Usage: unzstd-warc FILE', file = sys.stderr)
+	print('Decompresses FILE and writes its contents to stdout', file = sys.stderr)
+	sys.exit(1)
+
+
+with open(sys.argv[1], 'rb') as 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'
+	d = fp.read(dictSize)
+	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: {p.stderr!r}'
+		d = out
+	#elif d.startswith(b'\x37\xA4\x30\xEC'): # Uncompressed dict, nothing to do
+
+with tempfile.NamedTemporaryFile() as dfp:
+	dfp.write(d)
+	pzstd = subprocess.Popen(['zstdcat', '-D', dfp.name, sys.argv[1]])
+	pzstd.communicate()