Add automated checksum generation

Signed-off-by: Knut Ahlers <knut@ahlers.me>

Makefile

@@ -0,0 +1,5 @@
+default:
+
+auto-hook-pre-commit:
+	python3 checksum.py
+	git status --porcelain | grep -q '^M  filters.txt' && git add filters.txt || true

checksum.py

@@ -0,0 +1,36 @@
+import sys
+import re
+import codecs
+import hashlib
+import base64
+
+checksumRegexp = re.compile(
+    r'^\s*!\s*checksum[\s\-:]+([\w\+\/=]+).*\n', re.I | re.M)
+
+
+def addChecksum(data):
+    checksum = calculateChecksum(data)
+    data = re.sub(checksumRegexp, '', data)
+    data = re.sub(r'(\r?\n)', r'\1! Checksum: %s\1' % checksum, data, 1)
+    return data
+
+
+def calculateChecksum(data):
+    data = normalize(data)
+
+    hash = hashlib.md5()
+    hash.update(data.encode('utf-8'))
+    return base64.b64encode(hash.digest()).decode('utf-8').rstrip('=')
+
+
+def normalize(data):
+    data = re.sub(r'\r', '', data)
+    data = re.sub(r'\n+', '\n', data)
+    data = re.sub(checksumRegexp, '', data)
+    return data
+
+
+if __name__ == '__main__':
+    data = addChecksum(open('filters.txt').read())
+    with open('filters.txt', 'w') as f:
+        f.write(data)