Day 19

2020/day19.py

@@ -0,0 +1,60 @@
+import re
+import sys
+
+
+rules, frules, messages = {}, {}, []
+srules = {}
+
+
+def main():
+    lines = [line.rstrip() for line in sys.stdin]
+    step = 0
+    for line in lines:
+        if not line:
+            step += 1
+            continue
+        if step == 0:
+            rid, rcontent = line.split(": ")
+            if '"' in rcontent:
+                frules[int(rid)] = rcontent.strip('"')
+            else:
+                rules[int(rid)] = tuple(map(
+                    lambda seq: tuple(map(int, seq.split())),
+                    rcontent.split(" | ")
+                ))
+        else:
+            messages.append(line)
+
+    # Part 1
+    rule_0 = re.compile("^" + get_regex(0) + "$")
+    valids = list(filter(lambda m: rule_0.match(m) is not None, messages))
+    print("Num valids:", len(valids))
+
+    # Part 2
+    del rules[8]
+    rule_42 = get_regex(42)
+    srules[8] = rule_42 + "+"
+    del rules[11]
+    rule_31 = get_regex(31)
+    srules[11] = "(" + "|".join(
+        f"{rule_42}{{{i}}}{rule_31}{{{i}}}"
+        for i in range(1, 10)  # haha
+    ) + ")"
+    rule_0 = re.compile("^" + get_regex(0) + "$")
+    valids = list(filter(lambda m: rule_0.match(m) is not None, messages))
+    print("Num valids with fake loop:", len(valids))
+
+
+def get_regex(rule_id):
+    if (char := frules.get(rule_id)):
+        return char
+    if (regex := srules.get(rule_id)):
+        return regex
+    seq_res = []
+    for seq in rules[rule_id]:
+        seq_res.append("".join(get_regex(r) for r in seq))
+    return f"({'|'.join(seq_res)})"
+        
+
+if __name__ == "__main__":
+    main()