[py] Solve 2021_14

py/2021/14.py

@@ -0,0 +1,51 @@
+import argparse
+from pathlib import Path
+from collections import Counter
+
+def step(pairs, rules):
+    result = Counter()
+    for pair, amount in pairs.items():
+        if pair in rules:
+            a, b = rules[pair]
+            result[a] += amount
+            result[b] += amount
+        else:
+            result[pair] += amount
+    return result
+
+def count_chars(template, pairs):
+    chars = Counter()
+    for pair, amount in pairs.items():
+        chars[pair[0]] += amount
+    chars[template[-1]] += 1
+    return chars
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument("file", type=Path)
+    args = parser.parse_args()
+
+    with open(args.file) as f:
+        template = next(f).strip()
+        next(f)
+        rules = {}
+        for line in f:
+            pair, char = line.strip().split(" -> ")
+            rules[pair] = (pair[0] + char, char + pair[1])
+
+    pairs = Counter(template[i:i+2] for i in range(len(template) - 1))
+
+    for i in range(10):
+        pairs = step(pairs, rules)
+
+    chars = count_chars(template, pairs).most_common()
+    print(f"Part 1: {chars[0][1] - chars[-1][1]}")
+
+    for i in range(30): # 40 steps total
+        pairs = step(pairs, rules)
+
+    chars = count_chars(template, pairs).most_common()
+    print(f"Part 2: {chars[0][1] - chars[-1][1]}")
+
+if __name__ == "__main__":
+    main()