[py] Port 2021_14

py/aoc/y2021/d14.py

@@ -0,0 +1,44 @@
+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 solve(inputstr):
+    template, rest = inputstr.split("\n\n", maxsplit=1)
+    rules = {}
+    for line in rest.splitlines():
+        pair, char = line.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]}")