stack

y2021/p14.py

+from collections import Counter
+
+
+def sum_counts(counts):
+    """
+    >>> sum_counts([('a', 20), ('b', 10), ('a', 30)])
+    {'a': 50, 'b': 10}
+    """
+    final = Counter()
+    for k, v in counts:
+        final[k] += v
+    return final
+
+
+polymer, eqs = text.split('\n\n')
+mapping = dict(eq.split(' -> ') for eq in eqs.splitlines())
+grow = {(a, b): ((a, x), (x, b)) for (a, b), x in mapping.items()}
+
+duos = Counter(duo for duo in zip(polymer, polymer[1:]))
+scores = {}
+for n in range(40):
+    duos = sum_counts((new, N) for old, N in duos.items() for new in grow[old])
+
+    # account for double-counting when scoring, except at edges
+    singles = sum_counts((k, N) for ab, N in duos.items() for k in ab)
+    singles[polymer[0]] += 1
+    singles[polymer[-1]] += 1
+    xmax, *_, xmin = [N // 2 for _, N in singles.most_common()]
+    scores[n + 1] = xmax - xmin
+ans1 = scores[10]
+ans2 = scores[40]