Day 14

2021/day14.py

@@ -0,0 +1,32 @@
+from collections import defaultdict
+from math import ceil
+
+with open("input14.txt") as f:
+    lines = [line.rstrip() for line in f]
+template = lines[0]
+rules = {k: v for k, v in map(lambda s: s.split(" -> "), lines[2:])}
+pairs = defaultdict(int)
+for i in range(len(template) - 1):
+    pairs[template[i:i+2]] += 1
+# N  B  B  B  C  N  C  C  N  B  B  N  B  N  B  B  C  H  B  H  H  B  C  H  B
+#  NB BB BB BC CN NC CC CN NB BB BN NB BN NB BB BC CH HB BH HH HB BC CH HB
+# N=5, B=11, C=5, H=4, but in pairs N=9, B=21, C=10, H=8
+# so if odd: n+1 / 2 elif even: n / 2, which is ceil(n / 2) for both
+for step in range(40):
+    for pair, n in list(pairs.items()):
+        if pairs[pair] == 0:
+            continue
+        mid = rules[pair]
+        new_pair1, new_pair2 = pair[0] + mid, mid + pair[1]
+        pairs[pair] -= n
+        pairs[new_pair1] += n
+        pairs[new_pair2] += n
+    if step in (9, 39):
+        pcount = defaultdict(int)
+        for pair, n in pairs.items():
+            for p in pair:
+                pcount[p] += n
+        tcount = {p: ceil(n / 2) for p, n in pcount.items()}
+        most_common = max(tcount, key=lambda p: tcount[p])
+        least_common = min(tcount, key=lambda p: tcount[p])
+        print(tcount[most_common] - tcount[least_common])