14

2021/14/README.md

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+# 14
+
+This was nice! Similar to the Lanterfish one. The naive way is to have a big string, but that becomes unfeasible quickly.
+
+The solution was to keep track of the number of combinations available and at the end divide by 2 the counted elements. Rounding up so that the first and last element is not neglected as they are not doubled.
+
+<details>
+  <summary>Solutions</summary>
+  <ol>
+    <li>2988</li>
+    <li>3572761917024</li>
+  </ol>
+</details>

2021/14/python/main.py

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+#!/usr/bin/env python
+
+import math
+from collections import defaultdict
+from os.path import dirname, join
+
+# Day 14
+
+# Common
+
+
+def read_input(filename):
+    data = join(dirname(__file__), '..', filename)
+    with open(data) as f:
+        return f.read().strip()
+
+
+test = read_input('test.txt')
+data = read_input('input.txt')
+
+
+class Polymer:
+    def __init__(self, state: str, insertions: dict[str, str]):
+        self.state = state
+        self.insertions = insertions
+        self.states: defaultdict[str, int] = defaultdict(lambda: 0)
+
+        for i in range(len(self) - 1):
+            pair = self.state[i:i+2]
+            self.states[pair] += 1
+
+    def __len__(self):
+        return len(self.state)
+
+    def step(self):
+        new = defaultdict(lambda: 0)
+        for state, count in self.states.items():
+            if count == 0:
+                continue
+            insertion = self.insertions[state]
+            for key in [state[0] + insertion, insertion + state[1]]:
+                new[key] += count
+        self.states = new
+
+    def score(self):
+        elements = defaultdict(lambda: 0)
+        for state, count in self.states.items():
+            for element in state:
+                elements[element] += count
+        for element, count in elements.items():
+            elements[element] = math.ceil(elements[element]/2)
+        return max(elements.values()) - min(elements.values())
+
+    def flag(self, steps: int):
+        for _ in range(steps):
+            self.step()
+        print(self.score())
+
+    @staticmethod
+    def parse(data: str):
+        state, raw = data.split('\n\n')
+        lines = [
+            l.split(' -> ')
+            for l in raw.split('\n')
+        ]
+        insertions = {
+            i[0]: i[1]
+            for i in lines
+        }
+        return Polymer(state.strip(), insertions)
+
+
+# 1
+print('1.')
+
+polymer = Polymer.parse(test)
+polymer.flag(10)
+polymer = Polymer.parse(data)
+polymer.flag(10)
+
+# 2
+print('\n2.')
+polymer = Polymer.parse(test)
+polymer.flag(40)
+polymer = Polymer.parse(data)
+polymer.flag(40)