diff --git a/2021/06/README.md b/2021/06/README.md
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+++ b/2021/06/README.md
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+# 06
+
+The naive way to do it would be to keep a list. A way more performant ways way to do this is just having an array with 9 entries (0-8) which count the states of the fish. Then you actually just rotate the array each day and add the `0` entry to the `6` additionally.
+
+<details>
+  <summary>Solutions</summary>
+  <ol>
+    <li>352195</li>
+    <li>1600306001288</li>
+  </ol>
+</details>
diff --git a/2021/06/input.txt b/2021/06/input.txt
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+++ b/2021/06/input.txt
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+4,1,1,1,5,1,3,1,5,3,4,3,3,1,3,3,1,5,3,2,4,4,3,4,1,4,2,2,1,3,5,1,1,3,2,5,1,1,4,2,5,4,3,2,5,3,3,4,5,4,3,5,4,2,5,5,2,2,2,3,5,5,4,2,1,1,5,1,4,3,2,2,1,2,1,5,3,3,3,5,1,5,4,2,2,2,1,4,2,5,2,3,3,2,3,4,4,1,4,4,3,1,1,1,1,1,4,4,5,4,2,5,1,5,4,4,5,2,3,5,4,1,4,5,2,1,1,2,5,4,5,5,1,1,1,1,1,4,5,3,1,3,4,3,3,1,5,4,2,1,4,4,4,1,1,3,1,3,5,3,1,4,5,3,5,1,1,2,2,4,4,1,4,1,3,1,1,3,1,3,3,5,4,2,1,1,2,1,2,3,3,5,4,1,1,2,1,2,5,3,1,5,4,3,1,5,2,3,4,4,3,1,1,1,2,1,1,2,1,5,4,2,2,1,4,3,1,1,1,1,3,1,5,2,4,1,3,2,3,4,3,4,2,1,2,1,2,4,2,1,5,2,2,5,5,1,1,2,3,1,1,1,3,5,1,3,5,1,3,3,2,4,5,5,3,1,4,1,5,2,4,5,5,5,2,4,2,2,5,2,4,1,3,2,1,1,4,4,1,5
+
diff --git a/2021/06/python/main.py b/2021/06/python/main.py
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+#!/usr/bin/env python
+
+from os.path import join, dirname
+from typing import Dict, List
+
+# Day 06
+
+# 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 School:
+    def __init__(self, fishes: List[int]) -> None:
+        self.fishes = [0] * 9
+        for fish in fishes:
+            self.fishes[fish] += 1
+
+    @staticmethod
+    def parse(raw: str):
+        return School([int(x) for x in raw.split(',')])
+
+    def __len__(self) -> int:
+        return sum(self.fishes)
+
+    def tick(self):
+        born, *rest = self.fishes.copy()
+        self.fishes = rest + [born]
+        self.fishes[6] += born
+
+    def simulate(self, days: int):
+        for _ in range(days):
+            self.tick()
+
+
+# 1
+print('1.')
+
+school = School.parse(test)
+school.simulate(18)
+print(len(school))
+school.simulate(80)
+print(f'Test: {len(school)}')
+
+school = School.parse(data)
+school.simulate(80)
+print(f'Data: {len(school)}')
+
+# 2
+print('\n2.')
+
+school = School.parse(test)
+school.simulate(256)
+print(f'Test: {len(school)}')
+
+school = School.parse(data)
+school.simulate(256)
+print(f'Data: {len(school)}')
diff --git a/2021/06/test.txt b/2021/06/test.txt
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index 0000000..55129f1
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+++ b/2021/06/test.txt
@@ -0,0 +1 @@
+3,4,3,1,2