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2021/07/README.md

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+# 07
+
+The first ist quite east, just find the median, and then calculate cost.
+
+For the second exercise we need triangle numbers. And the median won't work anymore.
+
+<details>
+  <summary>Solutions</summary>
+  <ol>
+    <li>349769</li>
+    <li>99540554</li>
+  </ol>
+</details>

2021/07/python/main.py

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+#!/usr/bin/env python
+
+from os.path import join, dirname
+
+# Day 07
+
+# 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')
+
+# 1
+
+
+def get_cheapest_alignment(raw: str) -> int:
+    crabs = [int(x) for x in raw.split(',')]
+    median = sorted(crabs)[len(crabs) // 2]
+    costs = [abs(x - median) for x in crabs]
+    return sum(costs)
+
+
+print('1.')
+result = get_cheapest_alignment(test)
+print(result)
+result = get_cheapest_alignment(data)
+print(result)
+
+# 2
+
+
+def triangle_number(distance: int) -> int:
+    return distance * (distance + 1) // 2
+
+
+def get_cheapest_alignment_complex(raw: str) -> int:
+    crabs = [int(x) for x in raw.split(',')]
+    lower = min(crabs)
+    upper = max(crabs)
+    lowest = -1
+    for target in range(lower, upper + 1):
+        cost = sum([triangle_number(abs(x - target)) for x in crabs])
+        if lowest == -1 or cost < lowest:
+            lowest = cost
+    return lowest
+
+
+print('2.')
+result = get_cheapest_alignment_complex(test)
+print(result)
+result = get_cheapest_alignment_complex(data)
+print(result)