1 files changed, 30 insertions, 0 deletions
diff --git a/aoc2021/Day07.cs b/aoc2021/Day07.cs
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+++ b/aoc2021/Day07.cs
@@ -0,0 +1,30 @@
+namespace aoc2021;
+
+/// <summary>
+/// Day 7: <see href="https://adventofcode.com/2021/day/7"/>
+/// </summary>
+public sealed class Day07 : Day
+{
+    private readonly List<int> _tape;
+    
+    public Day07() : base(7, "The Treachery of Whales")
+    {
+        _tape = Input.First().Split(',').Select(int.Parse).OrderBy(i => i).ToList();
+    }
+
+    private static int ArithmeticSumTo(int n) => n * (n + 1) / 2;
+
+    public override string Part1()
+    {
+        var i = _tape[_tape.Count / 2];
+        return $"{_tape.Select(t => Math.Abs(t - i)).Sum()}";
+    }
+
+    public override string Part2()
+    {
+        var avg = (decimal)_tape.Sum() / _tape.Count;
+        var floor = _tape.Select(t => ArithmeticSumTo(Math.Abs(t - (int)Math.Floor(avg)))).Sum();
+        var ceil = _tape.Select(t => ArithmeticSumTo(Math.Abs(t - (int)Math.Ceiling(avg)))).Sum();
+        return $"{Math.Min(floor, ceil)}";
+    }
+}