Day 6 solution

Took me a while to come to the fast solution, but I got there.

advent-of-code-2021.csproj

@@ -30,6 +30,9 @@
 	  <None Update="inputs\05.txt">
 	    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
 	  </None>
+	  <None Update="inputs\06.txt">
+	    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+	  </None>
 	</ItemGroup>
 
 </Project>

inputs/06.txt

@@ -0,0 +1 @@
+4,1,4,1,3,3,1,4,3,3,2,1,1,3,5,1,3,5,2,5,1,5,5,1,3,2,5,3,1,3,4,2,3,2,3,3,2,1,5,4,1,1,1,2,1,4,4,4,2,1,2,1,5,1,5,1,2,1,4,4,5,3,3,4,1,4,4,2,1,4,4,3,5,2,5,4,1,5,1,1,1,4,5,3,4,3,4,2,2,2,2,4,5,3,5,2,4,2,3,4,1,4,4,1,4,5,3,4,2,2,2,4,3,3,3,3,4,2,1,2,5,5,3,2,3,5,5,5,4,4,5,5,4,3,4,1,5,1,3,4,4,1,3,1,3,1,1,2,4,5,3,1,2,4,3,3,5,4,4,5,4,1,3,1,1,4,4,4,4,3,4,3,1,4,5,1,2,4,3,5,1,1,2,1,1,5,4,2,1,5,4,5,2,4,4,1,5,2,2,5,3,3,2,3,1,5,5,5,4,3,1,1,5,1,4,5,2,1,3,1,2,4,4,1,1,2,5,3,1,5,2,4,5,1,2,3,1,2,2,1,2,2,1,4,1,3,4,2,1,1,5,4,1,5,4,4,3,1,3,3,1,1,3,3,4,2,3,4,2,3,1,4,1,5,3,1,1,5,3,2,3,5,1,3,1,1,3,5,1,5,1,1,3,1,1,1,1,3,3,1

src/06.cs

@@ -0,0 +1,81 @@
+using System.Diagnostics;
+
+namespace aoc2021
+{
+    internal class Day06
+    {
+        [DebuggerDisplay("{State}")]
+        struct Fish
+        {
+            public int State;
+        }
+
+        internal static void Go()
+        {
+            Logger.Log("Day 6");
+            Logger.Log("-----");
+            var input = File.ReadAllText("inputs/06.txt");
+            List<Fish> fish = new();
+            foreach (var state in input.Split(','))
+            {
+                fish.Add(new Fish() { State = Convert.ToInt32(state) });
+            }
+            Part1(fish);
+            Part2(fish);
+            Logger.Log("");
+        }
+
+        private static void Part1(IEnumerable<Fish> fish)
+        {
+            using var t = new Timer();
+
+            var list = fish.ToList();
+            // brute force method (my initial solution)
+            for (int day = 0; day < 80; day++)
+            {
+                for (int i = list.Count - 1; i >= 0; i--)
+                {
+                    Fish f = list[i];
+                    if (f.State == 0)
+                    {
+                        list.Add(new Fish() { State = 8 });
+                        f.State = 7;
+                    }
+
+                    f.State--;
+                    list[i] = f;
+                }
+            }
+
+#pragma warning disable CA1829 // Use Length/Count property instead of Count() when available - Count is of type int, list might be longer than that
+            Logger.Log($"part1: #fish={list.LongCount()}");
+#pragma warning restore CA1829 // Use Length/Count property instead of Count() when available
+        }
+
+        private static void Part2(IEnumerable<Fish> fish)
+        {
+            using var t = new Timer();
+
+            // fast method (when brute force threatened to blow RAM and take way too long)
+            Dictionary<int, long> fishAtState = new();
+            for (int i = 0; i <= 8; i++)
+            {
+                fishAtState[i] = fish.Count(x => x.State == i);
+            }
+
+            for (int day = 0; day < 256; day++)
+            {
+                var adders = fishAtState[0];
+                for (int i = 0; i < 8; i++)
+                {
+                    fishAtState[i] = fishAtState[i + 1];
+                }
+
+                fishAtState[6] += adders;
+                fishAtState[8] = adders;
+            }
+
+            Logger.Log($"part2: #fish={fishAtState.Values.Sum()}");
+        }
+    }
+}

src/main.cs

@@ -1,3 +1,4 @@
 aoc2021.Day02.Go();
 aoc2021.Day03.Go();
 aoc2021.Day05.Go();
+aoc2021.Day06.Go();