day 23 part 1

aoc2020.test/DayTests.cs

@@ -29,7 +29,7 @@ public class DayTests
     //[DataRow(typeof(Day20), "21599955909991", "")]
     [DataRow(typeof(Day21), "2436", "dhfng,pgblcd,xhkdc,ghlzj,dstct,nqbnmzx,ntggc,znrzgs")]
     [DataRow(typeof(Day22), "32856", "33805")]
-    //[DataRow(typeof(Day23), "", "")]
+    [DataRow(typeof(Day23), "36542897", "")]
     //[DataRow(typeof(Day24), "", "")]
     //[DataRow(typeof(Day25), "", "")]
     public void CheckAllDays(Type dayType, string part1, string part2)

aoc2020/Day23.cs

@@ -5,11 +5,80 @@ namespace aoc2020;
 /// </summary>
 public sealed class Day23 : Day
 {
+    private readonly Dictionary<long, long> cups = new();
+    private readonly ImmutableList<long> initialCups;
+    private readonly long[] move;
+    private long current;
+
     public Day23() : base(23, "Crab Cups")
     {
+        initialCups = Input.First().Select(c => long.Parse(c.ToString())).ToImmutableList();
+        current = initialCups.First();
+        move = new long[3];
     }
 
-    public override string Part1() => "";
+    private void DoMoves(int turns)
+    {
+        for (var turn = 0; turn < turns; turn++)
+        {
+            var dest = current - 1;
+            if (dest == 0) dest = cups.Count;
 
-    public override string Part2() => "";
+            for (var i = 0; i <= 2; i++)
+            {
+                var id = cups[current];
+                var removedNext = cups[id];
+                cups.Remove(id);
+                cups[current] = removedNext;
+
+                move[i] = id;
+            }
+
+            while (move.Contains(dest))
+            {
+                dest--;
+                if (dest == 0) dest = cups.Count + 3;
+            }
+
+            for (var i = 0; i <= 2; i++)
+            {
+                var id = cups[dest];
+                cups[dest] = move[i];
+                cups.Add(move[i], id);
+                dest = cups[dest];
+            }
+
+            current = cups[current];
+        }
+    }
+
+    public override string Part1()
+    {
+        for (var i = 0; i < initialCups.Count; i++)
+            cups[initialCups[i]] = initialCups[(i + 1) % initialCups.Count];
+
+        DoMoves(100);
+
+        current = 1;
+        var result = new StringBuilder();
+        while (cups[current] != 1)
+        {
+            result.Append(cups[current]);
+            current = cups[current];
+        }
+
+        return result.ToString();
+    }
+
+    public override string Part2()
+    {
+        cups[initialCups.Last()] = 10;
+        for (var i = 10; i < 1_000_000; i++)
+            cups.Add(i, i + 1);
+        cups[1_000_000] = initialCups.First();
+
+        DoMoves(10_000_000);
+
+        return $"{cups[1] * cups[cups[1]]}";
+    }
 }