day 10 part 2

adds the socket and device to the list and reverts the 1s and 3s counts
back to 0.

algorithm design from
- https://math.stackexchange.com/q/1118670
- https://oeis.org/A249631

aoc2020.test/DayTests.cs

@@ -17,7 +17,7 @@ namespace aoc2020.test
         [DataRow(typeof(Day07), "169", "82372")]
         [DataRow(typeof(Day08), "1654", "833")]
         [DataRow(typeof(Day09), "138879426", "23761694")]
-        [DataRow(typeof(Day10), "1980", "")]
+        [DataRow(typeof(Day10), "1980", "4628074479616")]
         public void CheckAllDays(Type dayType, string part1, string part2)
         {
             // create day instance

aoc2020/Day10.cs

@@ -1,5 +1,4 @@
 using System;
-using System.IO.Enumeration;
 using System.Linq;
 
 namespace aoc2020
@@ -10,34 +9,51 @@ namespace aoc2020
     public sealed class Day10 : Day
     {
         private readonly int[] _adapters;
+        private readonly long[] _memo;
 
         public Day10() : base(10)
         {
-            _adapters = Input.Select(int.Parse).OrderBy(i => i).ToArray();
+            var parsed = Input.Select(int.Parse).ToArray();
+            // add socket and device to the list
+            _adapters = parsed.Concat(new[] {0, parsed.Max() + 3}).OrderBy(i => i).ToArray();
+            _memo = new long[_adapters.Length];
+        }
+
+        private long Connections(int i)
+        {
+            if (i == _adapters.Length - 1) _memo[i] = 1;
+            if (_memo[i] > 0) return _memo[i];
+
+            for (var j = i + 1; j <= i + 3 && j < _adapters.Length; j++)
+                if (_adapters[j] - _adapters[i] <= 3)
+                    _memo[i] += Connections(j);
+
+            return _memo[i];
         }
 
         public override string Part1()
         {
-            var ones = 1;
-            var threes = 1;
+            var ones = 0;
+            var threes = 0;
 
             for (var i = 0; i < _adapters.Length - 1; i++)
-            {
-                var difference = _adapters[i + 1] - _adapters[i];
-                switch (difference)
+                switch (_adapters[i + 1] - _adapters[i])
                 {
-                    case 1: ones++; break;
-                    case 3: threes++; break;
+                    case 1:
+                        ones++;
+                        break;
+                    case 3:
+                        threes++;
+                        break;
                     default: throw new Exception("something went wrong");
                 }
-            }
 
             return $"{ones * threes}";
         }
 
         public override string Part2()
         {
-            return "";
+            return $"{Connections(0)}";
         }
     }
-}
+}