day 9 part 1

aoc2020/Day09.cs

@@ -1,17 +1,28 @@
+using System.Collections.Generic;
+using System.Linq;
+
 namespace aoc2020
 {
     /// <summary>
-    ///     Day 9: <see href="https://adventofcode.com/2020/day/9"></see>
+    ///     Day 9: <see href="https://adventofcode.com/2020/day/9">Encoding Error</see>
     /// </summary>
     public sealed class Day09 : Day
     {
+        private readonly long[] _list;
+
         public Day09() : base(9)
         {
+            _list = Input.Select(long.Parse).ToArray();
         }
 
         public override string Part1()
         {
-            return "";
+            var i = 25;
+            while (_list.Skip(i - 25).Take(25).DifferentCombinations(2).Select(c => c.Sum()).Contains(_list[i]))
+            {
+                i++;
+            }
+            return $"{_list[i]}";
         }
 
         public override string Part2()

aoc2020/Extensions.cs

@@ -1,5 +1,7 @@
 using System;
+using System.Collections.Generic;
 using System.Diagnostics;
+using System.Linq;
 
 namespace aoc2020
 {
@@ -12,6 +14,14 @@ namespace aoc2020
                 : source.Remove(source.LastIndexOf(value, StringComparison.Ordinal));
         }
 
+        public static IEnumerable<IEnumerable<T>> DifferentCombinations<T>(this IEnumerable<T> elements, int k)
+        {
+            var enumerable = elements as T[] ?? elements.ToArray();
+            return k == 0 ? new[] { Array.Empty<T>() } :
+                enumerable.SelectMany((e, i) =>
+                    enumerable.Skip(i + 1).DifferentCombinations(k - 1).Select(c => (new[] {e}).Concat(c)));
+        }
+
         /// <summary>
         ///     increased accuracy for stopwatch based on frequency.
         ///     <see