day 9 part 2

aoc.tests/DayTests.cs

@@ -17,7 +17,7 @@ namespace aoc.tests
         [DataRow(typeof(Day06), "6273", "3254")]
         [DataRow(typeof(Day07), "169", "82372")]
         [DataRow(typeof(Day08), "1654", "833")]
-        [DataRow(typeof(Day09), "", "")]
+        [DataRow(typeof(Day09), "138879426", "23761694")]
         public void CheckAllDays(Type dayType, string part1, string part2)
         {
             // create day instance

aoc2020/Day09.cs

@@ -1,4 +1,3 @@
-using System.Collections.Generic;
 using System.Linq;
 
 namespace aoc2020
@@ -9,6 +8,7 @@ namespace aoc2020
     public sealed class Day09 : Day
     {
         private readonly long[] _list;
+        private long _part1;
 
         public Day09() : base(9)
         {
@@ -18,16 +18,27 @@ namespace aoc2020
         public override string Part1()
         {
             var i = 25;
-            while (_list.Skip(i - 25).Take(25).DifferentCombinations(2).Select(c => c.Sum()).Contains(_list[i]))
-            {
-                i++;
-            }
+            while (_list.Skip(i - 25).Take(25).Combinations(2).Select(c => c.Sum()).Contains(_list[i])) i++;
+            _part1 = _list[i];
             return $"{_list[i]}";
         }
 
         public override string Part2()
         {
+            for (var i = 0; i < _list.Length - 1; i++)
+            {
+                var offset = 1;
+                while (_list.Skip(i).Take(offset).Sum() < _part1) offset++;
+
+                var subset = _list.Skip(i).Take(offset).ToArray();
+                if (subset.Sum() != _part1) continue;
+
+                var min = subset.Min();
+                var max = subset.Max();
+                return $"{min + max}";
+            }
+
             return "";
         }
     }
-}
+}

aoc2020/Extensions.cs

@@ -14,12 +14,13 @@ namespace aoc2020
                 : source.Remove(source.LastIndexOf(value, StringComparison.Ordinal));
         }
 
-        public static IEnumerable<IEnumerable<T>> DifferentCombinations<T>(this IEnumerable<T> elements, int k)
+        public static IEnumerable<IEnumerable<T>> Combinations<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)));
+            return k == 0
+                ? new[] {Array.Empty<T>()}
+                : enumerable.SelectMany((e, i) =>
+                    enumerable.Skip(i + 1).Combinations(k - 1).Select(c => new[] {e}.Concat(c)));
         }
 
         /// <summary>