day 15

aoc2020.test/DayTests.cs

@@ -22,6 +22,7 @@ namespace aoc2020.test
         [DataRow(typeof(Day12), "1710", "62045")]
         [DataRow(typeof(Day13), "171", "539746751134958")]
         [DataRow(typeof(Day14), "17481577045893", "4160009892257")]
+        [DataRow(typeof(Day15), "257", "8546398")]
         public void CheckAllDays(Type dayType, string part1, string part2)
         {
             // create day instance

aoc2020/Day15.cs

@@ -0,0 +1,48 @@
+using System.Linq;
+
+namespace aoc2020
+{
+    /// <summary>
+    ///     Day 15: <see href="https://adventofcode.com/2020/day/15">Rambunctious Recitation</see>
+    /// </summary>
+    public sealed class Day15 : Day
+    {
+        private readonly int[] _turns;
+        private int _current;
+        private int _i;
+
+        public Day15() : base(15)
+        {
+            var initial = Input.First().Split(',').Select(int.Parse).ToArray();
+            _turns = new int[30_000_000];
+
+            // seed array with initial values
+            for (_i = 1; _i < initial.Length + 1; _i++)
+                _turns[initial[_i - 1]] = _i;
+        }
+
+        public override string Part1()
+        {
+            for (; _i != 2020; _i++)
+            {
+                var next = _turns[_current] > 0 ? _i - _turns[_current] : 0;
+                _turns[_current] = _i;
+                _current = next;
+            }
+
+            return $"{_current}";
+        }
+
+        public override string Part2()
+        {
+            for (; _i != 30_000_000; _i++)
+            {
+                var next = _turns[_current] > 0 ? _i - _turns[_current] : 0;
+                _turns[_current] = _i;
+                _current = next;
+            }
+
+            return $"{_current}";
+        }
+    }
+}

aoc2020/input/day15.in

@@ -0,0 +1 @@
+0,14,6,20,1,4