day 13 part 2

aoc2020.test/DayTests.cs

@@ -20,7 +20,7 @@ namespace aoc2020.test
         [DataRow(typeof(Day10), "1980", "4628074479616")]
         [DataRow(typeof(Day11), "2303", "2057")]
         [DataRow(typeof(Day12), "1710", "62045")]
-        [DataRow(typeof(Day13), "171", "")]
+        [DataRow(typeof(Day13), "171", "539746751134958")]
         public void CheckAllDays(Type dayType, string part1, string part2)
         {
             // create day instance

aoc2020/Day.cs

@@ -29,7 +29,7 @@ namespace aoc2020
             var s = Stopwatch.StartNew();
             var part1 = Part1();
             s.Stop();
-            Console.Write($"Part1: {part1,-14} ");
+            Console.Write($"Part1: {part1,-15} ");
             Console.WriteLine(verbose ? $"{s.ScaleMilliseconds()}ms elapsed" : "");
 
             s.Reset();
@@ -37,7 +37,7 @@ namespace aoc2020
             s.Start();
             var part2 = Part2();
             s.Stop();
-            Console.Write($"Part2: {part2,-14} ");
+            Console.Write($"Part2: {part2,-15} ");
             Console.WriteLine(verbose ? $"{s.ScaleMilliseconds()}ms elapsed" : "");
 
             Console.WriteLine();

aoc2020/Day13.cs

@@ -1,5 +1,3 @@
-using System;
-using System.Collections.Generic;
 using System.Linq;
 
 namespace aoc2020
@@ -9,9 +7,9 @@ namespace aoc2020
     /// </summary>
     public sealed class Day13 : Day
     {
+        private readonly long[] _buses;
         private readonly long _earliest;
         private readonly string[] _fullSchedule;
-        private readonly long[] _buses;
 
         public Day13() : base(13)
         {
@@ -22,19 +20,35 @@ namespace aoc2020
 
         public override string Part1()
         {
-            for (var i = _earliest; ; i++)
-            {
+            for (var i = _earliest;; i++)
                 if (_buses.Any(b => i % b == 0))
                 {
                     var bus = _buses.First(b => i % b == 0);
                     return $"{bus * (i - _earliest)}";
                 }
-            }
         }
 
         public override string Part2()
         {
-            return "";
+            var i = 0;
+            long result = 1, multiplier = 1;
+
+            foreach (var id in _fullSchedule)
+            {
+                if (id != "x")
+                {
+                    var increment = long.Parse(id);
+                    while (((result += multiplier) + i) % increment != 0)
+                    {
+                    }
+
+                    multiplier *= increment;
+                }
+
+                i++;
+            }
+
+            return $"{result}";
         }
     }
-}
+}