2015 day 20

AOC.Test/Test2015.cs

@@ -25,6 +25,7 @@ public class Test2015
     [DataRow(typeof(Day17), "1304", "18")]
     [DataRow(typeof(Day18), "1061", "1006")]
     [DataRow(typeof(Day19), "576", "207")]
+    [DataRow(typeof(Day20), "665280", "705600")]
     public void CheckAllDays(Type dayType, string part1, string part2)
     {
         Common.CheckDay(dayType, part1, part2);

AOC2015/Day20.cs

@@ -5,11 +5,45 @@ namespace AOC2015;
 /// </summary>
 public sealed class Day20() : Day(2015, 20, "Infinite Elves and Infinite Houses")
 {
+    private int _input;
+
     public override void ProcessInput()
     {
+        _input = int.Parse(Input.First());
     }
 
-    public override object Part1() => "";
+    public override object Part1()
+    {
+        var houses = new int[1_000_000];
+        for (var i = 1; i < houses.Length; i++)
+        {
+            for (var j = i; j < houses.Length; j += i)
+            {
+                houses[j] += i * 10;
+            }
+        }
 
-    public override object Part2() => "";
+        for (var i = 1; i < houses.Length; i++)
+        {
+            if (houses[i] >= _input) return i;
+        }
+
+        return "";
+    }
+
+    public override object Part2()
+    {
+        var min = int.MaxValue;
+        var houses = new int[1_000_000];
+        for (var i = 1; i < houses.Length; i++)
+        {
+            for (int j = i, c = 0; j < houses.Length && c < 50; j += i, c++)
+            {
+                if ((houses[j] += i * 11) >= _input)
+                    min = Math.Min(min, j);
+            }
+        }
+
+        return min;
+    }
 }