day 25 part 1

aoc2020.test/DayTests.cs

@@ -31,7 +31,7 @@ public class DayTests
     [DataRow(typeof(Day22), "32856", "33805")]
     [DataRow(typeof(Day23), "36542897", "562136730660")]
     [DataRow(typeof(Day24), "282", "3445")]
-    //[DataRow(typeof(Day25), "", "")]
+    [DataRow(typeof(Day25), "11707042", "")]
     public void CheckAllDays(Type dayType, string part1, string part2)
     {
         // create day instance

aoc2020/Day25.cs

@@ -9,7 +9,35 @@ public sealed class Day25 : Day
     {
     }
 
-    public override string Part1() => "";
+    public override string Part1()
+    {
+        var cardKey = int.Parse(Input.First());
+        var doorKey = int.Parse(Input.Last());
+        return $"{Transform(doorKey, FindLoopSize(7, cardKey))}";
+    }
 
     public override string Part2() => "";
+    
+    private static long Transform(long subject, int loopSize)
+    {
+        var value = 1L;
+        for (var i = 0; i < loopSize; i++)
+        {
+            value *= subject;
+            value %= 20201227;
+        }
+        return value;
+    }
+    private static int FindLoopSize(long subject, int target)
+    {
+        var value = 1L;
+        var loops = 0;
+        while (value != target)
+        {
+            value *= subject;
+            value %= 20201227;
+            loops++;
+        }
+        return loops;
+    }
 }