Terrible, awful day 13 solution

13input.txt

@@ -0,0 +1,2 @@
+1000299
+41,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,971,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,17,13,x,x,x,x,23,x,x,x,x,x,29,x,487,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,19

2020.csproj

@@ -47,6 +47,9 @@
     <None Update="12input.txt">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
+    <None Update="13input.txt">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
   </ItemGroup>
 
 </Project>

Program.cs

@@ -17,6 +17,7 @@
          Q10.Go();
          Q11.Go();
          Q12.Go();
+         Q13.Go();
          Util.Log($"Total time={(System.DateTime.Now - start).TotalMilliseconds}ms");
       }
    }

Q13.cs

@@ -0,0 +1,171 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace _2020
+{
+   class Q13
+   {
+      static int leaveTime;
+      static List<int> busses = new List<int>();
+
+      public static void Go()
+      {
+         var start = DateTime.Now;
+         MakeList();
+         Util.Log($"Q13 MakeList took {(DateTime.Now - start).TotalMilliseconds}ms");
+         var p1start = DateTime.Now;
+         Part1();
+         Util.Log($"Q13 part1 took {(DateTime.Now - p1start).TotalMilliseconds}ms");
+         var p2start = DateTime.Now;
+         Part2();
+         Util.Log($"Q13 part2 took {(DateTime.Now - p2start).TotalMilliseconds}ms");
+
+         Util.Log($"Q13 took {(DateTime.Now - start).TotalMilliseconds}ms");
+      }
+
+      static void MakeList()
+      {
+         var lines = File.ReadAllLines("13input.txt");
+         if (lines.Length != 2)
+         {
+            throw new Exception("Invalid input");
+         }
+
+         leaveTime = Convert.ToInt32(lines[0]);
+         foreach (var bus in lines[1].Split(','))
+         {
+            if (bus == "x")
+            {
+               busses.Add(-1);
+               continue;
+            }
+
+            busses.Add(Convert.ToInt32(bus));
+         }
+      }
+
+      static void Part1()
+      {
+         int smallestModulo = int.MaxValue;
+         int smallestBus = -1;
+         foreach (var bus in busses)
+         {
+            if (bus < 0)
+            {
+               continue;
+            }
+
+            var mod = bus - (leaveTime % bus);
+            if (mod < smallestModulo)
+            {
+               smallestModulo = mod;
+               smallestBus = bus;
+            }
+         }
+
+         Util.Log($"Q13Part1: soonest bus={smallestBus}, departs={leaveTime + smallestModulo}, wait time={smallestModulo}m, answer={smallestBus * smallestModulo}");
+      }
+
+      // taken from https://rosettacode.org/wiki/Chinese_remainder_theorem#C.23, modified for int64s. i don't know how this works. who am i, sun-tzu?
+      static long GetChineseRemainderTheorem(IEnumerable<int> n, IEnumerable<int> a)
+      {
+         long prod = n.Aggregate(1L, (i, j) => i * j);
+         long p;
+         long sm = 0;
+         for (int i = 0; i < n.Count(); i++)
+         {
+            p = prod / n.ElementAt(i);
+            sm += a.ElementAt(i) * ModularMultiplicativeInverse(p, n.ElementAt(i)) * p;
+         }
+         return sm % prod;
+      }
+
+      static long ModularMultiplicativeInverse(long a, long mod)
+      {
+         long b = a % mod;
+         for (long x = 1; x < mod; x++)
+         {
+            if ((b * x) % mod == 1)
+            {
+               return x;
+            }
+         }
+         return 1;
+      }
+
+      static void Part2()
+      {
+         var a = new List<int>();
+         var n = new List<int>();
+         for (int i = 0; i < busses.Count; i++)
+         {
+            if (busses[i] <= 0)
+            {
+               continue;
+            }
+
+            a.Add(busses[i] - i % busses[i]);
+            n.Add(busses[i]);
+         }
+
+         Util.Log($"Q13Part2: val={GetChineseRemainderTheorem(n, a)}");
+      }
+
+      // this is how i initially solved it. took ~30mins. i'm not proud of it, but the "real" answer apparently required specialized knowledge i did not have.
+      static void Part2BruteForce()
+      {
+         int root = busses.First(x => x > 0);
+         var sorted = busses.OrderBy(x => x);
+         var largest = sorted.Last();
+
+         long checkVal = 0;
+
+         var idx = busses.IndexOf(largest);
+         int answersFound = 0;
+         int numThreads = 13;
+         Parallel.For(0, numThreads, thread =>
+         {
+            var threadVal = checkVal + (thread * largest);
+            long rootCheckVal = 0;
+            while (answersFound == 0)
+            {
+               threadVal += numThreads * largest;
+               rootCheckVal = threadVal - idx;
+               if (rootCheckVal % root == 0)
+               {
+                  var gotit = false;
+                  for (int i = 1; i < busses.Count; i++)
+                  {
+                     if (busses[i] <= 0)
+                     {
+                        continue;
+                     }
+
+                     if ((rootCheckVal + i) % busses[i] == 0)
+                     {
+                        gotit = true;
+                     }
+                     else
+                     {
+                        gotit = false;
+                        break;
+                     }
+                  }
+
+                  if (gotit)
+                  {
+                     Interlocked.Increment(ref answersFound);
+                     break;
+                  }
+               }
+            }
+
+            Util.Log($"Q13Part2: val={rootCheckVal}");
+         });
+      }
+   }
+}