Day 5

Q05.cs

@@ -0,0 +1,144 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+
+namespace _2020
+{
+   class Q05
+   {
+      static List<string> list = new List<string>();
+
+      const int numRows = 128;
+      const int numCols = 8;
+
+      public static void Go()
+      {
+         MakeList();
+         Part1();
+         Part2();
+      }
+
+      static void MakeList()
+      {
+         list = new List<string>(System.IO.File.ReadAllLines("05input.txt"));
+      }
+
+      static int BinaryPartition(int upperExcl, IEnumerable<char> input, char frontHalf, char backHalf)
+      {
+         int retval = -1;
+
+         int lowerBoundIncl = 0;
+         int upperBoundExcl = upperExcl;
+
+         int i = 0;
+         for (; retval < 0; i++)
+         {
+            if (input.Count() <= i)
+            {
+               throw new Exception("Fell off the end of the input");
+            }
+
+            int range = upperBoundExcl - lowerBoundIncl;
+            int reduceBy = range / 2;
+
+            if (input.ElementAt(i) == frontHalf)
+            {
+               upperBoundExcl -= reduceBy;
+            }
+            else if (input.ElementAt(i) == backHalf)
+            {
+               lowerBoundIncl += reduceBy;
+            }
+            else
+            {
+               throw new Exception("Unexpected input");
+            }
+
+            if (upperBoundExcl - lowerBoundIncl == 1)
+            {
+               retval = lowerBoundIncl;
+            }
+         }
+
+         return retval;
+      }
+
+      static Tuple<int, int> GetRowCol(string ticket)
+      {
+         var fbs = ticket.TakeWhile((c) => c == 'F' || c == 'B');
+         int row = BinaryPartition(numRows, fbs, 'F', 'B');
+         var lrs = ticket.Skip(fbs.Count()).TakeWhile((c) => c == 'L' || c == 'R');
+         int col = BinaryPartition(numCols, lrs, 'L', 'R');
+
+         if (ticket.Length != fbs.Count() + lrs.Count())
+         {
+            throw new Exception("Invalid input");
+         }
+
+         return new Tuple<int, int>(row, col);
+      }
+
+      static int GetSeatId(int row, int col)
+      {
+         return (row * numCols) + col;
+      }
+
+      static void Part1()
+      {
+         int highestId = 0;
+         foreach (var ticket in list)
+         {
+            var (row, col) = GetRowCol(ticket);
+
+            int seatId = GetSeatId(row, col);
+            if (seatId > highestId)
+            {
+               highestId = seatId;
+            }
+         }
+
+         Debug.WriteLine($"Q05Part1: highest id={highestId}");
+      }
+
+      static void Part2()
+      {
+         var seatList = new bool[numRows, numCols];
+         foreach (var ticket in list)
+         {
+            var (row, col) = GetRowCol(ticket);
+            seatList[row, col] = true;
+         }
+
+         bool foundValid = false;
+         bool foundMine = false;
+         int myId = -1;
+         for (int row = 0; row < seatList.GetLength(0); row++)
+         {
+            for (int col = 0; col < seatList.GetLength(1); col++)
+            {
+               if (seatList[row, col])
+               {
+                  if (!foundValid && myId >= 0)
+                  {
+                     if (foundMine)
+                     {
+                        throw new Exception("Found multiple seats");
+                     }
+
+                     Debug.WriteLine($"Q05Part2: your seat={myId}");
+                     foundMine = true;
+                  }
+
+                  foundValid = true;
+               }
+               else if (foundValid)
+               {
+                  myId = GetSeatId(row, col);
+                  foundValid = false;
+               }
+            }
+         }
+      }
+   }
+}