Day 9

2020.csproj

@@ -35,6 +35,9 @@
     <None Update="08input.txt">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
+    <None Update="09input.txt">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
   </ItemGroup>
 
 </Project>

Program.cs

@@ -12,6 +12,7 @@
          Q06.Go();
          Q07.Go();
          Q08.Go();
+         Q09.Go();
       }
    }
 }

Q09.cs

@@ -0,0 +1,125 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+
+namespace _2020
+{
+   class Q09
+   {
+      static List<long> list = new List<long>();
+
+      const int chunkLength = 25;
+
+      public static void Go()
+      {
+         MakeList();
+         Part1();
+         Part2();
+      }
+
+      static void MakeList()
+      {
+         foreach (var line in File.ReadAllLines("09input.txt"))
+         {
+            list.Add(Convert.ToInt64(line));
+         }
+      }
+
+      static int GetIndexOfInvalidNumber()
+      {
+         int retval = -1;
+         for (int i = chunkLength; i < list.Count; i++)
+         {
+            var bFound = false;
+            for (int j = i - chunkLength; j < i && !bFound; j++)
+            {
+               for (int k = j + 1; k < i && !bFound; k++)
+               {
+                  if (list[j] + list[k] == list[i])
+                  {
+                     bFound = true;
+                  }
+               }
+            }
+
+            if (!bFound)
+            {
+               if (retval == -1)
+               {
+                  retval = i;
+               }
+               else
+               {
+                  throw new Exception("Found multiple invalid numbers");
+               }
+            }
+         }
+
+         return retval;
+      }
+
+      static void Part1()
+      {
+         var idx = GetIndexOfInvalidNumber();
+
+         if (idx >= 0)
+         {
+            Debug.WriteLine($"Q09Part1: found invalid number={list[idx]}");
+         }
+         else
+         {
+            throw new Exception("Didn't find an invalid number");
+         }
+      }
+
+      static void Part2()
+      {
+         var idx = GetIndexOfInvalidNumber();
+         var targetNum = list[idx];
+
+         int sumRangeStartIdx = -1;
+         int sumRangeEndIdx = -1;
+         for (int i = 0; i < list.Count; i++)
+         {
+            var val = list[i];
+            for (int j = i + 1; j < list.Count; j++)
+            {
+               val += list[j];
+               if (val == targetNum)
+               {
+                  if (sumRangeStartIdx != -1 || sumRangeEndIdx != -1)
+                  {
+                     throw new Exception("Found two invalid ranges");
+                  }
+
+                  sumRangeStartIdx = i;
+                  sumRangeEndIdx = j;
+               }
+            }
+         }
+
+         if (sumRangeStartIdx < 0 || sumRangeEndIdx < 0)
+         {
+            throw new Exception("Didn't find valid range");
+         }
+
+         long smallestInRange = long.MaxValue;
+         long largestInRange = long.MinValue;
+         for (int i = sumRangeStartIdx; i <= sumRangeEndIdx; i++)
+         {
+            if (list[i] < smallestInRange)
+            {
+               smallestInRange = list[i];
+            }
+
+            if (list[i] > largestInRange)
+            {
+               largestInRange = list[i];
+            }
+         }
+
+         Debug.WriteLine($"Q09Part2: encryption weakness={smallestInRange + largestInRange}");
+      }
+   }
+}