From e7b9d4e44e016b746198e011beed3a22a0a72e1d Mon Sep 17 00:00:00 2001
From: Colin McMillen <colin@semicolin.games>
Date: Wed, 9 Dec 2020 08:51:14 -0500
Subject: [PATCH] day 9, part 2 solution

---
 2020/Day09.cs | 38 ++++++++++++++++++++++++++++++++++----
 1 file changed, 34 insertions(+), 4 deletions(-)

diff --git a/2020/Day09.cs b/2020/Day09.cs
index 1519f39..6be0907 100644
--- a/2020/Day09.cs
+++ b/2020/Day09.cs
@@ -33,14 +33,43 @@ namespace AdventOfCode {
       throw new Exception("didn't find an invalid int");
     }
 
+    static long CalculateWeakness(List<long> longs, int minIndex, int maxIndex) {
+      long largest = 0;
+      long smallest = long.MaxValue;
+      for (int i = minIndex; i <= maxIndex; i++) {
+        if (longs[i] > largest) {
+          largest = longs[i];
+        }
+        if (longs[i] < smallest) {
+          smallest = longs[i];
+        }
+      }
+      return smallest + largest;
+    }
+
+    static long FindWeakness(List<long> longs, long target) {
+      for (int i = 0; i < longs.Count() - 1; i++) {
+        long total = 0;
+        for (int j = i; j < longs.Count() && total < target; j++) {
+          total += longs[j];
+          if (total == target) {
+            return CalculateWeakness(longs, i, j);
+          }
+        }
+      }
+      throw new Exception("didn't find a weakness");
+    }
+
     static long Part1() {
       string[] input = File.ReadAllLines(Util.RootDir + "day09.txt");
       List<long> longs = input.ToList().Select(long.Parse).ToList();
       return FindInvalid(longs, 25);
     }
 
-    static int Part2() {
-      return -1;
+    static long Part2(long target) {
+      string[] input = File.ReadAllLines(Util.RootDir + "day09.txt");
+      List<long> longs = input.ToList().Select(long.Parse).ToList();
+      return FindWeakness(longs, target);
     }
 
     [Fact]
@@ -69,8 +98,9 @@ namespace AdventOfCode {
       List<long> longs = example.ToList().Select(long.Parse).ToList();
       Assert.Equal(127, FindInvalid(longs, 5));
 
-      Assert.Equal(50047984, Part1());
-      Assert.Equal(-1, Part2());
+      long invalidNumber = Part1();
+      Assert.Equal(50047984, invalidNumber);
+      Assert.Equal(5407707, Part2(invalidNumber));
     }
   }
 }