day 9 part 1 solution

2020/Day09.cs

@@ -0,0 +1,76 @@
+using System;
+using static System.Console;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text.RegularExpressions;
+using Xunit;
+
+namespace AdventOfCode {
+
+  public class Day09 {
+
+    static bool Valid(List<long> ints, int preambleSize, int target) {
+      for (int i = target - preambleSize; i < target - 1; i++) {
+        for (int j = i + 1; j < target; j++) {
+          if (i == j) {
+            continue;
+          }
+          if (ints[i] + ints[j] == ints[target]) {
+            return true;
+          }
+        }
+      }
+      return false;
+    }
+
+    static long FindInvalid(List<long> ints, int preambleSize) {
+      for (int target = preambleSize; target < ints.Count(); target++) {
+        if (!Valid(ints, preambleSize, target)) {
+          return ints[target];
+        }
+      }
+      throw new Exception("didn't find an invalid int");
+    }
+
+    static long Part1() {
+      string[] input = File.ReadAllLines(Util.RootDir + "day09.txt");
+      List<long> ints = input.ToList().Select(long.Parse).ToList();
+      return FindInvalid(ints, 25);
+    }
+
+    static int Part2() {
+      return -1;
+    }
+
+    [Fact]
+    public static void Test() {
+      string[] example =
+@"35
+20
+15
+25
+47
+40
+62
+55
+65
+95
+102
+117
+150
+182
+127
+219
+299
+277
+309
+576".Split('\n');
+      List<long> ints = example.ToList().Select(long.Parse).ToList();
+      Assert.Equal(127, FindInvalid(ints, 5));
+
+      Assert.Equal(50047984, Part1());
+      Assert.Equal(-1, Part2());
+    }
+  }
+}