day(2): Red-Nosed Reports :reindeer:

src/AdventOfCode/Day2.cs

@@ -0,0 +1,52 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using AdventOfCode.Utilities;
+
+namespace AdventOfCode
+{
+    /// <summary>
+    /// Solver for Day 2
+    /// </summary>
+    public class Day2
+    {
+        public int Part1(string[] input)
+        {
+            return input.Select(l => l.Numbers<int>()).Count(IsSafe);
+        }
+
+        public int Part2(string[] input)
+        {
+            return input.Select(l => l.Numbers<int>()).Count(ReallySafe);
+        }
+
+        private static bool IsSafe(ICollection<int> numbers)
+        {
+            var pairs = numbers.Zip(numbers.Skip(1));
+            var diffs = pairs.Select(pair => pair.First - pair.Second).ToArray();
+
+            var signs = Math.Abs(diffs.Sum(Math.Sign));
+
+            if (signs != diffs.Length) return false;
+
+            if (diffs.Select(Math.Abs).Any(d => d is 0 or > 3)) return false;
+
+            return true;
+        }
+
+        private static bool ReallySafe(ICollection<int> numbers)
+        {
+            if (IsSafe(numbers)) return true;
+
+            for (int i = 0; i < numbers.Count; i++)
+            {
+                var clone = numbers.ToList();
+                clone.RemoveAt(i);
+
+                if (IsSafe(clone)) return true;
+            }
+
+            return false;
+        }
+    }
+}