1 files changed, 63 insertions, 0 deletions
diff --git a/Day4.cs b/Day4.cs
new file mode 100644
index 0000000..8e57b00
--- /dev/null
+++ b/Day4.cs
@@ -0,0 +1,63 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+
+namespace aoc2019
+{
+    internal class Day4 : Day
+    {
+        int start, end;
+
+        public Day4()
+        {
+            var range = File.ReadLines("input/day4.in")
+                .First()
+                .Split('-')
+                .Select(i => int.Parse(i))
+                .ToList();
+
+            start = range[0]; end = range[1];
+        }
+
+        private bool IsValid(int i)
+        {
+            return Math.Floor(Math.Log10(i) + 1) == 6
+                && i >= start
+                && i <= end
+                && IsIncreasingDigits(i);
+        }
+
+        private bool IsIncreasingDigits(int i)
+        {
+            int prev = 0;
+            bool hasDup = false;
+            foreach (var c in i.ToString())
+            {
+                int curr = c - '0';
+                if (curr < prev)
+                    return false;
+                if (curr == prev)
+                    hasDup = true;
+                prev = curr;
+            }
+            return hasDup;
+        }
+
+        private bool Part2Criterion(int i)
+        {
+            var s = i.ToString();
+            return s.Select(c => s.Count(j => j == c)).Any(c => c == 2);
+        }
+
+        public override void Part1()
+        {
+            Console.WriteLine(Enumerable.Range(start, end).Count(i => IsValid(i)));
+        }
+
+        public override void Part2()
+        {
+            Console.WriteLine(Enumerable.Range(start,end).Count(i => IsValid(i) && Part2Criterion(i)));
+        }
+    }
+}