1 files changed, 49 insertions, 0 deletions
diff --git a/Day04.cs b/Day04.cs
new file mode 100644
index 0000000..f4ddcf9
--- /dev/null
+++ b/Day04.cs
@@ -0,0 +1,49 @@
+using System.Linq;
+
+namespace aoc2019
+{
+    internal sealed class Day04 : Day
+    {
+        private readonly int end;
+
+        private readonly int start;
+
+        public Day04() : base(4, "Secure Container")
+        {
+            var range = Input.First().Split('-').Select(int.Parse).ToList();
+            start = range[0];
+            end = range[1];
+        }
+
+        private bool IsValid(int i)
+        {
+            var prev = 0;
+            var hasDup = false;
+            foreach (var c in i.ToString())
+            {
+                var curr = c - '0';
+                if (curr < prev) return false;
+                if (curr == prev) hasDup = true;
+                prev = curr;
+            }
+
+            return i >= start && i <= end && hasDup;
+        }
+
+        private bool HasOnePair(int i)
+        {
+            var s = i.ToString();
+            return IsValid(i) && s.Select(c => s.Count(j => j == c)).Any(c => c == 2);
+        }
+
+        protected override string Part1()
+        {
+            return $"{Enumerable.Range(start, end).Count(IsValid)}";
+        }
+
+        protected override string Part2()
+        {
+            return $"{Enumerable.Range(start, end).Count(HasOnePair)}";
+        }
+    }
+}
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