feat: Add day 2

AdventOfCode2026/Day2.cs

@@ -0,0 +1,102 @@
+namespace AdventOfCode2026;
+
+public class Day2 : Solution
+{
+    public override string SolvePart1()
+    {
+        return InputReader
+            .ReadLines(2)
+            .SelectMany(ToRanges)
+            .Select(ToLimit)
+            .SelectMany(GetIds)
+            .Where(id => IsTwiceSequence(id.ToString()))
+            .Sum()
+            .ToString();
+    }
+
+    public override string SolvePart2()
+    {
+        return InputReader
+            .ReadLines(2)
+            .SelectMany(ToRanges)
+            .Select(ToLimit)
+            .SelectMany(GetIds)
+            .Where(id => IsTwiceOrMoreSequence(id.ToString()))
+            .Sum()
+            .ToString();
+    }
+
+    private static IEnumerable<long> GetIds((string from, string to) limit) => GetIds(limit.from, limit.to);
+
+    private static IEnumerable<long> GetIds(string from, string to)
+    {
+        var current = long.Parse(from);
+        var end = long.Parse(to);
+        while (current <= end)
+        {
+            yield return current;
+            current++;
+        }
+    }
+
+    private static bool IsTwiceOrMoreSequence(string input)
+    {
+        if (input.Length < 2)
+        {
+            return false;
+        }
+
+        for (var subLength = 1; subLength <= input.Length / 2; subLength++)
+        {
+            if (input.Length % subLength != 0)
+            {
+                continue;
+            }
+
+            var testBlock = input[..subLength];
+            var theoreticalString = string.Concat(Enumerable.Repeat(testBlock, input.Length / subLength));
+            if (theoreticalString == input)
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    private static bool IsTwiceSequence(string input)
+    {
+        if (input.Length < 2)
+        {
+            return false;
+        }
+
+        if (input.Length % 2 != 0)
+        {
+            return false;
+        }
+
+        var firstHalfIndex = 0;
+        var secondHalfIndex = input.Length / 2;
+        while (secondHalfIndex < input.Length)
+        {
+            if (input[firstHalfIndex] != input[secondHalfIndex])
+            {
+                return false;
+            }
+
+            firstHalfIndex++;
+            secondHalfIndex++;
+        }
+
+        return true;
+    }
+
+    private static (string from, string to) ToLimit(string range)
+    {
+        var parts = range.Split('-');
+        return (parts[0], parts[1]);
+    }
+
+    private static IEnumerable<string> ToRanges(string line) => line.Split(',');
+}