Day01 part 2

AdventOfCode2017/Day01.cs

@@ -1,10 +1,4 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
-
-namespace AdventOfCode2017
+namespace AdventOfCode2017
 {
     /*
      * 
@@ -31,6 +25,22 @@ namespace AdventOfCode2017
 
     What is the solution to your captcha?
 
+    --- Part Two ---
+
+    You notice a progress bar that jumps to 50% completion. Apparently, the door isn't yet satisfied, but it did emit a star as encouragement. The instructions change:
+
+    Now, instead of considering the next digit, it wants you to consider the digit halfway around the circular list. That is, if your list contains 10 items, only include a digit in your sum if the digit 10/2 = 5 steps forward matches it. Fortunately, your list has an even number of elements.
+
+    For example:
+
+        1212 produces 6: the list contains 4 items, and all four digits match the digit 2 items ahead.
+        1221 produces 0, because every comparison is between a 1 and a 2.
+        123425 produces 4, because both 2s match each other, but no other digit has a match.
+        123123 produces 12.
+        12131415 produces 4.
+
+    What is the solution to your new captcha?
+
     */
 
     public class Day01 : IDay
@@ -52,7 +62,19 @@ namespace AdventOfCode2017
 
         public string ResolvePart2(string[] inputs)
         {
-            return null;
+            string input = inputs[0];
+            int value = 0;
+            int inputLength = input.Length;
+            int inputHalfLength = inputLength / 2;
+            for (int i = 0; i < inputLength; i++)
+            {
+                int nextI = (i + inputHalfLength) % inputLength;
+                if (input[i] == input[nextI])
+                {
+                    value += (input[i] - '0');
+                }
+            }
+            return value.ToString();
         }
     }
 }