Add hexadecimal exercise

Author: bressain

config.json

@@ -45,7 +45,8 @@
     "linked-list",
     "wordy",
     "tournament",
-    "acronym"
+    "acronym",
+    "hexadecimal"
   ],
   "deprecated": [
 

hexadecimal/Example.cs

@@ -0,0 +1,41 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text.RegularExpressions;
+
+public class Hexadecimal
+{
+    private static readonly Dictionary<char, int> AlphaValues = new Dictionary<char, int>
+        {
+            { 'a', 10 },
+            { 'b', 11 },
+            { 'c', 12 },
+            { 'd', 13 },
+            { 'e', 14 },
+            { 'f', 15 }
+        };
+
+    public static int ToDecimal(string value)
+    {
+        if (IsNotValidHexadecimal(value)) return 0;
+
+        return value.Select((c, i) => GetNumericValue(c) * SixteenToThePowerOf(value.Length - i - 1)).Sum();
+    }
+
+    private static bool IsNotValidHexadecimal(string value)
+    {
+        return Regex.IsMatch(value, "[^0-9abcdef]", RegexOptions.IgnoreCase);
+    }
+
+    private static int GetNumericValue(char value)
+    {
+        if (char.IsNumber(value))
+            return (int)char.GetNumericValue(value);
+        return AlphaValues[value];
+    }
+
+    private static int SixteenToThePowerOf(int power)
+    {
+        return (int)Math.Pow(16, power);
+    }
+}

hexadecimal/HexadecimalTest.cs

@@ -0,0 +1,33 @@
+using NUnit.Framework;
+
+[TestFixture]
+public class HexadecimalTest
+{
+    // change Ignore to false to run test case or just remove 'Ignore = true'
+    [TestCase("1", Result = 1)]
+    [TestCase("c", Result = 12, Ignore = true)]
+    [TestCase("10", Result = 16, Ignore = true)]
+    [TestCase("af", Result = 175, Ignore = true)]
+    [TestCase("100", Result = 256, Ignore = true)]
+    [TestCase("19ace", Result = 105166, Ignore = true)]
+    [TestCase("19ace", Result = 105166, Ignore = true)]
+    public int Hexadecimal_converts_to_decimal(string hexadecimal)
+    {
+        return Hexadecimal.ToDecimal(hexadecimal);
+    }
+
+    [Ignore]
+    [Test]
+    public void Invalid_hexadecimal_is_decimal_0()
+    {
+        Assert.That(Hexadecimal.ToDecimal("carrot"), Is.EqualTo(0));
+    }
+
+    [TestCase("000000", Result = 0, Ignore = true)]
+    [TestCase("ffffff", Result = 16777215, Ignore = true)]
+    [TestCase("ffff00", Result = 16776960, Ignore = true)]
+    public int Octal_can_convert_formatted_strings(string hexidecimal)
+    {
+        return Hexadecimal.ToDecimal(hexidecimal);
+    }
+}