Initial release, all features working as expected

Keygen.cs

@@ -0,0 +1,72 @@
+using System;
+
+namespace Longhorn_TN3270
+{
+    /**
+     * This was a fun project, had me racking my brain for about a week
+     * Thanks for the puzzle!
+     */
+    internal class Keygen
+    {
+        private readonly Random _rnd = new Random();
+        public string Generate(string userName)
+        {
+            // licence much have a total length of 10
+            char[] licence = {' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' '};
+            
+            // populate every other char with random bytes
+            for (var i = 2; i < licence.Length; i += 2)
+            {
+                licence[i] = Convert.ToBoolean(_rnd.Next() % 2) ? (char)(_rnd.Next() % 10 + '0') : (char)(_rnd.Next() % 10 + 'D');
+            }
+
+            // populate every other character with the licence
+            var iCalcLicence = LicenseCalc(Sanitize(userName.ToUpper()));
+            var sCalcLicence = string.Format("{0:D5}", iCalcLicence);
+            for (int i = 0, j = 1; i < sCalcLicence.Length; i++, j += 2)
+            {
+                licence[j] = sCalcLicence[i];
+            }
+
+            // first character must be 0-9
+            var sum = 0;
+            for (var i = 1; i < licence.Length; i++)
+            {
+                sum += licence[i] - 48;
+            }
+
+            licence[0] = (char)(sum % 10 + 48);
+
+            return new string(licence);
+        }
+
+        /**
+         * Not gonna lie, this function seems pretty silly
+         * but it looks like the origional author (of the C function)
+         * seemed to have an off by 2 error and was getting garbage
+         * while generating licenses
+         */
+        private static string Sanitize(string stringIn)
+        {
+            char[] t = {' '};
+            return stringIn.TrimStart(t);
+        }
+
+        /**
+         * Man oh MAN did I go through a whole LOT
+         * for 4 measley lines of code
+         * Y'all better appreciate this
+         */
+        private static int LicenseCalc(string sUserName)
+        {
+            var sum = 0x5217;
+            for (var i = 0; i < sUserName.Length; i++)
+            {
+                sum += sUserName[i] * 0xa3;
+            }
+
+            return sum & 0xffff;
+        }
+
+    }
+}