Add function to extract information from season keys

MultiversalDiplomacy/Model/TimelineFactory.cs

@@ -34,19 +34,33 @@ public class TimelineFactory(int nextTimeline)
     /// <summary>
     /// Convert a timeline serial number to its string identifier.
     /// </summary>
-    /// <param name="designation">Integer.</param>
+    /// <param name="serial">Integer.</param>
     /// <returns>Timeline identifier.</returns>
-    public static string IntToString(int designation) {
+    public static string IntToString(int serial) {
         static int downshift(int i ) => (i - (i % 26)) / 26;
-        IEnumerable<char> result = [Letters[designation % 26]];
+        IEnumerable<char> result = [Letters[serial % 26]];
-        for (int remainder = downshift(designation); remainder > 0; remainder = downshift(remainder) - 1) {
+        for (int remainder = downshift(serial); remainder > 0; remainder = downshift(remainder) - 1) {
             // We subtract 1 after downshifting for the same reason we add 1 above after upshifting.
-            // 
             result = result.Prepend(Letters[(remainder % 26 + 25) % 26]);
         }
         return new string(result.ToArray());
     }
 
+    /// <summary>
+    /// Extract the timeline and turn components of a season designation.
+    /// </summary>
+    /// <param name="seasonKey">A timeline-turn season designation.</param>
+    /// <returns>The timeline and turn components.</returns>
+    /// <exception cref="FormatException"></exception>
+    public static (string timeline, int turn) SplitKey(string seasonKey)
+    {
+        int i = 1;
+        for (; !char.IsAsciiDigit(seasonKey[i]) && i < seasonKey.Length; i++);
+        return int.TryParse(seasonKey.AsSpan(i), out int turn)
+            ? (seasonKey[..i], turn)
+            : throw new FormatException($"Could not parse turn from {seasonKey}");
+    }
+
     public TimelineFactory() : this(0) { }
 
     public int nextTimeline = nextTimeline;

MultiversalDiplomacyTests/Model/TimelineFactoryTest.cs

@@ -22,6 +22,16 @@ public class TimelineFactoryTest
         Assert.That(TimelineFactory.StringToInt(designation), Is.EqualTo(number), "Incorrect number");
     }
 
+    [TestCase("a0", "a", 0)]
+    [TestCase("a1", "a", 1)]
+    [TestCase("a10", "a", 10)]
+    [TestCase("aa2", "aa", 2)]
+    [TestCase("aa22", "aa", 22)]
+    public void SeasonKeySplit(string key, string timeline, int turn)
+    {
+        Assert.That(TimelineFactory.SplitKey(key), Is.EqualTo((timeline, turn)), "Failed to split key");
+    }
+
     [Test]
     public void NoSharedFactoryState()
     {