Remove Season.Past so all lookups go through World

MultiversalDiplomacy/Adjudicate/PathFinder.cs

@@ -114,17 +114,17 @@ public static class PathFinder
         List<Season> adjacents = [];
 
         // The immediate past and all immediate futures are adjacent.
-        if (season.Past != null) adjacents.Add(season.Past);
+        if (season.PastId != null) adjacents.Add(world.GetSeason(season.PastId));
         adjacents.AddRange(season.Futures);
 
         // Find all adjacent timelines by finding all timelines that branched off of this season's
         // timeline, i.e. all futures of this season's past that have different timelines. Also
         // include any timelines that branched off of the timeline this timeline branched off from.
-        List<Season> adjacentTimelineRoots = new();
+        List<Season> adjacentTimelineRoots = [];
         Season? current;
         for (current = season;
-            current?.Past?.Timeline != null && current.Past.Timeline == current.Timeline;
+            current?.PastId != null && world.GetSeason(current.PastId).Timeline == current.Timeline;
-            current = current.Past)
+            current = world.GetSeason(current.PastId))
         {
             adjacentTimelineRoots.AddRange(
                 current.Futures.Where(s => s.Timeline != current.Timeline));
@@ -135,10 +135,10 @@ public static class PathFinder
         // then current is the branch timeline's root season (current.past.timeline !=
         // current.timeline). There are co-branches if this season is in a branched timeline, since
         // the first timeline by definition cannot have co-branches.
-        if (current?.Past != null)
+        if (current?.PastId != null && world.GetSeason(current.PastId) is Season past)
         {
-            IEnumerable<Season> cobranchRoots = current.Past.Futures
+            IEnumerable<Season> cobranchRoots = past.Futures
-                .Where(s => s.Timeline != current.Timeline && s.Timeline != current.Past.Timeline);
+                .Where(s => s.Timeline != current.Timeline && s.Timeline != past.Timeline);
             adjacentTimelineRoots.AddRange(cobranchRoots);
         }
 

MultiversalDiplomacy/Model/Season.cs

@@ -10,13 +10,6 @@ public class Season
     /// </summary>
     public const int FIRST_TURN = 0;
 
-    /// <summary>
-    /// The season immediately preceding this season.
-    /// If this season is an alternate timeline root, the past is from the origin timeline.
-    /// The initial season does not have a past.
-    /// </summary>
-    public Season? Past { get; }
-
     /// <summary>
     /// The designation of the season immediately preceding this season.
     /// If this season is an alternate timeline root, the past is from the origin timeline.
@@ -54,7 +47,6 @@ public class Season
 
     private Season(Season? past, int turn, string timeline, TimelineFactory factory)
     {
-        this.Past = past;
         this.PastId = past?.ToString();
         this.Turn = turn;
         this.Timeline = timeline;

MultiversalDiplomacy/Model/World.cs

@@ -245,9 +245,12 @@ public class World
     /// </summary>
     public Season GetTimelineRoot(Season season)
     {
-        Season? past = season.Past;
+        if (season.PastId is null) {
-        return past != null && season.Timeline == past.Timeline
+            return season;
-            ? GetTimelineRoot(season.Past!)
+        }
+        Season past = SeasonLookup[season.PastId];
+        return season.Timeline == past.Timeline
+            ? GetTimelineRoot(past)
             : season;
     }
 

MultiversalDiplomacyTests/MovementAdjudicatorTest.cs

@@ -171,7 +171,7 @@ public class MovementAdjudicatorTest
         // Confirm the future was created
         Assert.That(updated.Seasons.Count, Is.EqualTo(2));
         Season future = updated.Seasons.Single(s => s != updated.RootSeason);
-        Assert.That(future.Past, Is.EqualTo(updated.RootSeason));
+        Assert.That(future.PastId, Is.EqualTo(updated.RootSeason.ToString()));
         Assert.That(future.Futures, Is.Empty);
         Assert.That(future.Timeline, Is.EqualTo(updated.RootSeason.Timeline));
         Assert.That(future.Turn, Is.EqualTo(Season.FIRST_TURN + 1));
@@ -200,7 +200,7 @@ public class MovementAdjudicatorTest
 
         // Confirm the future was created
         Season s2 = updated.GetSeason("a", 1);
-        Assert.That(s2.Past, Is.EqualTo(s1));
+        Assert.That(s2.PastId, Is.EqualTo(s1.ToString()));
         Assert.That(s2.Futures, Is.Empty);
         Assert.That(s2.Timeline, Is.EqualTo(s1.Timeline));
         Assert.That(s2.Turn, Is.EqualTo(s1.Turn + 1));
@@ -250,7 +250,7 @@ public class MovementAdjudicatorTest
 
         // Confirm the future was created
         Season s2 = updated.GetSeason(s1.Timeline, s1.Turn + 1);
-        Assert.That(s2.Past, Is.EqualTo(s1));
+        Assert.That(s2.PastId, Is.EqualTo(s1.ToString()));
         Assert.That(s2.Futures, Is.Empty);
         Assert.That(s2.Timeline, Is.EqualTo(s1.Timeline));
         Assert.That(s2.Turn, Is.EqualTo(s1.Turn + 1));