Implement coastal accessibility checks for support-move orders

MultiversalDiplomacy/Model/OrderParser.cs

@@ -403,36 +403,47 @@ public class OrderParser(World world)
             : target.Season.Turn;
 
         var destProvince = world.Map.Provinces.Single(province => province.Is(support.destProvince));
+        string? destLocationKey = null;
 
-        // DATC 4.B.6 requires that "irrelevant" locations like army to Spain nc be ignored.
+        // DATC 4.B specifies how to interpret orders with missing or incorrect locations. These issues arise because
-        // To satisfy this, any location of the wrong type is categorically ignored, so for an army the
+        // of provinces with multiple locations of the same type, i.e. two-coast provinces in Classical. In general,
-        // "north coast" location effectively doesn't exist here.
+        // DATC's only concern is to disambiguate the order, failing the order only when it is ineluctably ambiguous
-        // Note that target is used instead of subject, since it is possible to support a move to an inaccessible
+        // (4.B.1) or explicitly incorrect (4.B.3). Irrelevant or nonexistent locations can be ignored.
-        // coast as long as the subject can reach the province and the target can reach the location.
+        
+        // If there is only one possible location for the moving unit, that location is used. The idea of land and
+        // water locations is an implementation detail of 5dplomacy and not part of the Diplomacy rules, so they will
+        // usually be omitted, and so moving an army to any land province or a fleet to a non-multi-coast province is
+        // naturally unambiguous even without the location.
         var unitLocations = destProvince.Locations.Where(loc => loc.Type switch {
             LocationType.Land => target.Type == UnitType.Army,
             LocationType.Water => target.Type == UnitType.Fleet,
             _ => false,
         });
-        // DATC 4.6.B also requires that unknown coasts be ignored. To satisfy this, an additional filter by name.
+        if (!unitLocations.Any()) return false; // If *no* locations match, the move is illegal
-        // Doing both of these filters means "A - Spain/nc" is as meaningful as "F - Spain/wc".
+        if (unitLocations.Count() == 1) destLocationKey ??= unitLocations.Single().Key;
-        var matchingLocations = unitLocations.Where(loc => loc.Is(support.destLocation));
-
-        // If one location matched, use that location. If the coast is inaccessible to the target, the order will
-        // be invalidated by a path check later to satisfy DATC 4.B.3.
-        string? destLocationKey = matchingLocations.FirstOrDefault(defaultValue: null)?.Key;
 
+        // If more than one location is possible for the unit, the order must be disambiguated by the dest location
+        // or the physical realities of which coast is accessible. DATC 4.B.3 makes an order illegal if the location
+        // is specified but it isn't an accessible coast, so successfully specifying a location takes precedence over
+        // there being one accessible coast.
         if (destLocationKey is null) {
-            // If no location matched, location was omitted, nonexistent, or the wrong type.
+            var matchingLocations = unitLocations.Where(loc => loc.Is(support.destLocation));
-            // If one location is accessible, DATC 4.B.2 requires that it be used.
+            if (matchingLocations.Any()) destLocationKey ??= matchingLocations.Single().Key;
-            // If more than one location is accessible, DATC 4.B.1 requires the order fail.
-
-            // TODO check which locations are accessible per the above
-            destLocationKey = unitLocations.First().Key;
-
-            // return false;
         }
 
+        // If the order location didn't disambiguate the coasts, either because it's missing or it's nonsense, the
+        // order can be disambiguated by there being one accessible coast from the order source.
+        if (destLocationKey is null) {
+            Location source = world.Map.GetLocation(target.Location);
+            var accessibleLocations = destProvince.Locations.Where(loc => loc.Adjacents.Contains(source));
+            if (accessibleLocations.Count() == 1) destLocationKey ??= accessibleLocations.Single().Key;
+        }
+
+        // If the order is still ambiguous, fail per DATC 4.B.1. This also satisfies 4.B.4, which prefers for
+        // programmatic adjudicators with order validation to require the coasts instead of interpreting the ambiguous
+        // support by referring to the move order it supports.
+        if (destLocationKey is null) return false;
+
         var destLocation = world.Map.GetLocation(destLocationKey);
         order = new SupportMoveOrder(power, subject, target, new(destTimeline, destTurn), destLocation);
         return true;

MultiversalDiplomacyTests/OrderParserTest.cs

@@ -296,8 +296,40 @@ public class OrderParserTest
         World world = World.WithStandardMap().AddUnits("France F Portugal");
 
         Assert.That(
-            OrderParser.TryParseOrder(world, "France", "Portugal - Spain", out Order? northOrder),
+            OrderParser.TryParseOrder(world, "France", "Portugal - Spain", out Order? _),
             Is.False,
             "Should not parse ambiguous coastal move");
     }
+
+    [Test]
+    public void DisambiguateSupportToSingleAccessibleCoast()
+    {
+        World world = World.WithStandardMap().AddUnits("France F Gascony", "France F Marseilles");
+
+        Assert.That(
+            OrderParser.TryParseOrder(world, "France", "Gascony S Marseilles - Spain", out Order? northOrder),
+            "Failed to parse north coast order");
+        Assert.That(
+            OrderParser.TryParseOrder(world, "France", "Marseilles S Gascony - Spain", out Order? southOrder),
+            "Failed to parse south coast order");
+
+        Assert.That(northOrder, Is.TypeOf<SupportMoveOrder>(), "Unexpected north coast order");
+        Assert.That(southOrder, Is.TypeOf<SupportMoveOrder>(), "Unexpected south coast order");
+        Location northTarget = ((SupportMoveOrder)northOrder!).Location;
+        Location southTarget = ((SupportMoveOrder)southOrder!).Location;
+
+        Assert.That(northTarget.Name, Is.EqualTo("south coast"), "Unexpected disambiguation");
+        Assert.That(southTarget.Name, Is.EqualTo("north coast"), "Unexpected disambiguation");
+    }
+
+    [Test]
+    public void DisambiguateSupportToMultipleAccessibleCoasts()
+    {
+        World world = World.WithStandardMap().AddUnits("France F Portugal", "France F Marseilles");
+
+        Assert.That(
+            OrderParser.TryParseOrder(world, "France", "Marseilles S Portugal - Spain", out Order? _),
+            Is.False,
+            "Should not parse ambiguous coastal support");
+    }
 }