From aaf2e78730432ea08699b309e089a9fafe66a42f Mon Sep 17 00:00:00 2001
From: Tim Van Baak <tim.vanbaak@gmail.com>
Date: Fri, 6 Sep 2024 15:09:30 +0000
Subject: [PATCH] Implement coastal accessibility checks for move orders

---
 MultiversalDiplomacy/Model/OrderParser.cs | 47 ++++++++++++++---------
 1 file changed, 29 insertions(+), 18 deletions(-)

diff --git a/MultiversalDiplomacy/Model/OrderParser.cs b/MultiversalDiplomacy/Model/OrderParser.cs
index 6878f3b..fb99bc8 100644
--- a/MultiversalDiplomacy/Model/OrderParser.cs
+++ b/MultiversalDiplomacy/Model/OrderParser.cs
@@ -307,33 +307,44 @@ public class OrderParser(World world)
             : subject.Season.Turn;
 
         var destProvince = world.Map.Provinces.Single(province => province.Is(move.destProvince));
+        string? destLocationKey = null;
 
-        // DATC 4.B.6 requires that "irrelevant" locations like army to Spain nc be ignored.
-        // To satisfy this, any location of the wrong type is categorically ignored, so for an army the
-        // "north coast" location effectively doesn't exist here.
+        // DATC 4.B specifies how to interpret orders with missing or incorrect locations. These issues arise because
+        // of provinces with multiple locations of the same type, i.e. two-coast provinces in Classical. In general,
+        // DATC's only concern is to disambiguate the order, failing the order only when it is ineluctably ambiguous
+        // (4.B.1) or explicitly incorrect (4.B.3). Irrelevant or nonexistent locations can be ignored.
+        
+        // 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 => subject.Type == UnitType.Army,
             LocationType.Water => subject.Type == UnitType.Fleet,
             _ => false,
         });
-        // DATC 4.6.B also requires that unknown coasts be ignored. To satisfy this, an additional filter by name.
-        // Doing both of these filters means "A - Spain/nc" is as meaningful as "F - Spain/wc".
-        var matchingLocations = unitLocations.Where(loc => loc.Is(move.destLocation));
-
-        // If one location matched, use that location. If the coast is inaccessible to the subject, the order will
-        // be invalidated by a path check later to satisfy DATC 4.B.3.
-        string? destLocationKey = matchingLocations.FirstOrDefault(defaultValue: null)?.Key;
+        if (!unitLocations.Any()) return false; // If *no* locations match, the move is illegal
+        if (unitLocations.Count() == 1) destLocationKey ??= unitLocations.Single().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.
-            // If one location is accessible, DATC 4.B.2 requires that it be used.
-            // 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;
+            var matchingLocations = unitLocations.Where(loc => loc.Is(move.destLocation));
+            if (matchingLocations.Any()) destLocationKey ??= matchingLocations.Single().Key;
         }
+
+        // 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(subject.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.
+        if (destLocationKey is null) return false;
         
         order = new MoveOrder(power, subject, new(destTimeline, destTurn), destLocationKey);
         return true;