Map events apart from the map: map_events/<map>/*.json of any mod merged by id (RpgMapEventResolver, RpgMapEventMerge); item Icon look for events; rpg_walk checks a pickup across leaving the map and loading a save

src/Data/RpgMapEventMerge.cs

@@ -0,0 +1,53 @@
+using System;
+using System.Collections.Generic;
+
+namespace Cthangover.Mods.Rpg
+{
+    /// <summary>
+    /// Builds the event list of every map from event files read in mod load order.
+    /// A new id is appended; an id already present (ignoring case) is replaced in
+    /// place, so the later file wins and keeps the original's drawing order. Mods
+    /// load after their dependencies, so the engine's replacement rule holds:
+    /// ship the same event id and declare <c>depends</c> on the owner to replace
+    /// it, ship a new id to add to any mod's map.
+    /// <para/>
+    /// Pure data, so the merge rules are tested without a mod set.
+    /// </summary>
+    public class RpgMapEventMerge
+    {
+        /// <summary>Events per map id (case-insensitive).</summary>
+        public Dictionary<string, List<RpgMapEvent>> Maps { get; } = new(StringComparer.OrdinalIgnoreCase);
+
+        /// <summary>
+        /// Adds the events of one file of <paramref name="modId"/> for map <paramref name="map"/>;
+        /// each event gets <see cref="RpgMapEvent.ModId"/> and <see cref="RpgMapEvent.File"/>.
+        /// Returns the ids that replaced an earlier event.
+        /// </summary>
+        public List<string> Add(string map, string modId, string file, IEnumerable<RpgMapEvent> events)
+        {
+            var replaced = new List<string>();
+            if (!Maps.TryGetValue(map, out var list))
+            {
+                list = new List<RpgMapEvent>();
+                Maps[map] = list;
+            }
+
+            foreach (var mapEvent in events ?? Array.Empty<RpgMapEvent>())
+            {
+                if (mapEvent == null || string.IsNullOrWhiteSpace(mapEvent.Id))
+                    continue;
+                mapEvent.ModId = modId;
+                mapEvent.File = file;
+                var index = list.FindIndex(e => string.Equals(e.Id, mapEvent.Id, StringComparison.OrdinalIgnoreCase));
+                if (index < 0)
+                {
+                    list.Add(mapEvent);
+                    continue;
+                }
+                list[index] = mapEvent;
+                replaced.Add(mapEvent.Id);
+            }
+            return replaced;
+        }
+    }
+}