Issue 6540: Towing from MM lobby

megamek/src/megamek/client/Client.java

@@ -476,6 +476,13 @@ public void sendLoadEntity(int id, int loaderId, int bayNumber) {
         send(new Packet(PacketCommand.ENTITY_LOAD, id, loaderId, bayNumber));
     }
 
+    /**
+     * Sends a "tow entity" packet
+     */
+    public void sendTowEntity(int id, int tractorId) {
+        send(new Packet(PacketCommand.ENTITY_TOW, id, tractorId));
+    }
+
     public void sendExplodeBuilding(Building.DemolitionCharge charge) {
         send(new Packet(PacketCommand.BLDG_EXPLODE, charge));
     }

megamek/src/megamek/client/ui/swing/DeploymentDisplay.java

@@ -246,6 +246,7 @@ public void selectEntity(int en) {
             clientgui.updateFiringArc(ce());
             clientgui.showSensorRanges(ce());
             computeCFWarningHexes(ce());
+            computeTowLinkBreakageHexes(ce());
         } else {
             disableButtons();
             setNextEnabled(true);
@@ -260,6 +261,12 @@ private void computeCFWarningHexes(Entity ce) {
         clientgui.showCollapseWarning(warnList);
     }
 
+    private void computeTowLinkBreakageHexes(Entity ce) {
+        Game game = clientgui.getClient().getGame();
+        List<Coords> warnList = TowLinkWarning.findTowLinkIssues(game, ce, game.getBoard());
+        clientgui.showCollapseWarning(warnList);
+    }
+
     /** Enables relevant buttons and sets up for your turn. */
     private void beginMyTurn() {
         clientgui.maybeShowUnitDisplay();

megamek/src/megamek/client/ui/swing/TowLinkWarning.java

@@ -0,0 +1,184 @@
+/*
+ * Copyright (c) 2025 - The MegaMek Team. All Rights Reserved.
+ *
+ * This file is part of MegaMek.
+ *
+ * MegaMek is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * MegaMek is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with MegaMek. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package megamek.client.ui.swing;
+
+import megamek.common.*;
+import megamek.logging.MMLogger;
+
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+
+/**
+ * When deploying units that are towing units, let's mark hexes that would break the tow linkage
+ * with warning. or something
+ */
+public class TowLinkWarning {
+    private final static MMLogger logger = MMLogger.create(TowLinkWarning.class);
+
+    /**
+     *
+     * This is
+     * used by the {@link MovementDisplay} class.
+     *
+     * @param game {@link Game} provided by the phase display class
+     * @param entity {@link Entity} currently selected in the deployment phase.
+     * @param board {@link Board} board object with building data.
+     *
+     * @return returns a list of {@link Coords} that where warning flags
+     *         should be placed.
+     */
+    public static List<Coords> findTowLinkIssues(Game game, Entity entity, Board board) {
+        List<Coords> warnList = new ArrayList<Coords>();
+
+        List<Coords> validTractorCoords = findCoordsForTractor(game, entity, board);
+        List<Coords> validTrailerCoords = findCoordsForTrailer(game, entity, board);
+
+        //int tractorId = entity.getTowedBy();
+        //Entity tractor = game.getEntity(tractorId);
+        //if (tractorId != Entity.NONE && tractor != null) {
+        //    validTrailerCoords = findCoordsForTrailer(game, tractor, board);
+        //}
+
+        if (validTractorCoords == null && validTrailerCoords == null) {
+            return warnList;
+        }
+        var boardHeight = board.getHeight();
+        var boardWidth = board.getWidth();
+        for (int x = 0; x < boardWidth; x++) {
+            for (int y = 0; y < boardHeight; y++) {
+                Coords coords = new Coords(x, y);
+                if (board.isLegalDeployment(coords, entity)) {
+                    if (validTractorCoords != null && !validTractorCoords.contains(coords)) {
+                        warnList.add(coords);
+                    } else if (validTrailerCoords != null && !validTrailerCoords.contains(coords)) {
+                        warnList.add(coords);
+                    }
+                }
+            }
+        }
+
+        return warnList;
+    }
+
+    /**
+     * When deploying a tractor, return the coords that would let it attach to its assigned trailer.
+     * @param game
+     * @param tractor
+     * @param board
+     * @return List of coords that a tractor could go, empty if there are none. Null if the tractor isn't a tractor or pulling anything
+     */
+    protected static List<Coords> findCoordsForTractor(Game game, Entity tractor, Board board) {
+        int trailerId = tractor.getTowing();
+        Entity trailer = game.getEntity(trailerId);
+        if (trailerId == Entity.NONE || trailer == null || trailer.getDeployRound() != tractor.getDeployRound()) {
+            return null;
+        }
+
+        List<Coords> validCoords = new ArrayList<Coords>();
+
+        if (trailer.isDeployed()) {
+            //Can they stack? If so, add the trailer's hex as valid
+            if (Compute.stackingViolation(game, tractor.getId(), tractor.getPosition(), false) == null) {
+                validCoords.add(trailer.getPosition());
+            }
+            validCoords.add(trailer.getPosition().translated(trailer.getFacing(), 1));
+            // Let's add the typical adjacent hexes
+            validCoords.addAll(trailer.getPosition().allAdjacent());
+
+            //Except the one behind the trailer, a tractor can't be there!
+            validCoords.remove(trailer.getPosition().translated(trailer.getFacing(), -1));
+
+        } else {
+            var boardHeight = board.getHeight();
+            var boardWidth = board.getWidth();
+            for (int x = 0; x < boardWidth; x++) {
+                for (int y = 0; y < boardHeight; y++) {
+                    Coords coords = new Coords(x, y);
+                    if (board.isLegalDeployment(coords, tractor)) {
+                        int facing = tractor.getFacing();
+                        // Can our trailer deploy in any of the adjacent hexes?
+                        if (coords.allAdjacent().stream().anyMatch(c -> board.isLegalDeployment(c, trailer) && !trailer.isLocationProhibited(c))) {
+                            validCoords.add(coords);
+                        }
+                    }
+                }
+            }
+        }
+
+        return validCoords;
+    }
+
+    /**
+     * When deploying a tractor, return the coords that would let it attach to its assigned trailer.
+     * @param game
+     * @param trailer
+     * @param board
+     * @return List of coords that a tractor could go, empty if there are none. Null if the tractor isn't a tractor or pulling anything
+     */
+    protected static List<Coords> findCoordsForTrailer(Game game, Entity trailer, Board board) {
+        int tractorId = trailer.getTractor();
+        Entity tractor = game.getEntity(tractorId);
+        if (tractorId == Entity.NONE || tractor == null || tractor.getDeployRound() != trailer.getDeployRound()) {
+            return null;
+        }
+
+        List<Coords> validCoords = new ArrayList<Coords>();
+
+        if (tractor.isDeployed()) {
+            //Can they stack? If so, add the trailer's hex as valid
+            if (Compute.stackingViolation(game, trailer.getId(), trailer.getPosition(), false) == null) {
+                validCoords.add(tractor.getPosition());
+            }
+            // Let's add the hex behind us
+            validCoords.add(tractor.getPosition().translated(tractor.getFacing(), -1));
+            //validCoords.addAll(tractor.getPosition().allAdjacent());
+
+        } else {
+            var boardHeight = board.getHeight();
+            var boardWidth = board.getWidth();
+            for (int x = 0; x < boardWidth; x++) {
+                for (int y = 0; y < boardHeight; y++) {
+                    Coords coords = new Coords(x, y);
+                    if (board.isLegalDeployment(coords, trailer)) {
+                        // Can our trailer deploy in any of the adjacent hexes?
+                        if (coords.allAdjacent().stream().anyMatch(c -> board.isLegalDeployment(c, tractor) && !tractor.isLocationProhibited(c))) {
+                            validCoords.add(coords);
+                        }
+                    }
+                }
+            }
+        }
+
+        return validCoords;
+    }
+
+    private HashSet<Coords> getAllCoords(Board board) {
+        var boardHeight = board.getHeight();
+        var boardWidth = board.getWidth();
+        var coordsSet = new HashSet<Coords>();
+        for (int x = 0; x < boardWidth; x++) {
+            for (int y = 0; y < boardHeight; y++) {
+                coordsSet.add(new Coords(x, y));
+            }
+        }
+        return coordsSet;
+    }
+}

megamek/src/megamek/client/ui/swing/lobby/ChatLounge.java

@@ -1542,6 +1542,20 @@ void offloadFromDifferentOwner(Entity entity, Collection<Entity> updateCandidate
         }
     }
 
+    /**
+     * Set the provided trailer to be towed by the tractor.
+     */
+    void towBy(Entity trailer, int tractorId) {
+        Entity tractor = game().getEntity(tractorId);
+        if (tractor == null || !tractor.canTow(trailer.getId())) {
+            return;
+        }
+
+        getLocalClient(trailer).sendTowEntity(trailer.getId(), tractor.getId());
+        // TODO: it would probably be a good idea
+        // to disable some settings for loaded units in customMekDialog
+    }
+
     /**
      * Sends the entities in the given Collection to the Server.
      * Sends only those that can be edited, i.e. the player's own

megamek/src/megamek/client/ui/swing/lobby/LobbyActions.java

@@ -661,6 +661,37 @@
         }
     }
 
+    /**
+     *
+     * @param trailer Entity to be towed
+     * @param info
+     */
+    public void tow(Entity trailer, String info) {
+        StringTokenizer sTow = new StringTokenizer(info, ":");
+        int tractorId = Integer.parseInt(sTow.nextToken());
+        Entity tractor = game().getEntity(tractorId);
+        // Remove those entities from the candidates that are already carried by that
+        // loader
+        if (trailer == null || tractor == null || trailer.getTractor() == tractorId) {
+            return;
+        }
+
+        // If a unit of the selected units is currently loaded onto another, 2nd unit of
+        // the selected
+        // units, do not continue. The player should unload units first. This would
+        // require
+        // a server update offloading that second unit AND embarking it. Currently not
+        // possible
+        // as a single server update and updates for one unit shouldn't be chained.
+        if (tractor.getTowing() != Entity.NONE || trailer.getTractor() != Entity.NONE) {
+            LobbyErrors.showNoDualTow(frame());
+        }
+
+        StringBuilder errorMsg = new StringBuilder();
+
+            lobby.towBy(trailer, tractorId);
+    }
+
     /** Asks for a new name for the provided forceId and applies it. */
     void forceRename(int forceId) {
         if (forceId == Force.NO_FORCE) {