feat: add FallingPlugin to simulate gravity

Plugins like Walking or Leaping assume that entities are not affected by gravity.

To keep this assumption we introduce a new plugin to explicitly enable gravity for entities. This also needs to be reflected in the respective Behaviors movement plugin.

src/main/java/org/terasology/flexiblepathfinding/plugins/basic/FallingPlugin.java

@@ -0,0 +1,56 @@
+// Copyright 2022 The Terasology Foundation
+// SPDX-License-Identifier: Apache-2.0
+
+package org.terasology.flexiblepathfinding.plugins.basic;
+
+import org.joml.Vector3i;
+import org.joml.Vector3ic;
+import org.terasology.engine.world.WorldProvider;
+import org.terasology.engine.world.block.BlockRegion;
+
+/**
+ * A pathfinding plugin to model entities falling down based on gravity.
+ * <br>
+ * All blocks between {@code from} and {@code to} position must be penetrable.
+ * The target block must be {@link #isWalkable(Vector3ic) walkable}.
+ * <br>
+ * <pre>
+ *            o
+ *      +---+ v
+ *          | o
+ *          +---+
+ * </pre>
+ */
+// TODO: do we need this, or can we mimic the same behavior in WalkingMovementPlugin?
+public class FallingPlugin extends WalkingPlugin {
+
+    //TODO: definition of walkable is a bit questionable - how can you fall into liquid blocks?
+
+    public FallingPlugin(WorldProvider world, float width, float height) {
+        super(world, width, height);
+    }
+
+    @Override
+    public boolean isReachable(Vector3ic to, Vector3ic from) {
+        // only allowed to move 1 unit in negative y axis
+        if (to.x() - from.x() != 0 || to.z() - from.z() != 0 || to.y() - from.y() != -1) {
+            return false;
+        }
+
+        // check that all blocks passed through by this movement are penetrable
+        for (Vector3ic occupiedBlock : getOccupiedRegionRelative()) {
+            // the start/stop for this block in the occupied region
+            Vector3i occupiedBlockTo = new Vector3i(to).add(occupiedBlock);
+            Vector3i occupiedBlockFrom = new Vector3i(from).add(occupiedBlock);
+
+            BlockRegion movementBounds = new BlockRegion(occupiedBlockTo).union(occupiedBlockFrom);
+            for (Vector3ic block : movementBounds) {
+                if (!world.getBlock(block).isPenetrable()) {
+                    return false;
+                }
+            }
+        }
+
+        return isWalkable(to);
+    }
+}