Implemented ForestGenerator that performs the "forestgen" or "//forest" commands.

src/main/java/com/sk89q/worldedit/EditSession.java

@@ -27,6 +27,7 @@ import com.sk89q.worldedit.blocks.BlockType;
 import com.sk89q.worldedit.expression.Expression;
 import com.sk89q.worldedit.expression.ExpressionException;
 import com.sk89q.worldedit.expression.runtime.RValue;
+import com.sk89q.worldedit.generator.ForestGenerator;
 import com.sk89q.worldedit.interpolation.Interpolation;
 import com.sk89q.worldedit.interpolation.KochanekBartelsInterpolation;
 import com.sk89q.worldedit.interpolation.Node;
@@ -2753,38 +2754,16 @@ public class EditSession {
      * @throws MaxChangedBlocksException
      */
     public int makeForest(Iterable<Vector2D> it, int upperY, int lowerY,
-                          double density, TreeGenerator treeGenerator) throws MaxChangedBlocksException {
-        if (upperY < lowerY) {
-            throw new IllegalArgumentException("upperY must be greater than or equal to lowerY");
-        }
+                          double density, TreeGenerator treeGenerator)
+            throws WorldEditException {
 
+        ForestGenerator generator = new ForestGenerator(this, treeGenerator, lowerY, upperY);
+        generator.setDensity(density);
         int affected = 0;
 
         for (Vector2D pt : it) {
-            // Don't want to be in the ground
-            if (!getBlock(pt.toVector(upperY)).isAir()) {
-                continue;
-            }
-
-            // The gods don't want a tree here
-            if (Math.random() >= density) {
-                continue;
-            } // def 0.05
-
-            for (int y = upperY; y >= lowerY; --y) {
-                // Check if we hit the ground
-                Vector testPt = pt.toVector(y);
-                int t = getBlock(testPt).getType();
-
-                if (t == BlockID.GRASS || t == BlockID.DIRT) {
-                    treeGenerator.generate(this, testPt.add(0, 1, 0));
-                    ++affected;
-                    break;
-                } else if (t == BlockID.SNOW) {
-                    setBlock(testPt, new BaseBlock(BlockID.AIR));
-                } else if (t != BlockID.AIR) { // Trees won't grow on this!
-                    break;
-                }
+            if (generator.apply(pt)) {
+                affected++;
             }
         }
 

src/main/java/com/sk89q/worldedit/generator/ForestGenerator.java

@@ -0,0 +1,96 @@
+/*
+ * WorldEdit, a Minecraft world manipulation toolkit
+ * Copyright (C) sk89q <http://www.sk89q.com>
+ * Copyright (C) WorldEdit team and contributors
+ *
+ * This program 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.
+ *
+ * This program 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 this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.sk89q.worldedit.generator;
+
+import com.sk89q.worldedit.EditSession;
+import com.sk89q.worldedit.MaxChangedBlocksException;
+import com.sk89q.worldedit.Vector;
+import com.sk89q.worldedit.Vector2D;
+import com.sk89q.worldedit.blocks.BaseBlock;
+import com.sk89q.worldedit.blocks.BlockID;
+import com.sk89q.worldedit.operation.GroundFindingFunction;
+import com.sk89q.worldedit.util.TreeGenerator;
+
+/**
+ * Generates forests by searching for the ground starting from the given upper Y
+ * coordinate for every column given.
+ */
+public class ForestGenerator extends GroundFindingFunction {
+
+    private final TreeGenerator treeGenerator;
+    private final EditSession editSession;
+    private double density;
+
+    /**
+     * Create a new instance.
+     *
+     * @param editSession the edit session
+     * @param treeGenerator a tree generator
+     * @param lowerY the lower Y boundary to end the ground search from
+     * @param upperY the upper Y boundary to start the ground search at
+     */
+    public ForestGenerator(EditSession editSession, TreeGenerator treeGenerator, int lowerY, int upperY) {
+        super(editSession, lowerY, upperY);
+        this.editSession = editSession;
+        this.treeGenerator = treeGenerator;
+    }
+
+    /**
+     * Set the density (0 <= density <= 1) which indicates the percentage chance
+     * that a tree will spawn in each column.
+     *
+     * @return the density
+     */
+    public double getDensity() {
+        return density;
+    }
+
+    /**
+     * Get the density (0 <= density <= 1) which indicates the percentage chance
+     * that a tree will spawn in each column.
+     *
+     * @param density the density
+     */
+    public void setDensity(double density) {
+        this.density = density;
+    }
+
+    @Override
+    protected boolean shouldContinue(Vector2D pt) {
+        return Math.random() < density;
+    }
+
+    @Override
+    protected boolean apply(Vector pt, BaseBlock block) throws MaxChangedBlocksException {
+        int t = block.getType();
+
+        if (t == BlockID.GRASS || t == BlockID.DIRT) {
+            treeGenerator.generate(editSession, pt.add(0, 1, 0));
+            return true;
+        } else if (t == BlockID.SNOW) {
+            editSession.setBlock(pt, new BaseBlock(BlockID.AIR));
+            return false;
+        } else { // Trees won't grow on this!
+            return false;
+        }
+    }
+
+
+}