Added lazy mode (untested)

src/main/java/morlok8k/MinecraftLandGenerator/MinecraftLandGenerator.java

@@ -7,6 +7,7 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
+import java.util.stream.Collectors;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
@@ -209,13 +210,31 @@ public static class ForceloadChunks extends CommandLineHelper {
 						+ "in slower generation. Smaller values will result in starting the server more often, which has some overhead and takes time. "
 						+ "Set this as high as possible without totally filling up your RAM.")
 		private int maxLoaded;
+		@Option(names = { "--lazy", "-l" },
+				description = "Scan the world for existing chunks and skip them.")
+		private boolean lazy;
 
 		@Override
 		protected void runGenerate() {
 			ArrayList<Vector2i> loadedChunks = new ArrayList<>();
 			for (int x = bounds.x; x < bounds.x + bounds.w; x++)
 				for (int z = bounds.z; z < bounds.z + bounds.h; z++)
 					loadedChunks.add(new Vector2i(x, z));
+			if (lazy) {
+				log.info("Searching for existing chunks to be skipped");
+				/*
+				 * All chunks to load -> The set of region files containing them -> (The chunks in those region files -> to world coordinates)
+				 * -> to list -> remove from original list
+				 */
+				int size = loadedChunks.size();
+				loadedChunks.removeAll(
+						loadedChunks.stream().map(v -> new Vector2i(v.x >> 5, v.y >> 5)).distinct()
+								.flatMap(v -> world.availableChunks(v, dimension)
+										.map(w -> new Vector2i((v.x << 5) | w.x, (v.y << 5) | w.y)))
+								.collect(Collectors.toList()));
+				log.debug("Removed " + (size - loadedChunks.size())
+						+ " chunks that are already present");
+			}
 			log.info("Generating world");
 			if (loadedChunks.size() < 5000)
 				log.debug("Chunks to generate: " + loadedChunks);
@@ -224,7 +243,7 @@ protected void runGenerate() {
 			for (int i = 0; i < stepCount; i++) {
 				List<Vector2i> batch = loadedChunks.subList(i * maxLoaded,
 						Math.min((i + 1) * maxLoaded, loadedChunks.size() - 1));
-				log.info("Generating batch " + i + " / " + stepCount + " with " + batch.size()
+				log.info("Generating batch " + (i + 1) + " / " + stepCount + " with " + batch.size()
 						+ " chunks");
 				try {
 					world.setLoadedChunks(batch, dimension);

src/main/java/morlok8k/MinecraftLandGenerator/World.java

@@ -1,19 +1,24 @@
 package morlok8k.MinecraftLandGenerator;
 
 import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.channels.FileChannel;
 import java.nio.file.FileAlreadyExistsException;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.Paths;
+import java.nio.file.StandardOpenOption;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
 import java.util.stream.Collectors;
 import java.util.stream.IntStream;
+import java.util.stream.Stream;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
@@ -198,4 +203,47 @@ private static List<Integer> generateLinearSpawnpoints(int start, int length, in
 		return IntStream.rangeClosed(0, (int) stepCount).mapToObj(i -> start + (int) (realStep * i))
 				.collect(Collectors.toList());
 	}
+
+	/**
+	 * List all available chunks in a region file.
+	 * 
+	 * @param regionX
+	 *            the x coordinate of the region file in the world
+	 * @param regionZ
+	 *            the z coordinate of the region file in the world
+	 * @param dimension
+	 *            the dimension of the region file in the world
+	 * @return a stream of all chunks in the given region file, in coordinates relative to the region file's origin (not in world coordinates)
+	 * @see #availableChunks(Vector2i, Dimension)
+	 */
+	public List<Vector2i> availableChunks(int regionX, int regionZ, Dimension dimension) {
+		return availableChunks(new Vector2i(regionX, regionZ), dimension)
+				.collect(Collectors.toList());
+	}
+
+	/**
+	 * List all available chunks in a region file.
+	 * 
+	 * @param regionCoords
+	 *            the coordinates of the region file
+	 * @param dimension
+	 *            the dimension of the region file in the world
+	 * @return a stream of all chunks in the given region file, in coordinates relative to the region file's origin (not in world coordinates)
+	 * @see #availableChunks(int, int, Dimension)
+	 */
+	public Stream<Vector2i> availableChunks(Vector2i regionCoords, Dimension dimension) {
+		Path path = world.resolve(dimension.path)
+				.resolve("r." + regionCoords.x + "." + regionCoords.y + ".mca");
+		if (!Files.exists(path)) return Collections.<Vector2i> emptyList().stream();
+		try (FileChannel channel = FileChannel.open(path, StandardOpenOption.READ)) {
+			ByteBuffer buffer = ByteBuffer.allocate(4096);
+			channel.read(buffer);
+			buffer.flip();
+			return IntStream.range(0, buffer.capacity()).filter(i -> buffer.get(i) != 0)
+					.mapToObj(i -> new Vector2i(i & 31, i >> 5));
+		} catch (IOException e) {
+			log.warn("Could not open region file " + path + ", assuming it contains no chunks", e);
+			return Collections.<Vector2i> emptyList().stream();
+		}
+	}
 }