add a pointer to the root node

Before the offset of the root node was hard-coded.
Now the offset of the pointer to the root node is hard-coded.
That allows us to replace the root node.

block-storage/src/main/java/org/lucares/pdb/diskstorage/DiskStorage.java

@@ -250,4 +250,8 @@ public class DiskStorage implements AutoCloseable {
 	public long size() throws IOException {
 		return fileChannel.size();
 	}
+
+	public int minAllocationSize() {
+		return FREE_LIST_NODE_SIZE;
+	}
 }

block-storage/src/main/java/org/lucares/pdb/map/PersistentMap.java

@@ -18,7 +18,7 @@ public class PersistentMap {
 
 	private static final Charset UTF8 = StandardCharsets.UTF_8;
 	static final int BLOCK_SIZE = 4096;
-	private static final int ROOT_NODE_OFFEST = 4096;
+	static final long NODE_OFFSET_TO_ROOT_NODE = 8;
 
 	private final DiskStorage diskStore;
 
@@ -29,14 +29,20 @@ public class PersistentMap {
 
 	private void initIfNew() throws IOException {
 		if (diskStore.size() < BLOCK_SIZE) {
-			// this map is new:
+			final long nodeOffsetToRootNode = diskStore.allocateBlock(diskStore.minAllocationSize());
-			// 1. make sure that new blocks are aligned to the block size (for faster disk
+			Preconditions.checkEqual(nodeOffsetToRootNode, NODE_OFFSET_TO_ROOT_NODE,
+					"The offset of the pointer to the root node must be at a well known location. "
+							+ "Otherwise we would not be able to find it in an already existing file.");
+
+			// 2. make sure that new blocks are aligned to the block size (for faster disk
 			// IO)
 			diskStore.ensureAlignmentForNewBlocks(BLOCK_SIZE);
 
-			// 2. initialize an empty root node
+			// 3. initialize an empty root node
 			final long blockOffset = diskStore.allocateBlock(BLOCK_SIZE);
-			assert blockOffset == ROOT_NODE_OFFEST : "offset was: " + blockOffset;
+
+			// 4. upate pointer to root node
+			writeNodeOffsetOfRootNode(blockOffset);
 		}
 	}
 
@@ -62,11 +68,13 @@ public class PersistentMap {
 	}
 
 	public byte[] put(final byte[] key, final byte[] value) throws IOException {
-		return insert(ROOT_NODE_OFFEST, key, value);
+		final long rootNodeOffset = readNodeOffsetOfRootNode();
+		return insert(rootNodeOffset, key, value);
 	}
 
 	public byte[] get(final byte[] key) throws IOException {
-		final NodeEntry entry = findNodeEntry(ROOT_NODE_OFFEST, key);
+		final long rootNodeOffset = readNodeOffsetOfRootNode();
+		final NodeEntry entry = findNodeEntry(rootNodeOffset, key);
 
 		return entry == null ? null : entry.getValue();
 	}
@@ -180,7 +188,8 @@ public class PersistentMap {
 	}
 
 	public void visitPreOrder(final VisitorCallback visitor) throws IOException {
-		visitPreOrderRecursively(ROOT_NODE_OFFEST, visitor, 0);
+		final long rootNodeOffset = readNodeOffsetOfRootNode();
+		visitPreOrderRecursively(rootNodeOffset, visitor, 0);
 	}
 
 	private void visitPreOrderRecursively(final long nodeOffset, final VisitorCallback visitor, final int depth)
@@ -196,4 +205,16 @@ public class PersistentMap {
 			}
 		}
 	}
+
+	private long readNodeOffsetOfRootNode() throws IOException {
+		final DiskBlock diskBlock = diskStore.getDiskBlock(NODE_OFFSET_TO_ROOT_NODE, diskStore.minAllocationSize());
+
+		return diskBlock.getByteBuffer().getLong(0);
+	}
+
+	private void writeNodeOffsetOfRootNode(final long newNodeOffsetToRootNode) throws IOException {
+		final DiskBlock diskBlock = diskStore.getDiskBlock(NODE_OFFSET_TO_ROOT_NODE, diskStore.minAllocationSize());
+		diskBlock.getByteBuffer().putLong(0, newNodeOffsetToRootNode);
+		diskBlock.force();
+	}
 }

block-storage/src/test/java/org/lucares/pdb/map/PersistentMapTest.java

@@ -5,6 +5,7 @@ import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.HashMap;
 import java.util.UUID;
+import java.util.concurrent.atomic.AtomicInteger;
 
 import org.lucares.pdb.diskstorage.DiskStorage;
 import org.lucares.utils.file.FileUtils;
@@ -30,17 +31,21 @@ public class PersistentMapTest {
 
 	public void testSingleValue() throws Exception {
 		final Path file = dataDirectory.resolve("map.db");
+		final String value = "value1";
+		final String key = "key1";
 
 		try (final DiskStorage ds = new DiskStorage(file)) {
 			final PersistentMap map = new PersistentMap(ds);
 
-			final String value = "value1";
-			final String key = "key1";
-
 			Assert.assertNull(map.getAsString(key));
 
 			Assert.assertNull(map.put(key, value));
 
+			Assert.assertEquals(map.getAsString(key), value);
+		}
+		try (final DiskStorage ds = new DiskStorage(file)) {
+			final PersistentMap map = new PersistentMap(ds);
+
 			Assert.assertEquals(map.getAsString(key), value);
 		}
 	}
@@ -52,7 +57,7 @@ public class PersistentMapTest {
 		try (final DiskStorage ds = new DiskStorage(file)) {
 			final PersistentMap map = new PersistentMap(ds);
 
-			for (int i = 0; i < 100; i++) {
+			for (int i = 0; i < 200; i++) {
 
 				final String key = UUID.randomUUID().toString() + "__" + i;
 				final String value = "long value to waste some bytes " + i;
@@ -72,11 +77,19 @@ public class PersistentMapTest {
 
 		try (final DiskStorage ds = new DiskStorage(file)) {
 			final PersistentMap map = new PersistentMap(ds);
+
 			map.visitPreOrder((nodeEntry, depth) -> {
 				if (nodeEntry.isInnerNode()) {
 					System.out.println("    ".repeat(depth) + nodeEntry);
 				}
 			});
+			final AtomicInteger counter = new AtomicInteger();
+			map.visitPreOrder((nodeEntry, depth) -> counter.addAndGet(nodeEntry.isInnerNode() ? 1 : 0));
+
+//			Assert.assertEquals(counter.get(), 3,
+//					"number of nodes should be small. Any number larger than 3 indicates, "
+//							+ "that new inner nodes are created even though the existing inner "
+//							+ "nodes could hold the values");
 
 			for (final var entry : insertedValues.entrySet()) {
 				final String actualValue = map.getAsString(entry.getKey());

pdb-utils/src/main/java/org/lucares/utils/Preconditions.java

@@ -29,4 +29,21 @@ public class Preconditions {
 			throw new IllegalStateException();
 		}
 	}
+
+	/**
+	 * Check that the given values are equal. The check is done with
+	 * {@link Objects#equals(Object, Object)}
+	 *
+	 * @param actual   the actual value
+	 * @param expected the expected value
+	 * @param message  formatted with {@link MessageFormat}
+	 * @param args     arguments for the message
+	 */
+	public static void checkEqual(final Object actual, final Object expected, final String message,
+			final Object... args) {
+		if (!Objects.equals(actual, expected)) {
+			throw new IllegalStateException(
+					MessageFormat.format(message, args) + " Expected: " + actual + " equals " + expected);
+		}
+	}
 }