add method isEmpty to PersistentMap

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

@@ -188,6 +188,13 @@ public class PersistentMap<K, V> implements AutoCloseable {
 
     private long version;
 
+    /**
+     * 
+     * @param path            file relative to {@code storageBasePath}
+     * @param storageBasePath base path
+     * @param keyEncoder      encoder for keys
+     * @param valueEncoder    encoder for values
+     */
     public PersistentMap(final Path path, final Path storageBasePath, final EncoderDecoder<K> keyEncoder,
             final EncoderDecoder<V> valueEncoder) {
         this.path = path;
@@ -633,6 +640,14 @@ public class PersistentMap<K, V> implements AutoCloseable {
         return stats;
     }
 
+    public synchronized boolean isEmpty() {
+        final long rootNodeOffset = readNodeOffsetOfRootNode();
+        final PersistentMapDiskNode node = getNode(rootNodeOffset);
+
+        final List<NodeEntry> entries = node.getEntries();
+        return entries.size() == 1; // the empty map has a single NodeEntry for the PersistentMapDiskNode.MAX_KEY
+    }
+
     private void swapFiles(final Path newFile) throws IOException {
         final Path backupFile = path.getParent().resolve(path.getFileName() + "."
                 + DateTimeFormatter.ofPattern("yyyyMMdd-HHmmss").format(OffsetDateTime.now()) + ".backup");