Add SequentialDataBase as a better SimpleDatabase

src/main/java/betterquesting/api2/storage/SequentialDataBase.java

@@ -0,0 +1,52 @@
+package betterquesting.api2.storage;
+
+import java.util.BitSet;
+
+/**
+ * a database implementation specialized for ids that're sequential, dense and not randomly generated
+ * <p>
+ * see {@link RandomIndexDatabase} for database specialized in handling randomly generated ids
+ */
+public class SequentialDataBase<T> extends AbstractDatabase<T> {
+
+    private final BitSet ids = new BitSet();
+    /**
+     * the "smallest acceptable" id, any id smaller than this is considered as used and can
+     * be skipped when searching for new id, this condition can be maintained by:
+     * - refresh lowerBound after new id is generated: {@link #nextID()}
+     * - refresh lowerBound after one id is removed: {@link #removeID(int)}
+     */
+    private int lowerBound = 0;
+
+    @Override
+    public synchronized int nextID() {
+        int next = ids.nextClearBit(lowerBound);
+        lowerBound = next + 1;
+        return next;
+    }
+
+    @Override
+    public synchronized DBEntry<T> add(int id, T value) {
+        DBEntry<T> result = super.add(id, value);
+        // Don't add when an exception is thrown
+        ids.set(id);
+        // lowerBound = id; //no, lowerBound will not be refreshed here, delay to next `nextID()` call instead
+        return result;
+    }
+
+    @Override
+    public synchronized boolean removeID(int key) {
+        boolean result = super.removeID(key);
+        if (result) {
+            ids.clear(key);
+            lowerBound = Math.min(key, lowerBound);
+        }
+        return result;
+    }
+
+    @Override
+    public synchronized void reset() {
+        super.reset();
+        ids.clear();
+    }
+}

src/main/java/betterquesting/api2/storage/SimpleDatabase.java

@@ -1,38 +1,8 @@
 package betterquesting.api2.storage;
 
-import java.util.BitSet;
-import java.util.Collections;
-import java.util.List;
-import java.util.TreeMap;
-
-public class SimpleDatabase<T> extends AbstractDatabase<T> {
-
-    private final BitSet idMap = new BitSet();
-
-    @Override
-    public synchronized int nextID() {
-        return idMap.nextClearBit(0);
-    }
-
-    @Override
-    public synchronized DBEntry<T> add(int id, T value) {
-        DBEntry<T> result = super.add(id, value);
-        // Don't add when an exception is thrown
-        idMap.set(id);
-        return result;
-    }
-
-    @Override
-    public synchronized boolean removeID(int key) {
-        boolean result = super.removeID(key);
-        if (result) idMap.clear(key);
-        return result;
-    }
-
-    @Override
-    public synchronized void reset() {
-        super.reset();
-        idMap.clear();
-    }
+/**
+ * @see SequentialDataBase
+ */
+public class SimpleDatabase<T> extends SequentialDataBase<T> {
 
 }