A concurrent list, to replace the array list of old roots (work in progress)

h2/src/main/org/h2/mvstore/ConcurrentLinkedList.java

@@ -7,70 +7,51 @@ package org.h2.mvstore;
 
 import java.util.Iterator;
 
+
 /**
  * A very simple linked list that supports concurrent access. 
+ * Internally, it uses immutable objects. 
+ * It uses recursion and is not meant for long lists.
  * 
  * @param <K> the key type
  */
 public class ConcurrentLinkedList<K> {
     
-    volatile Entry<K> head;
-    private volatile Entry<K> tail;
+    static final Entry<?> NULL = new Entry<Object>(null, null);
+
+    @SuppressWarnings("unchecked")
+    volatile Entry<K> head = (Entry<K>) NULL;
     
     public K peekFirst() {
         Entry<K> x = head;
-        return x == null ? null : x.obj;
+        return x.obj;
     }
 
     public K peekLast() {
-        Entry<K> x = tail;
-        return x == null ? null : x.obj;
-    }
-
-    public void add(K obj) {
-        Entry<K> x = new Entry<K>(obj);
-        Entry<K> t = tail;
-        if (t != null) {
-            t.next = x;
+        Entry<K> x = head;
+        while (x != NULL && x.next != NULL) {
+            x = x.next;
         }
-        tail = x;
-        if (head == null) {
-            head = x;
+        return x.obj;
     }
+
+    public synchronized void add(K obj) {
+        head = Entry.append(head, obj);
     }
     
-    public boolean removeFirst(K obj) {
-        Entry<K> x = head;
-        if (x == null || x.obj != obj) {
+    public synchronized boolean removeFirst(K obj) {
+        if (head.obj != obj) {
             return false;
         }
-        if (head == tail) {
-            tail = x.next;
-        }
-        head = x.next;
+        head = head.next;
         return true;
     }
 
-    public boolean removeLast(K obj) {
-        Entry<K> x = head;
-        if (x == null) {
+    public synchronized boolean removeLast(K obj) {
+        if (peekLast() != obj) {
             return false;
         }
-        Entry<K> prev = null;
-        while (x.next != null) {
-            prev = x;
-            x = x.next;
-        }
-        if (x.obj != obj) {
-            return false;
-        }
-        if (prev != null) {
-            prev.next = null;
-        }
-        if (head == tail) {
-            head = prev;
-        }
-        tail = prev;
+        head = Entry.removeLast(head);
         return true;
     }
 
@@ -81,7 +62,7 @@ public class ConcurrentLinkedList<K> {
 
             @Override
             public boolean hasNext() {
-                return current != null;
+                return current != NULL;
             }
 
             @Override
@@ -106,9 +87,27 @@ public class ConcurrentLinkedList<K> {
         final K obj;
         Entry<K> next;
         
-        Entry(K obj) {
+        Entry(K obj, Entry<K> next) {
             this.obj = obj;
+            this.next = next;
+        }
+
+        @SuppressWarnings("unchecked")
+        static <K> Entry<K> append(Entry<K> list, K obj) {
+            if (list == NULL) {
+                return new Entry<K>(obj, (Entry<K>) NULL);
             }
+            return new Entry<K>(list.obj, append(list.next, obj));
+        }
+        
+        @SuppressWarnings("unchecked")
+        static <K> Entry<K> removeLast(Entry<K> list) {
+            if (list == NULL || list.next == NULL) {
+                return (Entry<K>) NULL;
+            }
+            return new Entry<K>(list.obj, removeLast(list.next));
+        }
+
     }
 
 }

h2/src/main/org/h2/mvstore/ConcurrentRing.java

-/*
- * Copyright 2004-2014 H2 Group. Multiple-Licensed under the MPL 2.0,
- * and the EPL 1.0 (http://h2database.com/html/license.html).
- * Initial Developer: H2 Group
- */
-package org.h2.mvstore;
-
-import java.util.Iterator;
-
-/**
- * A ring buffer that supports concurrent access.
- * 
- * @param <K> the key type
- */
-public class ConcurrentRing<K> {
-
-    K[] buffer;
-    volatile int readPos;
-    volatile int writePos;
-
-    @SuppressWarnings("unchecked")
-    public ConcurrentRing() {
-        buffer = (K[]) new Object[4];
-    }
-    
-    public K peekFirst() {
-        return buffer[getIndex(readPos)];
-    }
-
-    public K peekLast() {
-        return buffer[getIndex(writePos - 1)];
-    }
-
-    public void add(K obj) {
-        buffer[getIndex(writePos)] = obj;
-        writePos++;
-        if (writePos - readPos >= buffer.length) {
-            // double the capacity
-            @SuppressWarnings("unchecked")
-            K[] b2 = (K[]) new Object[buffer.length * 2];
-            for (int i = readPos; i < writePos; i++) {
-                K x = buffer[getIndex(i)];
-                int i2 = i & b2.length - 1;
-                b2[i2] = x;
-            }
-            buffer = b2;
-        }
-    }
-    
-    public boolean removeFirst(K obj) {
-        int p = readPos;
-        int idx = getIndex(p);
-        if (buffer[idx] != obj) {
-            return false;
-        }
-        buffer[idx] = null;
-        readPos = p + 1;
-        return true;
-    }
-
-    public boolean removeLast(K obj) {
-        int p = writePos;
-        int idx = getIndex(p - 1);
-        if (buffer[idx] != obj) {
-            return false;
-        }
-        buffer[idx] = null;
-        writePos = p - 1;
-        return true;
-    }
-    
-    int getIndex(int pos) {
-        return pos & buffer.length - 1;
-    }
-
-    public Iterator<K> iterator() {
-        return new Iterator<K>() {
-
-            int pos = readPos;
-
-            @Override
-            public boolean hasNext() {
-                return pos != writePos;
-            }
-
-            @Override
-            public K next() {
-                return buffer[getIndex(pos++)];
-            }
-
-            @Override
-            public void remove() {
-                throw DataUtils.newUnsupportedOperationException("remove");
-            }
-
-        };
-    }
-    
-}

h2/src/main/org/h2/mvstore/MVMap.java

@@ -8,7 +8,6 @@ package org.h2.mvstore;
 import java.util.AbstractList;
 import java.util.AbstractMap;
 import java.util.AbstractSet;
-import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.List;
@@ -54,8 +53,8 @@ public class MVMap<K, V> extends AbstractMap<K, V>
     private final DataType keyType;
     private final DataType valueType;
     
-    private ConcurrentRing<Page> oldRoots = 
-            new ConcurrentRing<Page>();
+    private ConcurrentLinkedList<Page> oldRoots = 
+            new ConcurrentLinkedList<Page>();
 
     private boolean closed;
     private boolean readOnly;