pp.A4-HandOverHandLocking/src/main/java/pp/ThreadsafeSimplifiedList.java

@@ -1,144 +1,97 @@
 package pp;
 
-import java.util.concurrent.locks.Lock;
+public class ThreadsafeSimplifiedList<T> implements SimplifiedList<T> {
-import java.util.concurrent.locks.ReentrantLock;
+  private Node<T> first;
+  private final Object headLock = new Object();
 
-public class ThreadsafeSimplifiedList<T>
+  private static class Node<U> {
-        implements SimplifiedList<T> {
+    U element;
+    Node<U> prev;
+    Node<U> next;
+    final Object lock = new Object();
 
-  private Node<T> head;
+    Node(U element, Node<U> prev, Node<U> next) {
-  private final Lock listLock = new ReentrantLock();
-
-  private class Node<U> {
-    private U element;
-    private Node<U> next;
-    private final Lock nodeLock;
-
-    private Node(U element, Node<U> prev, Node<U> next) {
-      super();
       this.element = element;
+      this.prev = prev;
       this.next = next;
-      this.nodeLock = new ReentrantLock();
     }
   }
 
-  /**
+  public ThreadsafeSimplifiedList() {
-   * Returns the element at the specified position in this list.
+    this.first = null;
-   *
-   * @param index index of the element to return
-   * @return the element at the specified position in this list
-   */
-  @Override
-  public T get(int index) {
-    this.listLock.lock();
-    var ptr = this.head;
-    if (ptr == null) {
-      this.listLock.unlock();
-      throw new IndexOutOfBoundsException(index + " out of bounds (empty list)");
-    }
-    ptr.nodeLock.lock();
-    this.listLock.unlock();
-    for (var i = 0; i < index; i++) {
-      if (ptr.next != null) {
-        ptr.next.nodeLock.lock();
-        var savePtr = ptr;
-        ptr = ptr.next;
-        savePtr.nodeLock.unlock();
-      } else {
-        ptr.nodeLock.unlock();
-        throw new IndexOutOfBoundsException(index + " out of bounds");
-      }
   }
 
-    try {
-      return delay(ptr.element);
-    } finally {
-      ptr.nodeLock.unlock();
-    }
-  }
-
-  /**
-   * Appends the specified element to the end of this list. There are no
-   * limitations on what elements may be added to this list.
-   *
-   * @param element element to be appended to this list
-   * @return true
-   * @see java.util.Collection#add(Object)
-   *
-   */
   @Override
   public boolean add(T element) {
-    this.listLock.lock();
+    Node<T> curr;
-    if (this.head != null) {
+
-      var ptr = this.head;
+    synchronized (headLock) {
-      ptr.nodeLock.lock();
+      if (first == null) {
-      this.listLock.unlock();
+        first = new Node<>(element, null, null);
-      while (ptr.next != null) {
-        ptr.next.nodeLock.lock();
-        var savePtr = ptr;
-        ptr = ptr.next;
-        savePtr.nodeLock.unlock();
-      }
-      ptr.next = new Node<>(element, ptr, null);
-      ptr.nodeLock.unlock();
-    } else {
-      this.head = new Node<>(element, null, null);
-      this.listLock.unlock();
-    }
         return true;
       }
+      curr = first;
+    }
+
+    while (true) {
+      Node<T> next;
+      synchronized (curr.lock) {
+        next = curr.next;
+        if (next == null) {
+          curr.next = new Node<>(element, curr, null);
+          return true;
+        }
+      }
+      curr = next;
+    }
+  }
+
+  @Override
+  public T get(int index) {
+    Node<T> curr;
+    synchronized (headLock) {
+      if (first == null) throw new IndexOutOfBoundsException();
+      curr = first;
+    }
+
+    synchronized (curr.lock) {
+      for (int i = 0; i < index; i++) {
+        Node<T> next = curr.next;
+        if (next == null) throw new IndexOutOfBoundsException();
+        synchronized (next.lock) {
+          curr = next;
+        }
+      }
+      return delay(curr.element);
+    }
+  }
 
-  /**
-   * Replaces the element at the specified position in this list with the
-   * specified element.
-   *
-   * @param index   index of the element to replace
-   * @param element element to be stored at the specified position
-   * @return the element previously at the specified position
-   */
   @Override
   public T set(int index, T element) {
-    this.listLock.lock();
+    Node<T> curr;
-    var ptr = this.head;
+    synchronized (headLock) {
-    if (ptr == null) {
+      if (first == null) throw new IndexOutOfBoundsException();
-      this.listLock.unlock();
+      curr = first;
-      throw new IndexOutOfBoundsException(index + " out of bounds (empty list)");
-    }
-    ptr.nodeLock.lock();
-    this.listLock.unlock();
-    for (var i = 0; i < index; i++) {
-      if (ptr.next != null) {
-        ptr.next.nodeLock.lock();
-        var savePtr = ptr;
-        ptr = ptr.next;
-        savePtr.nodeLock.unlock();
-      } else {
-        ptr.nodeLock.unlock();
-        throw new IndexOutOfBoundsException(index + " out of bounds");
-      }
     }
 
-    try {
+    synchronized (curr.lock) {
-      T old = ptr.element;
+      for (int i = 0; i < index; i++) {
-      ptr.element = element;
+        Node<T> next = curr.next;
+        if (next == null) throw new IndexOutOfBoundsException();
+        synchronized (next.lock) {
+          curr = next;
+        }
+      }
+      T old = curr.element;
+      curr.element = element;
       return old;
-    } finally {
-      ptr.nodeLock.unlock();
     }
   }
 
-  /**
-   * Returns true if this list contains no elements.
-   *
-   * @return true if this list contains no elements
-   */
   @Override
   public boolean isEmpty() {
-    this.listLock.lock();
+    synchronized (headLock) {
-    try {
+      return first == null;
-      return this.head == null;
-    } finally {
-      this.listLock.unlock();
     }
   }
 }