Copyright 2010 the original author or authors.
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
/**
 * Copyright 2010 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.I0Itec.zkclient;

import java.io.Serializable;
import java.util.List;

import org.I0Itec.zkclient.exception.ZkNoNodeException;

public class DistributedQueue<T extends Serializable> {

    private static class Element<T> {
        private String _name;
        private T _data;

        public Element(String name, T data) {
            _name = name;
            _data = data;
        }

        public String getName() {
            return _name;
        }

        public T getData() {
            return _data;
        }
    }

    private ZkClient _zkClient;
    private String _root;

    private static final String ELEMENT_NAME = "element";

    public DistributedQueue(ZkClient zkClient, String root) {
        _zkClient = zkClient;
        _root = root;
    }

    public boolean offer(T element) {
        try {
            _zkClient.createPersistentSequential(_root + "/" + ELEMENT_NAME + "-", element);
        } catch (Exception e) {
            throw ExceptionUtil.convertToRuntimeException(e);
        }
        return true;
    }

    public T poll() {
        while (true) {
            Element<T> element = getFirstElement();
            if (element == null) {
                return null;
            }

            try {
                _zkClient.delete(element.getName());
                return element.getData();
            } catch (ZkNoNodeException e) {
                // somebody else picked up the element first, so we have to
                // retry with the new first element
            } catch (Exception e) {
                throw ExceptionUtil.convertToRuntimeException(e);
            }
        }
    }

    private String getSmallestElement(List<String> list) {
        String smallestElement = list.get(0);
        for (String element : list) {
            if (element.compareTo(smallestElement) < 0) {
                smallestElement = element;
            }
        }

        return smallestElement;
    }

    public boolean isEmpty() {
        return _zkClient.getChildren(_root).size() == 0;
    }

    @SuppressWarnings("unchecked")
    private Element<T> getFirstElement() {
        try {
            while (true) {
                List<String> list = _zkClient.getChildren(_root);
                if (list.size() == 0) {
                    return null;
                }
                String elementName = getSmallestElement(list);

                try {
                    return new Element<T>(_root + "/" + elementName, (T) _zkClient.readData(_root + "/" + elementName));
                } catch (ZkNoNodeException e) {
                    // somebody else picked up the element first, so we have to
                    // retry with the new first element
                }
            }
        } catch (Exception e) {
            throw ExceptionUtil.convertToRuntimeException(e);
        }
    }

    public T peek() {
        Element<T> element = getFirstElement();
        if (element == null) {
            return null;
        }
        return element.getData();
    }
}