/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.
 */

// Contributors:     Mathias Bogaert
//                   joelr@viair.com

package org.apache.log4j.helpers;

import org.apache.log4j.spi.LoggingEvent;

/**
 * <code>BoundedFIFO</code> serves as the bounded first-in-first-out buffer
 * heavily used by the {@link org.apache.log4j.AsyncAppender}.
 * 
 * @author Ceki G&uuml;lc&uuml;
 * @since version 0.9.1
 */
public class BoundedFIFO {

    LoggingEvent[] buf;
    int numElements = 0;
    int first = 0;
    int next = 0;
    int maxSize;

    /**
     * Instantiate a new BoundedFIFO with a maximum size passed as argument.
     */
    public BoundedFIFO(int maxSize) {
	if (maxSize < 1) {
	    throw new IllegalArgumentException("The maxSize argument (" + maxSize + ") is not a positive integer.");
	}
	this.maxSize = maxSize;
	buf = new LoggingEvent[maxSize];
    }

    /**
     * Get the first element in the buffer. Returns <code>null</code> if there are
     * no elements in the buffer.
     */
    public LoggingEvent get() {
	if (numElements == 0)
	    return null;

	LoggingEvent r = buf[first];
	buf[first] = null; // help garbage collection

	if (++first == maxSize) {
	    first = 0;
	}
	numElements--;
	return r;
    }

    /**
     * Place a {@link LoggingEvent} in the buffer. If the buffer is full then the
     * event is <b>silently dropped</b>. It is the caller's responsability to make
     * sure that the buffer has free space.
     */
    public void put(LoggingEvent o) {
	if (numElements != maxSize) {
	    buf[next] = o;
	    if (++next == maxSize) {
		next = 0;
	    }
	    numElements++;
	}
    }

    /**
     * Get the maximum size of the buffer.
     */
    public int getMaxSize() {
	return maxSize;
    }

    /**
     * Return <code>true</code> if the buffer is full, that is, whether the number
     * of elements in the buffer equals the buffer size.
     */
    public boolean isFull() {
	return numElements == maxSize;
    }

    /**
     * Get the number of elements in the buffer. This number is guaranteed to be in
     * the range 0 to <code>maxSize</code> (inclusive).
     */
    public int length() {
	return numElements;
    }

    int min(int a, int b) {
	return a < b ? a : b;
    }

    /**
     * Resize the buffer to a new size. If the new size is smaller than the old size
     * events might be lost.
     * 
     * @since 1.1
     */
    synchronized public void resize(int newSize) {
	if (newSize == maxSize)
	    return;

	LoggingEvent[] tmp = new LoggingEvent[newSize];

	// we should not copy beyond the buf array
	int len1 = maxSize - first;

	// we should not copy beyond the tmp array
	len1 = min(len1, newSize);

	// er.. how much do we actually need to copy?
	// We should not copy more than the actual number of elements.
	len1 = min(len1, numElements);

	// Copy from buf starting a first, to tmp, starting at position 0, len1
	// elements.
	System.arraycopy(buf, first, tmp, 0, len1);

	// Are there any uncopied elements and is there still space in the new array?
	int len2 = 0;
	if ((len1 < numElements) && (len1 < newSize)) {
	    len2 = numElements - len1;
	    len2 = min(len2, newSize - len1);
	    System.arraycopy(buf, 0, tmp, len1, len2);
	}

	this.buf = tmp;
	this.maxSize = newSize;
	this.first = 0;
	this.numElements = len1 + len2;
	this.next = this.numElements;
	if (this.next == this.maxSize) // this should never happen, but again, it just might.
	    this.next = 0;
    }

    /**
     * Returns <code>true</code> if there is just one element in the buffer. In
     * other words, if there were no elements before the last {@link #put} operation
     * completed.
     */
    public boolean wasEmpty() {
	return numElements == 1;
    }

    /**
     * Returns <code>true</code> if the number of elements in the buffer plus 1
     * equals the maximum buffer size, returns <code>false</code> otherwise.
     */
    public boolean wasFull() {
	return (numElements + 1 == maxSize);
    }

}
