package com.uas.reservation.util;

import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * generate unique code
 *
 * @author sunyj
 * @since 2018/2/10 11:01
 */
public class CodeGenerator {

    private final static int MAX_OFFSET = 16 * 16 * 16 - 16 * 16;
    private volatile static CodeGenerator generator;
    private int offset = 0;

    private CodeGenerator() {
    }

    public static CodeGenerator getGenerator() {
        if (generator == null) {
            synchronized (CodeGenerator.class) {
                if (generator == null) {
                    generator = new CodeGenerator();
                }
            }
        }
        return generator;
    }

    /**
     * generate one code: yyyyMMddHHmm + AAA(offset in 3 byte)
     * <p>
     * ensure that codes will not duplicate as long as generated codes' number is not greater than {@link CodeGenerator#MAX_OFFSET} every second
     *
     * @return code
     */
    public synchronized String generate() {
        // offset, less than MAX_OFFSET
        offset = (offset + 1) % MAX_OFFSET + 16 * 16;
        return new SimpleDateFormat("yyyyMMddHHmm").format(new Date()) + Integer.toHexString(offset).toUpperCase();
    }
}