package ca.uhn.hl7v2.util;

import java.io.*;

/**
 * Creates unique message IDs.  IDs are stored in a file called <hapi.home>/id_file for persistence
 * across JVM sessions.  Note that if one day you run the JVM with a new working directory,
 * you must move or copy id_file into this directory so that new ID numbers will begin
 * with the last one used, rather than starting over again.
 * @author Neal Acharya
 */
public class MessageIDGenerator {
    
    private static MessageIDGenerator messageIdGenerator;
    private long id;
    private FileWriter fileW;
    private static final String idFile = Home.getHomeDirectory().getAbsolutePath() + "/id_file";
    
    /**
     * Constructor
     * Creates an instance of the class
     * Its reads an id (longint#) from an external file, if one is not present then the external file
     * is created and initialized to zero.
     * This id is stored into the private field of id.
     */
    private  MessageIDGenerator() throws IOException {
        /*check external file for the last value unique id value generated by
        this class*/
        try{
            // We should check to see if the external file for storing the unique ids exists
            File extFile = new File(idFile);
            if (extFile.createNewFile()== true){
                /*there was no existing file so a new one has been created with createNewFile method.  The
                file is stored at  <hapi.home>/id_file.txt */
                // We can simply initialize the private id field to zero
                id = 0;
                
            }//end if
            else{
                /*The file does exist which is why we received false from the
                createNewFile method. We should now try to read from this file*/
                FileReader fileR = new FileReader(idFile);
                char[] charArray = new char[100];
                int e = fileR.read(charArray);
                if (e <= 0){
                    /*We know the file exists but it has no value stored in it. So at this point we can simply initialize the
                    private id field to zero*/
                    id = 0;
                }//end if
                else{
                    /* Here we know that the file exists and has a stored value. we should read this value and set the
                    private id field to it*/
                    String idStr = String.valueOf(charArray);
                    String idStrTrim = idStr.trim();
                    id = Long.parseLong(idStrTrim);
                }//end else
                //Fix for bug 1100881:  Close the file after writing.
                fileR.close();
            }//end else
        }//end try
        catch (FileNotFoundException e){
            System.out.println(e);
        }//end catch
    }//end constructor code
    
    /**
     * Synchronized method used to return the single (static) instance of the class
     */
    public static synchronized MessageIDGenerator getInstance() throws IOException {
        if (messageIdGenerator == null)
            messageIdGenerator = new MessageIDGenerator();
        return messageIdGenerator;
    }//end method
    
    /**
     * Synchronized method used to return the incremented id value
     */
    public synchronized String getNewID()throws IOException{
        //increment the private field
        id = id + 1;
        //create an instance of the Filewriter Object pointing to "C:\\extfiles\\Idfile.txt"
        fileW = new FileWriter(idFile, false);
        //write the id value to the file
        String idStr = String.valueOf(id);
        fileW.write(idStr);
        fileW.flush();
        fileW.close();
        return String.valueOf(id);
    }//end method
    
}