
import java.io.*;

import java.util.Map;
import java.util.TreeMap;
import java.util.Scanner;

/**
 * This program lets the user keep a persistent "phone book" that
 * contains names and phone numbers.  The data for the phone book
 * is stored in a file in the user's home directory.
 * 
 * The program is meant only as a demonstration of file use.  The phone
 * book data is stored is the form of Name/Number pairs, with
 * little error checking.  In particular, the "phone directory"
 * used in this program is not even close to what would be needed for
 * a real phone book application.
 */

public class PhoneDirectoryFileDemo {

    /**
     * The name of the file in which the phone book data is kept.  The
     * file is stored in the user's home directory.  The "." at the
     * beginning of the file name means that the file will be a
     * "hidden" file on Unix-based computers, including Linux and
     * MacOS.
     */
    private static String DATA_FILE_NAME = ".phone_book_demo";


    public static void main(String[] args) {

        String name, number;  // Name and number of an entry in the directory
                              // (used at various places in the program).

        TreeMap<String,String>  phoneBook;   // Phone directory data structure.
                                             // Entries are name/number pairs.

        phoneBook = new TreeMap<String,String>();


        /* Create a dataFile variable of type File to represent the
         * data file that is stored in the user's home directory.
         */

        File userHomeDirectory = new File( System.getProperty("user.home") );
        File dataFile = new File( userHomeDirectory, DATA_FILE_NAME );


        /* If the data file already exists, then the data in the file is
         * read and is used to initialize the phone directory.  The format
         * of the file must be as follows:  Each line of the file represents
         * one directory entry, with the name and the number for that entry
         * separated by the character '%'.  If a file exists but does not
         * have this format, then the program terminates; this is done to
         * avoid overwriting a file that is being used for another purpose.
         */

        if ( ! dataFile.exists() ) {
            System.out.println("No phone book data file found.  A new one");
            System.out.println("will be created, if you add any entries.");
            System.out.println("File name:  " + dataFile.getAbsolutePath());
        }
        else {
            System.out.println("Reading phone book data...");
            try( Scanner scanner = new Scanner(dataFile) ) {
                while (scanner.hasNextLine()) {
                    String phoneEntry = scanner.nextLine();
                    int separatorPosition = phoneEntry.indexOf('%');
                    if (separatorPosition == -1)
                        throw new IOException("File is not a phonebook data file.");
                    name = phoneEntry.substring(0, separatorPosition);
                    number = phoneEntry.substring(separatorPosition+1);
                    phoneBook.put(name,number);
                }
            }
            catch (IOException e) {
                System.out.println("Error in phone book data file.");
                System.out.println("File name:  " + dataFile.getAbsolutePath());
                System.out.println("This program cannot continue.");
                System.exit(1);
            }
        }


        /* Read commands from the user and carry them out, until the
         * user gives the "Exit from program" command.
         */

        Scanner in = new Scanner( System.in );
        boolean changed = false;  // Have any changes been made to the directory?

        mainLoop: while (true) {
            System.out.println("\nSelect the action that you want to perform:");
            System.out.println("   1.  Look up a phone number.");
            System.out.println("   2.  Add or change a phone number.");
            System.out.println("   3.  Remove an entry from your phone directory.");
            System.out.println("   4.  List the entire phone directory.");
            System.out.println("   5.  Exit from the program.");
            System.out.println("Enter action number (1-5):  ");
            int command;
            if ( in.hasNextInt() ) {
                command = in.nextInt();
                in.nextLine();
            }
            else {
                System.out.println("\nILLEGAL RESPONSE.  YOU MUST ENTER A NUMBER.");
                in.nextLine();
                continue;
            }
            switch(command) {
            case 1:
                System.out.print("\nEnter the name whose number you want to look up: ");
                name = in.nextLine().trim().toLowerCase();
                number = phoneBook.get(name);
                if (number == null)
                    System.out.println("\nSORRY, NO NUMBER FOUND FOR " + name);
                else
                    System.out.println("\nNUMBER FOR " + name + ":  " + number);
                break;
            case 2:
                System.out.print("\nEnter the name: ");
                name = in.nextLine().trim().toLowerCase();
                if (name.length() == 0)
                    System.out.println("\nNAME CANNOT BE BLANK.");
                else if (name.indexOf('%') >= 0)
                    System.out.println("\nNAME CANNOT CONTAIN THE CHARACTER \"%\".");
                else { 
                    System.out.print("Enter phone number: ");
                    number = in.nextLine().trim();
                    if (number.length() == 0)
                        System.out.println("\nPHONE NUMBER CANNOT BE BLANK.");
                    else {
                        phoneBook.put(name,number);
                        changed = true;
                    }
                }
                break;
            case 3:
                System.out.print("\nEnter the name whose entry you want to remove: ");
                name = in.nextLine().trim().toLowerCase();
                number = phoneBook.get(name);
                if (number == null)
                    System.out.println("\nSORRY, THERE IS NO ENTRY FOR " + name);
                else {
                    phoneBook.remove(name);
                    changed = true;
                    System.out.println("\nDIRECTORY ENTRY REMOVED FOR " + name);
                }
                break;
            case 4:
                System.out.println("\nLIST OF ENTRIES IN YOUR PHONE BOOK:\n");
                for ( Map.Entry<String,String> entry : phoneBook.entrySet() )
                    System.out.println("   " + entry.getKey() + ": " + entry.getValue() );
                break;
            case 5:
                System.out.println("\nExiting program.");
                break mainLoop;
            default:
                System.out.println("\nILLEGAL ACTION NUMBER.");
            }
        }


        /* Before ending the program, write the current contents of the
         * phone directory, but only if some changes have been made to
         * the directory.
         */

        if (changed) {
            System.out.println("Saving phone directory changes to file " + 
                    dataFile.getAbsolutePath() + " ...");
            PrintWriter out;
            try {
                out = new PrintWriter( new FileWriter(dataFile) );
            }
            catch (IOException e) {
                System.out.println("ERROR: Can't open data file for output.");
                return;
            }
            for ( Map.Entry<String,String> entry : phoneBook.entrySet() )
                out.println(entry.getKey() + "%" + entry.getValue() );
            out.flush();
            out.close();
            if (out.checkError())
                System.out.println("ERROR: Some error occurred while writing data file.");
            else
                System.out.println("Done.");
        }

    }

}
