/**
	Program to create random balanced properly formed expressions.
*/

import java.io.*;
import java.util.*;

public class balanced
{
	private static Scanner keyboard = new Scanner(System.in);
	private static Random rand = new Random();
	private static final String PAIRS = "{}[]()<>";

public static void main(String[] args) throws IOException
{
	int size;
	for (int count=1; count <= 5; count++)
	{	size = getLength();
		System.out.println("nested: " + nested(size/2));
		System.out.println("sequenced: " + sequenced(size/2));
		System.out.println("mixed: " + expr(size));
		System.out.println();
	}
}

/**	Get an even length from the user
	@return the length
*/
public static int getLength() throws IOException
{
	int size = 1;
	do {
		System.out.print("enter even expression length: ");
		size = keyboard.nextInt();
	} while (size%2 != 0);
	return size;
}


	// PHASES I & II
/**	Generate a random balanced nested expression
	@param length the number of pairs in the expression
	@return the generated expression
*/
public static String nested(int length)
{
	if (length == 0)	// base case!
		return "";

	int choice;		
	if (length == 1)	// another base case
	{			// pick a pair to use
		choice = rand.nextInt(4);
		return PAIRS.substring(choice*2, choice*2 + 2);
	}

	// if length > 1 - pick the outside pair and recurse inside

	choice = rand.nextInt(4);
	return  PAIRS.charAt(choice*2) + nested(length-1) + 
			PAIRS.charAt(choice*2+1);
}

	// PHASE IV ----------------------------------------------------------
/**	Generate a random balanced expression from sequencing only
	@param length the number of pairs in the expression
	@return the generated expression
*/
public static String sequenced(int length)
{
	if (length == 0)	// base case!
		return "";

	int choice;		
	if (length == 1)	// another base case, pick a pair to use
	{			        
		choice = rand.nextInt(4);
		return PAIRS.substring(choice*2, choice*2 + 2);
	}

	// otherwise generate the first pair, and the rest recursively
	return sequenced(1) + sequenced(length-1);
}

	// PHASE V ----------------------------------------------------------
/**	Generate a random balanced expression
	@param length the total number of characters in the expression
	@return the generated expression
*/
public static String expr(int length)
{
	if (length == 0)	// base case!
		return "";

	if (length%2 == 1)
		return "Can't form an expression of an odd length";
	
	int choice;		
	if (length == 2)	// another base case, pick a pair to use
	{
		choice = rand.nextInt(4);
		return PAIRS.substring(choice*2, choice*2 + 2);
	}

	// if length > 2, first decide on nest or sequence
	boolean nest = rand.nextBoolean();
	if (nest)
	{
		choice = rand.nextInt(4);
		return  PAIRS.charAt(choice*2) + expr(length-2) + 
			PAIRS.charAt(choice*2+1);
	}

	// else sequence - decide on split
	choice = rand.nextInt(length/2-1) + 1;	// #pairs for left part
	return expr(choice*2) + expr(length-choice*2);
}

}