import java.util.ArrayList;
import java.util.List;
import java.util.Set;
import java.util.TreeSet;

/**
 * This utility class contains some methods for lists of integers.
 * 
 * 
 * @author Franck van Breugel
 */
public class Test2E 
{
    private Test2E() {}

    /**
     * Returns an empty list of integers.
     * 
     * @return an empty list of integers.
     */
    public static List<Integer> empty()
    {
	return new ArrayList<Integer>();
    }
    
    /**
     * Returns the list containing the given number.
     * 
     * @param number an integer.
     * @return the list containing the given number.
     * @throws IllegalArgumentException if number <= 0
     */
    public static List<Integer> single(int number) throws IllegalArgumentException
    {
	if (number <= 0)
	{
	    throw new IllegalArgumentException();
	}
	else
	{
	    List<Integer> single = new ArrayList<Integer>();
	    single.add(number);
	    return single;
	}
    }
    
    /**
     * Returns the list consisting of the integer 1, 2, ..., number.
     * 
     * @param number an integer.
     * @pre. number > 0
     * @return the list consisting of the integer 1, 2, ..., number.
     */
    public static List<Integer> interval(int number)
    {
	List<Integer> interval = new ArrayList<Integer>();
	for (int n = 1; n <= number; n++)
	{
	    interval.add(n);
	}
	return interval;
    }
    
    /**
     * Given a string of digits, each pair of digits
     * represents an interval.  For example, the string<br/>
     * 14217944<br/>
     * represents the intervals<br/>
     * [1, 2, 3, 4], [], [7, 8, 9], [4]<br/>
     * Note that the second interval is empty since 2 > 1.  Combine
     * all these intervals and return the resulting sorted list.  For 
     * the above example, this gives us<br/>
     * [1, 2, 3, 4, 7, 8, 9]<br/>
     * For the string<br/>
     * 1446<br/>
     * the list [1, 2, 3, 4, 5, 6] is returned.  Note that the list
     * does not contain duplicates.
     * 
     * @param sequence a string of digits.
     * @pre. sequence != null && sequence.length() > 0 && sequence.length is even && sequence only contains digits
     * @return the union of the intervals specified by the given sequence.
     */
    public static List<Integer> union(String sequence)
    {
	Set<Integer> set = new TreeSet<Integer>();
	for (int i = 0; i < sequence.length(); i = i + 2)
	{
	    int first = sequence.charAt(i) - '0';
	    int second = sequence.charAt(i + 1) - '0';
	    for (int j = first; j <= second; j++)
	    {
		set.add(j);
	    }
	}

	List<Integer> list = new ArrayList<Integer>(set);
	return list;
    }
}