import java.util.Random;

/**
 * A random number generator that returns a number within the specified bound
 * and of alternating parity. The number returned from the first call is even,
 * then odd, then even and so on.
 * 
 * @convention numCalls >= 0
 * @correspondence numCalls = |history|
 * @author paolo
 */
public class UnfilteredRandom implements RandomWithParity {

	/**
	 * Number of times the generateNumber method has been called.
	 */
	private int numCalls;
	
	/**
	 * Generates a stream of random numbers.
	 */
	private Random generator;

	/**
	 * Initially, no calls have been made.
	 * 
	 * @ensures numCalls = 0
	 */
	UnfilteredRandom() {
		this.numCalls = 0;
		this.generator = new Random();
	}

	/**
	 * Generates a random number in the specified interval.
	 * 
	 * @param upperBound an upper limit on the number to be returned
	 * @requires upperBound >= 1
	 * @alters numCalls
	 * @ensures numCalls = #numCalls + 1
	 * @return 0 <= generateNumber <= upperBound </br>
	 * generateNumber is even <==> #numCalls is even
	 */
	public int generateNumber(int upperBound) {
		int randomNumber;
		do {
			randomNumber = this.generator.nextInt(upperBound+1);
		} while (randomNumber % 2 != this.numCalls % 2);
		this.numCalls++;
		return randomNumber;
	}

}