/**
 * Generates a binary coin (0 or 1) in strict alternating sequence. With every
 * flip of the coin, the coin changes from 0 to 1 or vice versa. The first call
 * returns a 0, then 1, then 0, and so on.
 * 
 * @correspondence |history| is even <==> doneEvenFlips
 * @author paolo
 */
public class AlternatingCoin implements RandomWithParity {

	/**
	 * True if and only if an even number of invocations to this AlternatingCoin
	 * have been made.
	 */
	private boolean doneEvenFlips;

	/**
	 * Initialially zero flips have been performed, hence
	 * doneEvenFlips is true;
	 * 
	 * @ensures doneEvenFlips = true;
	 */
	public AlternatingCoin() {
		this.doneEvenFlips = true;
	}
	
	/**
	 * Returns a 0 or a 1 (alternating) and ignoring upperBound parameter.
	 * 
	 * @param upperBound
	 *            an integer that is ignored
	 * @ensures doneEvenFlips = !#doneEvenFlips
	 * @return 0 iff #doneEvenFlips <br>
	 *          1 iff !#doneEvenFlips
	 * @see RandomWithParity#generateNumber(int)
	 */
	public int generateNumber(int upperBound) {
		if (this.doneEvenFlips) {
			this.doneEvenFlips = false;
			return 0;
		}
		this.doneEvenFlips = true;
		return 1;
	}
}