package rebelsky.exam2.solutions;

import java.util.Comparator;

/**
 * Something that knows how to do binary search on arrays using a
 * specified comparator.
 *
 * @author Samuel A. Rebelsky
 * @author YOUR NAME HERE
 */
public class BinarySearcher
{
// +--------+--------------------------------------------------
// | Fields |
// +--------+

	/** 
	 * The comparator used to determine the order of
	 * elements in an array.
	 */
	Comparator metric;

// +--------------+--------------------------------------------
// | Constructors |
// +--------------+

	/**
	 * Build a new searcher that uses a particular
	 * comparator.
	 */
	public BinarySearcher(Comparator _metric)
	{
		this.metric = _metric;
	} // BinarySearcher(Comparator)


// +----------------+------------------------------------------
// | Public Methods |
// +----------------+

	/**
	 * Searches for findMe in stuff.  This should take
	 * O(log_2(stuff.length)) steps.
	 *
	 * @pre
	 *    (1) stuff must be in increasing order according to the previously-provided metric.  metric.compare(stuff[i],stuff[i+1]) <= 0 for all "reasonable" i.
	 *    (2) findMe must be an object that metric can compare to elements of stuff.  In particular metric.compare(findMe,stuff[i]) and metric.compare(stuff[i],findMe) must be valid calls.
	 *    (3) stuff must be non-empty.  (It's stupid to look in an empty array.)
	 * @post
	 *    (1) stuff is not changed.
	 *    (2) Returns the index of something equal to findMe.  That is, returns an i such that metric.compare(stuff[i],findMe) == 0
	 * @exception Exception
	 *    If nothing equal to findMe appears in stuff.
	 */
	public int binarySearch(Object findMe, Object[] stuff)
		throws Exception
	{
		int lb = 0;	// The lower-bound of the portion of interest
		int ub = stuff.length - 1;	// The upper-bound
		while (lb <= ub) {
			int mid = (lb + ub)/2;
			int c = metric.compare(findMe, stuff[mid]);
			if (c == 0) // Found it!
				return mid;
			else if (c < 0)  // Middle element too big!
				ub = mid-1;
			else // Middle element too small!
				lb = mid+1;
		} // while
		// Whoops, doesn't seem to be there
		throw new Exception("Not found!");
	} // binarySearch(Object)

} // class BinarySearcher