/* -*- MaTX -*-
 *
 *  NAME
 *      diff_col() - Difference function for each column
 *      diff_row() - Difference function for each row
 *      diff()     - Same as diff_col
 *
 *  SYNOPSIS
 *     dx = diff_col(x)
 *         Matrix dx;
 *         Matrix x;
 *
 *     dx = diff_col(x, k)
 *         Matrix dx;
 *         Matrix x;
 *         Integer k;
 *
 *     dx = diff_row(x)
 *         Matrix dx;
 *         Matrix x;
 *
 *     dx = diff_row(x, k)
 *         Matrix dx;
 *         Matrix x;
 *         Integer k;
 *
 *     dx = diff(x)
 *         Matrix dx;
 *         Matrix x;
 *
 *     dx = diff(x, k)
 *         Matrix dx;
 *         Matrix x;
 *         Integer k;
 *
 *  DESCRIPTION
 *      If x is a vector, then diff(x) returns a vector of differences 
 *      between adjacent elements as [x(2)-x(1) x(3)-x(2) ... x(n)-x(n-1)].
 *
 *      If x is a matrix, 
 *
 *      diff_col(x) calculates the differences for each column:
 *      diff_col(x) = x(2:n,:) - x(1:n-1,:).
 *
 *      diff_row(x) calculates the differences for each row:
 *      diff_row(x) = x(:,2:n) - x(:,1:n-1).
 *
 *      diff(x,n), diff_col(x,n), and diff_row(x,n) are the n'th 
 *      difference functions.
 *
 *  SEE ALSO
 *      cumsum
 */


Func Matrix diff(x, k, ...)
	Matrix x;
	Integer k;
{

	error(nargchk(1, 2, nargs, "diff"));
	
	if (nargs == 1) {
		if (Rows(x) == 1 || Cols(x) == 1) {
			return diff_col(x);
		} else {
			error("diff(): Vector is expected.\n");
		}
	} else {
		if (Rows(x) == 1 || Cols(x) == 1) {
			return diff_col(x, k);
		} else {
			error("diff(): Vector is expected.\n");
		}
	}
}

Func Matrix diff_col(x, k, ...)
	Matrix x;
	Integer k;
{
	Matrix dx;
	Integer i,m,n;

	error(nargchk(1, 2, nargs, "diff_col"));

	if (nargs == 1) {
		k = 1;
	}
	
	if (k <= 0) {
		error("diff_col(): Index must be positive.\n");
	}

	for (i = 1; i <= k; i++) {
		{m,n} = size(x);
		if (m == 1) {
			dx = x(2:n) - x(1:n-1);
		} else {
			dx = x(2:m,:) - x(1:m-1,:);
		}
	}
	return dx;
}

Func Matrix diff_row(x, k, ...)
	Matrix x;
	Integer k;
{
	error(nargchk(1, 2, nargs, "diff_row"));
	
	if (nargs == 1) {
		return trans(diff_col(trans(x)));
	} else {
		return trans(diff_col(trans(x), k));
	}
}