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CSparseJ is a Java port of CSparse: a Concise Sparse matrix package.
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/*
 * CXSparse: a Concise Sparse matrix package.
 * Copyright (C) 2006-2011, Timothy A. Davis.
 * Copyright (C) 2011-2012, Richard W. Lincoln.
 * http://www.cise.ufl.edu/research/sparse/CXSparse
 *
 * -------------------------------------------------------------------------
 *
 * CXSparseJ is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * CXSparseJ is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this Module; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301
 *
 */

package edu.emory.mathcs.csparsej.tdcomplex;

import edu.emory.mathcs.csparsej.tdcomplex.DZcs_common.DZcs;

import static edu.emory.mathcs.csparsej.tdcomplex.DZcs_util.CS_CSC ;
import static edu.emory.mathcs.csparsej.tdcomplex.DZcs_util.CS_MARK ;
import static edu.emory.mathcs.csparsej.tdcomplex.DZcs_util.CS_MARKED ;

/**
 * Nonzero pattern of kth row of Cholesky factor, L(k,1:k-1).
 *
 * @author Piotr Wendykier ([email protected])
 * @author Richard Lincoln ([email protected])
 *
 */
public class DZcs_ereach {

	/**
	 * Find nonzero pattern of Cholesky L(k,1:k-1) using etree and triu(A(:,k)).
	 * If ok, s[top..n-1] contains pattern of L(k,:).
	 *
	 * @param A
	 *            column-compressed matrix; L is the Cholesky factor of A
	 * @param k
	 *            find kth row of L
	 * @param parent
	 *            elimination tree of A
	 * @param s
	 *            size n, s[top..n-1] is nonzero pattern of L(k,1:k-1)
	 * @param s_offset
	 *            the index of the first element in array s
	 * @param w
	 *            size n, work array, w[0..n-1]>=0 on input, unchanged on output
	 *
	 * @return top in successful, -1 on error
	 */
	public static int cs_ereach(DZcs A, int k, int[] parent, int[] s, int s_offset, int[] w)
	{
		int i, p, n, len, top, Ap[], Ai[] ;
		if (!CS_CSC (A) || parent == null || s == null || w == null) return (-1) ; /* check inputs */
		top = n = A.n ; Ap = A.p ; Ai = A.i ;
		CS_MARK (w, k) ;			/* mark node k as visited */
		for (p = Ap [k] ; p < Ap [k + 1] ; p++)
		{
			i = Ai [p] ;			/* A(i,k) is nonzero */
			if (i > k) continue ;		/* only use upper triangular part of A */
			for (len = 0 ; !CS_MARKED (w, i) ; i = parent [i]) /* traverse up etree*/
			{
				s [s_offset + len++] = i ; 	/* L(k,i) is nonzero */
				CS_MARK (w, i) ;		/* mark i as visited */
			}
			while (len > 0)
				s[s_offset + --top] = s[s_offset + --len] ;  /* push path onto stack */
		}
		for (p = top ; p < n ; p++) CS_MARK(w, s [s_offset + p]) ;  /* unmark all nodes */
		CS_MARK (w, k) ;			/* unmark node k */
		return (top) ;				/* s [top..n-1] contains pattern of L(k,:)*/
	}

}