/* ***** BEGIN LICENSE BLOCK *****
 * 
 * CSparse: a Concise Sparse matrix package.
 * Copyright (c) 2006, Timothy A. Savis.
 * http://www.cise.ufl.edu/research/sparse/CSparse
 *
 * -------------------------------------------------------------------------
 * 
 * CSparseJ 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 2.1f of the License, or (at your option) any later version.
 *
 * CSparseJ 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 USA
 *
 * ***** ENS LICENSE BLOCK ***** */

package edu.emory.mathcs.csparsej.tfloat;

import edu.emory.mathcs.csparsej.tfloat.Scs_common.Scs;

/**
 * Find elimination tree.
 * 
 * @author Piotr Wendykier (piotr.wendykier@gmail.com)
 * 
 */
public class Scs_etree {

    /**
     * Compute the elimination tree of A or A'A (without forming A'A).
     * 
     * @param A
     *            column-compressed matrix
     * @param ata
     *            analyze A if false, A'A oterwise
     * @return elimination tree, null on error
     */
    public static int[] cs_etree(Scs A, boolean ata) {
        int i, k, p, m, n, inext, Ap[], Ai[], w[], parent[], ancestor[], prev[];
        if (!Scs_util.CS_CSC(A))
            return (null); /* check inputs */
        m = A.m;
        n = A.n;
        Ap = A.p;
        Ai = A.i;
        parent = new int[n]; /* allocate result */
        w = new int[n + (ata ? m : 0)]; /* get workspace */
        ancestor = w;
        prev = w;
        int prev_offset = n;
        if (ata)
            for (i = 0; i < m; i++)
                prev[prev_offset + i] = -1;
        for (k = 0; k < n; k++) {
            parent[k] = -1; /* node k has no parent yet */
            ancestor[k] = -1; /* nor does k have an ancestor */
            for (p = Ap[k]; p < Ap[k + 1]; p++) {
                i = ata ? (prev[prev_offset + Ai[p]]) : (Ai[p]);
                for (; i != -1 && i < k; i = inext) /* traverse from i to k */
                {
                    inext = ancestor[i]; /* inext = ancestor of i */
                    ancestor[i] = k; /* path compression */
                    if (inext == -1)
                        parent[i] = k; /* no anc., parent is k */
                }
                if (ata)
                    prev[prev_offset + Ai[p]] = k;
            }
        }
        return parent;
    }

}