cs_spsolve.c

/*
 * CXSPARSE: a Concise Sparse Matrix package - Extended.
 * Copyright (c) 2006-2009, Timothy A. Davis.
 * http://www.cise.ufl.edu/research/sparse/CXSparse
 * 
 * CXSparse 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.
 * 
 * CXSparse 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
 */

#include "cs.h"
/* solve Gx=b(:,k), where G is either upper (lo=0) or lower (lo=1) triangular */
CS_INT cs_spsolve (cs *G, const cs *B, CS_INT k, CS_INT *xi, CS_ENTRY *x, const CS_INT *pinv,
    CS_INT lo)
{
    CS_INT j, J, p, q, px, top, n, *Gp, *Gi, *Bp, *Bi ;
    CS_ENTRY *Gx, *Bx ;
    if (!CS_CSC (G) || !CS_CSC (B) || !xi || !x) return (-1) ;
    Gp = G->p ; Gi = G->i ; Gx = G->x ; n = G->n ;
    Bp = B->p ; Bi = B->i ; Bx = B->x ;
    top = cs_reach (G, B, k, xi, pinv) ;        /* xi[top..n-1]=Reach(B(:,k)) */
    for (p = top ; p < n ; p++) x [xi [p]] = 0 ;    /* clear x */
    for (p = Bp [k] ; p < Bp [k+1] ; p++) x [Bi [p]] = Bx [p] ; /* scatter B */
    for (px = top ; px < n ; px++)
    {
        j = xi [px] ;                               /* x(j) is nonzero */
        J = pinv ? (pinv [j]) : j ;                 /* j maps to col J of G */
        if (J < 0) continue ;                       /* column J is empty */
        x [j] /= Gx [lo ? (Gp [J]) : (Gp [J+1]-1)] ;/* x(j) /= G(j,j) */
        p = lo ? (Gp [J]+1) : (Gp [J]) ;            /* lo: L(j,j) 1st entry */
        q = lo ? (Gp [J+1]) : (Gp [J+1]-1) ;        /* up: U(j,j) last entry */
        for ( ; p < q ; p++)
        {
            x [Gi [p]] -= Gx [p] * x [j] ;          /* x(i) -= G(i,j) * x(j) */
        }
    }
    return (top) ;                                  /* return top of stack */
}