cs_happly.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"
/* apply the ith Householder vector to x */
CS_INT cs_happly (const cs *V, CS_INT i, double beta, CS_ENTRY *x)
{
    CS_INT p, *Vp, *Vi ;
    CS_ENTRY *Vx, tau = 0 ;
    if (!CS_CSC (V) || !x) return (0) ;     /* check inputs */
    Vp = V->p ; Vi = V->i ; Vx = V->x ;
    for (p = Vp [i] ; p < Vp [i+1] ; p++)   /* tau = v'*x */
    {
        tau += CS_CONJ (Vx [p]) * x [Vi [p]] ;
    }
    tau *= beta ;                           /* tau = beta*(v'*x) */
    for (p = Vp [i] ; p < Vp [i+1] ; p++)   /* x = x - v*tau */
    {
        x [Vi [p]] -= Vx [p] * tau ;
    }
    return (1) ;
}