/*! \file
Copyright (c) 2003, The Regents of the University of California, through
Lawrence Berkeley National Laboratory (subject to receipt of any required 
approvals from U.S. Dept. of Energy) 

All rights reserved. 

The source code is distributed under BSD license, see the file License.txt
at the top-level directory.
*/

#include <math.h>
#include "slu_mt_sdefs.h"

int_t psgst04(int_t n, int_t nrhs, float *x, int_t ldx, float *xact,
	    int_t ldxact, float rcond, float *resid)
{
/*
 * -- SuperLU MT routine (version 2.0) --
 * Lawrence Berkeley National Lab, Univ. of California Berkeley,
 * and Xerox Palo Alto Research Center.
 * September 10, 2007
 *
 *  Purpose   
 *  =======   
 *
 *  psgst04() computes the difference between a computed solution and the   
 *  true solution to a system of linear equations.   
 *  RESID =  ( norm(X-XACT) * RCOND ) / ( norm(XACT) * EPS ),   
 *  where RCOND is the reciprocal of the condition number and EPS is the 
 *  machine epsilon.   
 *
 *  Arguments   
 *  =========   
 *
 *  N       (input) INT_T   
 *          The number of rows of the matrices X and XACT.  N >= 0.   
 *
 *  NRHS    (input) INT_T   
 *          The number of columns of the matrices X and XACT.  NRHS >= 0. 
 *
 *  X       (input) DOUBLE PRECISION array, dimension (LDX,NRHS)   
 *          The computed solution vectors.  Each vector is stored as a   
 *          column of the matrix X.   
 *
 *  LDX     (input) INT_T   
 *          The leading dimension of the array X.  LDX >= max(1,N).   
 *
 *  XACT    (input) DOUBLE PRECISION array, dimension( LDX, NRHS )   
 *          The exact solution vectors.  Each vector is stored as a   
 *          column of the matrix XACT.   
 *
 *  LDXACT  (input) INT_T   
 *          The leading dimension of the array XACT.  LDXACT >= max(1,N). 
 *
 *  RCOND   (input) DOUBLE PRECISION   
 *          The reciprocal of the condition number of the coefficient   
 *          matrix in the system of equations.   
 *
 *  RESID   (output) DOUBLE PRECISION   
 *          The maximum over the NRHS solution vectors of   
 *          ( norm(X-XACT) * RCOND ) / ( norm(XACT) * EPS )   
 *
 *  ===================================================================== 
*/
    /* Table of constant values */
    int c__1 = 1;

    /* System generated locals */
    float d__1, d__2, d__3, d__4;

    /* Local variables */
    int_t    i, j;
    int_t    ix;
    int      n__1;
    float xnorm;
    float eps;
    float diffnm;

    /* Function prototypes */
    extern int isamax_(int *, float *, int *);
    extern double slamch_(char *);

    /* Quick exit if N = 0 or NRHS = 0. */
   if ( n <= 0 || nrhs <= 0 ) {
	*resid = 0.;
	return 0;
    }

    /* Exit with RESID = 1/EPS if RCOND is invalid. */

    eps = slamch_("Epsilon");
    if ( rcond < 0. ) {
	*resid = 1. / eps;
	return 0;
    }

    /* Compute the maximum of norm(X - XACT) / ( norm(XACT) * EPS )   
       over all the vectors X and XACT . */

    *resid = 0.;
    for (j = 0; j < nrhs; ++j) {
	n__1 = n;
	ix = idamax_(&n__1, &xact[j*ldxact], &c__1);
        xnorm = (d__1 = xact[ix-1 + j*ldxact], fabs(d__1));

	diffnm = 0.;
	for (i = 0; i < n; ++i) {
	    /* Computing MAX */
	    d__3 = diffnm;
            d__4 = (d__1 = x[i+j*ldx]-xact[i+j*ldxact], fabs(d__1));
	    diffnm = SUPERLU_MAX(d__3,d__4);
	}
	if (xnorm <= 0.) {
	    if (diffnm > 0.) {
		*resid = 1. / eps;
	    }
	} else {
	    /* Computing MAX */
	    d__1 = *resid, d__2 = diffnm / xnorm * rcond;
	    *resid = SUPERLU_MAX(d__1,d__2);
	}
    }
    if (*resid * eps < 1.) {
	*resid /= eps;
    }

    return 0;

} /* psgst04 */