hypre/lapack/dlassq.c

#include "hypre_lapack.h"
#include "f2c.h"

/* Subroutine */ HYPRE_Int dlassq_(integer *n, doublereal *x, integer *incx, 
	doublereal *scale, doublereal *sumsq)
{
/*  -- LAPACK auxiliary routine (version 3.0) --   
       Univ. of Tennessee, Univ. of California Berkeley, NAG Ltd.,   
       Courant Institute, Argonne National Lab, and Rice University   
       June 30, 1999   


    Purpose   
    =======   

    DLASSQ  returns the values  scl  and  smsq  such that   

       ( scl**2 )*smsq = x( 1 )**2 +...+ x( n )**2 + ( scale**2 )*sumsq,   

    where  x( i ) = X( 1 + ( i - 1 )*INCX ). The value of  sumsq  is   
    assumed to be non-negative and  scl  returns the value   

       scl = max( scale, abs( x( i ) ) ).   

    scale and sumsq must be supplied in SCALE and SUMSQ and   
    scl and smsq are overwritten on SCALE and SUMSQ respectively.   

    The routine makes only one pass through the vector x.   

    Arguments   
    =========   

    N       (input) INTEGER   
            The number of elements to be used from the vector X.   

    X       (input) DOUBLE PRECISION array, dimension (N)   
            The vector for which a scaled sum of squares is computed.   
               x( i )  = X( 1 + ( i - 1 )*INCX ), 1 <= i <= n.   

    INCX    (input) INTEGER   
            The increment between successive values of the vector X.   
            INCX > 0.   

    SCALE   (input/output) DOUBLE PRECISION   
            On entry, the value  scale  in the equation above.   
            On exit, SCALE is overwritten with  scl , the scaling factor   
            for the sum of squares.   

    SUMSQ   (input/output) DOUBLE PRECISION   
            On entry, the value  sumsq  in the equation above.   
            On exit, SUMSQ is overwritten with  smsq , the basic sum of   
            squares from which  scl  has been factored out.   

   =====================================================================   


       Parameter adjustments */
    /* System generated locals */
    integer i__1, i__2;
    doublereal d__1;
    /* Local variables */
    static doublereal absxi;
    static integer ix;

    --x;

    /* Function Body */
    if (*n > 0) {
	i__1 = (*n - 1) * *incx + 1;
	i__2 = *incx;
	for (ix = 1; i__2 < 0 ? ix >= i__1 : ix <= i__1; ix += i__2) {
	    if (x[ix] != 0.) {
		absxi = (d__1 = x[ix], abs(d__1));
		if (*scale < absxi) {
/* Computing 2nd power */
		    d__1 = *scale / absxi;
		    *sumsq = *sumsq * (d__1 * d__1) + 1;
		    *scale = absxi;
		} else {
/* Computing 2nd power */
		    d__1 = absxi / *scale;
		    *sumsq += d__1 * d__1;
		}
	    }
/* L10: */
	}
    }
    return 0;

/*     End of DLASSQ */

} /* dlassq_ */