slasq6.c

#include "clapack.h"

/* Subroutine */ int slasq6_(integer *i0, integer *n0, real *z__, integer *pp, 
	 real *dmin__, real *dmin1, real *dmin2, real *dn, real *dnm1, real *
	dnm2)
{
    /* System generated locals */
    integer i__1;
    real r__1, r__2;

    /* Local variables */
    real d__;
    integer j4, j4p2;
    real emin, temp;
    extern doublereal slamch_(char *);
    real safmin;


/*  -- LAPACK auxiliary routine (version 3.1) -- */
/*     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd.. */
/*     November 2006 */

/*     .. Scalar Arguments .. */
/*     .. */
/*     .. Array Arguments .. */
/*     .. */

/*  Purpose */
/*  ======= */

/*  SLASQ6 computes one dqd (shift equal to zero) transform in */
/*  ping-pong form, with protection against underflow and overflow. */

/*  Arguments */
/*  ========= */

/*  I0    (input) INTEGER */
/*        First index. */

/*  N0    (input) INTEGER */
/*        Last index. */

/*  Z     (input) REAL array, dimension ( 4*N ) */
/*        Z holds the qd array. EMIN is stored in Z(4*N0) to avoid */
/*        an extra argument. */

/*  PP    (input) INTEGER */
/*        PP=0 for ping, PP=1 for pong. */

/*  DMIN  (output) REAL */
/*        Minimum value of d. */

/*  DMIN1 (output) REAL */
/*        Minimum value of d, excluding D( N0 ). */

/*  DMIN2 (output) REAL */
/*        Minimum value of d, excluding D( N0 ) and D( N0-1 ). */

/*  DN    (output) REAL */
/*        d(N0), the last value of d. */

/*  DNM1  (output) REAL */
/*        d(N0-1). */

/*  DNM2  (output) REAL */
/*        d(N0-2). */

/*  ===================================================================== */

/*     .. Parameter .. */
/*     .. */
/*     .. Local Scalars .. */
/*     .. */
/*     .. External Function .. */
/*     .. */
/*     .. Intrinsic Functions .. */
/*     .. */
/*     .. Executable Statements .. */

    /* Parameter adjustments */
    --z__;

    /* Function Body */
    if (*n0 - *i0 - 1 <= 0) {
	return 0;
    }

    safmin = slamch_("Safe minimum");
    j4 = (*i0 << 2) + *pp - 3;
    emin = z__[j4 + 4];
    d__ = z__[j4];
    *dmin__ = d__;

    if (*pp == 0) {
	i__1 = *n0 - 3 << 2;
	for (j4 = *i0 << 2; j4 <= i__1; j4 += 4) {
	    z__[j4 - 2] = d__ + z__[j4 - 1];
	    if (z__[j4 - 2] == 0.f) {
		z__[j4] = 0.f;
		d__ = z__[j4 + 1];
		*dmin__ = d__;
		emin = 0.f;
	    } else if (safmin * z__[j4 + 1] < z__[j4 - 2] && safmin * z__[j4 
		    - 2] < z__[j4 + 1]) {
		temp = z__[j4 + 1] / z__[j4 - 2];
		z__[j4] = z__[j4 - 1] * temp;
		d__ *= temp;
	    } else {
		z__[j4] = z__[j4 + 1] * (z__[j4 - 1] / z__[j4 - 2]);
		d__ = z__[j4 + 1] * (d__ / z__[j4 - 2]);
	    }
	    *dmin__ = dmin(*dmin__,d__);
/* Computing MIN */
	    r__1 = emin, r__2 = z__[j4];
	    emin = dmin(r__1,r__2);
/* L10: */
	}
    } else {
	i__1 = *n0 - 3 << 2;
	for (j4 = *i0 << 2; j4 <= i__1; j4 += 4) {
	    z__[j4 - 3] = d__ + z__[j4];
	    if (z__[j4 - 3] == 0.f) {
		z__[j4 - 1] = 0.f;
		d__ = z__[j4 + 2];
		*dmin__ = d__;
		emin = 0.f;
	    } else if (safmin * z__[j4 + 2] < z__[j4 - 3] && safmin * z__[j4 
		    - 3] < z__[j4 + 2]) {
		temp = z__[j4 + 2] / z__[j4 - 3];
		z__[j4 - 1] = z__[j4] * temp;
		d__ *= temp;
	    } else {
		z__[j4 - 1] = z__[j4 + 2] * (z__[j4] / z__[j4 - 3]);
		d__ = z__[j4 + 2] * (d__ / z__[j4 - 3]);
	    }
	    *dmin__ = dmin(*dmin__,d__);
/* Computing MIN */
	    r__1 = emin, r__2 = z__[j4 - 1];
	    emin = dmin(r__1,r__2);
/* L20: */
	}
    }

/*     Unroll last two steps. */

    *dnm2 = d__;
    *dmin2 = *dmin__;
    j4 = (*n0 - 2 << 2) - *pp;
    j4p2 = j4 + (*pp << 1) - 1;
    z__[j4 - 2] = *dnm2 + z__[j4p2];
    if (z__[j4 - 2] == 0.f) {
	z__[j4] = 0.f;
	*dnm1 = z__[j4p2 + 2];
	*dmin__ = *dnm1;
	emin = 0.f;
    } else if (safmin * z__[j4p2 + 2] < z__[j4 - 2] && safmin * z__[j4 - 2] < 
	    z__[j4p2 + 2]) {
	temp = z__[j4p2 + 2] / z__[j4 - 2];
	z__[j4] = z__[j4p2] * temp;
	*dnm1 = *dnm2 * temp;
    } else {
	z__[j4] = z__[j4p2 + 2] * (z__[j4p2] / z__[j4 - 2]);
	*dnm1 = z__[j4p2 + 2] * (*dnm2 / z__[j4 - 2]);
    }
    *dmin__ = dmin(*dmin__,*dnm1);

    *dmin1 = *dmin__;
    j4 += 4;
    j4p2 = j4 + (*pp << 1) - 1;
    z__[j4 - 2] = *dnm1 + z__[j4p2];
    if (z__[j4 - 2] == 0.f) {
	z__[j4] = 0.f;
	*dn = z__[j4p2 + 2];
	*dmin__ = *dn;
	emin = 0.f;
    } else if (safmin * z__[j4p2 + 2] < z__[j4 - 2] && safmin * z__[j4 - 2] < 
	    z__[j4p2 + 2]) {
	temp = z__[j4p2 + 2] / z__[j4 - 2];
	z__[j4] = z__[j4p2] * temp;
	*dn = *dnm1 * temp;
    } else {
	z__[j4] = z__[j4p2 + 2] * (z__[j4p2] / z__[j4 - 2]);
	*dn = z__[j4p2 + 2] * (*dnm1 / z__[j4 - 2]);
    }
    *dmin__ = dmin(*dmin__,*dn);

    z__[j4 + 2] = *dn;
    z__[(*n0 << 2) - *pp] = emin;
    return 0;

/*     End of SLASQ6 */

} /* slasq6_ */