dlasq6.c

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/* dlasq6.f -- translated by f2c (version 20061008).
   You must link the resulting object file with libf2c:
        on Microsoft Windows system, link with libf2c.lib;
        on Linux or Unix systems, link with .../path/to/libf2c.a -lm
        or, if you install libf2c.a in a standard place, with -lf2c -lm
        -- in that order, at the end of the command line, as in
                cc *.o -lf2c -lm
        Source for libf2c is in /netlib/f2c/libf2c.zip, e.g.,

                http://www.netlib.org/f2c/libf2c.zip
*/

#include "f2c.h"
#include "blaswrap.h"

/* Subroutine */ int dlasq6_(integer *i0, integer *n0, doublereal *z__, 
        integer *pp, doublereal *dmin__, doublereal *dmin1, doublereal *dmin2, 
         doublereal *dn, doublereal *dnm1, doublereal *dnm2)
{
    /* System generated locals */
    integer i__1;
    doublereal d__1, d__2;

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


/*  -- LAPACK routine (version 3.2)                                    -- */

/*  -- Contributed by Osni Marques of the Lawrence Berkeley National   -- */
/*  -- Laboratory and Beresford Parlett of the Univ. of California at  -- */
/*  -- Berkeley                                                        -- */
/*  -- November 2008                                                   -- */

/*  -- LAPACK is a software package provided by Univ. of Tennessee,    -- */
/*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..-- */

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

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

/*  DLASQ6 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) DOUBLE PRECISION 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) DOUBLE PRECISION */
/*        Minimum value of d. */

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

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

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

/*  DNM1  (output) DOUBLE PRECISION */
/*        d(N0-1). */

/*  DNM2  (output) DOUBLE PRECISION */
/*        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 = dlamch_("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.) {
                z__[j4] = 0.;
                d__ = z__[j4 + 1];
                *dmin__ = d__;
                emin = 0.;
            } 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__ = min(*dmin__,d__);
/* Computing MIN */
            d__1 = emin, d__2 = z__[j4];
            emin = min(d__1,d__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.) {
                z__[j4 - 1] = 0.;
                d__ = z__[j4 + 2];
                *dmin__ = d__;
                emin = 0.;
            } 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__ = min(*dmin__,d__);
/* Computing MIN */
            d__1 = emin, d__2 = z__[j4 - 1];
            emin = min(d__1,d__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.) {
        z__[j4] = 0.;
        *dnm1 = z__[j4p2 + 2];
        *dmin__ = *dnm1;
        emin = 0.;
    } 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__ = min(*dmin__,*dnm1);

    *dmin1 = *dmin__;
    j4 += 4;
    j4p2 = j4 + (*pp << 1) - 1;
    z__[j4 - 2] = *dnm1 + z__[j4p2];
    if (z__[j4 - 2] == 0.) {
        z__[j4] = 0.;
        *dn = z__[j4p2 + 2];
        *dmin__ = *dn;
        emin = 0.;
    } 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__ = min(*dmin__,*dn);

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

/*     End of DLASQ6 */

} /* dlasq6_ */