slaic1.c

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/* slaic1.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"

/* Table of constant values */

static integer c__1 = 1;
static real c_b5 = 1.f;

/* Subroutine */ int slaic1_(integer *job, integer *j, real *x, real *sest, 
        real *w, real *gamma, real *sestpr, real *s, real *c__)
{
    /* System generated locals */
    real r__1, r__2, r__3, r__4;

    /* Builtin functions */
    double sqrt(doublereal), r_sign(real *, real *);

    /* Local variables */
    real b, t, s1, s2, eps, tmp, sine;
    extern doublereal sdot_(integer *, real *, integer *, real *, integer *);
    real test, zeta1, zeta2, alpha, norma, absgam, absalp;
    extern doublereal slamch_(char *);
    real cosine, absest;


/*  -- LAPACK auxiliary routine (version 3.2) -- */
/*     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd.. */
/*     November 2006 */

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

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

/*  SLAIC1 applies one step of incremental condition estimation in */
/*  its simplest version: */

/*  Let x, twonorm(x) = 1, be an approximate singular vector of an j-by-j */
/*  lower triangular matrix L, such that */
/*           twonorm(L*x) = sest */
/*  Then SLAIC1 computes sestpr, s, c such that */
/*  the vector */
/*                  [ s*x ] */
/*           xhat = [  c  ] */
/*  is an approximate singular vector of */
/*                  [ L     0  ] */
/*           Lhat = [ w' gamma ] */
/*  in the sense that */
/*           twonorm(Lhat*xhat) = sestpr. */

/*  Depending on JOB, an estimate for the largest or smallest singular */
/*  value is computed. */

/*  Note that [s c]' and sestpr**2 is an eigenpair of the system */

/*      diag(sest*sest, 0) + [alpha  gamma] * [ alpha ] */
/*                                            [ gamma ] */

/*  where  alpha =  x'*w. */

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

/*  JOB     (input) INTEGER */
/*          = 1: an estimate for the largest singular value is computed. */
/*          = 2: an estimate for the smallest singular value is computed. */

/*  J       (input) INTEGER */
/*          Length of X and W */

/*  X       (input) REAL array, dimension (J) */
/*          The j-vector x. */

/*  SEST    (input) REAL */
/*          Estimated singular value of j by j matrix L */

/*  W       (input) REAL array, dimension (J) */
/*          The j-vector w. */

/*  GAMMA   (input) REAL */
/*          The diagonal element gamma. */

/*  SESTPR  (output) REAL */
/*          Estimated singular value of (j+1) by (j+1) matrix Lhat. */

/*  S       (output) REAL */
/*          Sine needed in forming xhat. */

/*  C       (output) REAL */
/*          Cosine needed in forming xhat. */

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

/*     .. Parameters .. */
/*     .. */
/*     .. Local Scalars .. */
/*     .. */
/*     .. Intrinsic Functions .. */
/*     .. */
/*     .. External Functions .. */
/*     .. */
/*     .. Executable Statements .. */

    /* Parameter adjustments */
    --w;
    --x;

    /* Function Body */
    eps = slamch_("Epsilon");
    alpha = sdot_(j, &x[1], &c__1, &w[1], &c__1);

    absalp = dabs(alpha);
    absgam = dabs(*gamma);
    absest = dabs(*sest);

    if (*job == 1) {

/*        Estimating largest singular value */

/*        special cases */

        if (*sest == 0.f) {
            s1 = dmax(absgam,absalp);
            if (s1 == 0.f) {
                *s = 0.f;
                *c__ = 1.f;
                *sestpr = 0.f;
            } else {
                *s = alpha / s1;
                *c__ = *gamma / s1;
                tmp = sqrt(*s * *s + *c__ * *c__);
                *s /= tmp;
                *c__ /= tmp;
                *sestpr = s1 * tmp;
            }
            return 0;
        } else if (absgam <= eps * absest) {
            *s = 1.f;
            *c__ = 0.f;
            tmp = dmax(absest,absalp);
            s1 = absest / tmp;
            s2 = absalp / tmp;
            *sestpr = tmp * sqrt(s1 * s1 + s2 * s2);
            return 0;
        } else if (absalp <= eps * absest) {
            s1 = absgam;
            s2 = absest;
            if (s1 <= s2) {
                *s = 1.f;
                *c__ = 0.f;
                *sestpr = s2;
            } else {
                *s = 0.f;
                *c__ = 1.f;
                *sestpr = s1;
            }
            return 0;
        } else if (absest <= eps * absalp || absest <= eps * absgam) {
            s1 = absgam;
            s2 = absalp;
            if (s1 <= s2) {
                tmp = s1 / s2;
                *s = sqrt(tmp * tmp + 1.f);
                *sestpr = s2 * *s;
                *c__ = *gamma / s2 / *s;
                *s = r_sign(&c_b5, &alpha) / *s;
            } else {
                tmp = s2 / s1;
                *c__ = sqrt(tmp * tmp + 1.f);
                *sestpr = s1 * *c__;
                *s = alpha / s1 / *c__;
                *c__ = r_sign(&c_b5, gamma) / *c__;
            }
            return 0;
        } else {

/*           normal case */

            zeta1 = alpha / absest;
            zeta2 = *gamma / absest;

            b = (1.f - zeta1 * zeta1 - zeta2 * zeta2) * .5f;
            *c__ = zeta1 * zeta1;
            if (b > 0.f) {
                t = *c__ / (b + sqrt(b * b + *c__));
            } else {
                t = sqrt(b * b + *c__) - b;
            }

            sine = -zeta1 / t;
            cosine = -zeta2 / (t + 1.f);
            tmp = sqrt(sine * sine + cosine * cosine);
            *s = sine / tmp;
            *c__ = cosine / tmp;
            *sestpr = sqrt(t + 1.f) * absest;
            return 0;
        }

    } else if (*job == 2) {

/*        Estimating smallest singular value */

/*        special cases */

        if (*sest == 0.f) {
            *sestpr = 0.f;
            if (dmax(absgam,absalp) == 0.f) {
                sine = 1.f;
                cosine = 0.f;
            } else {
                sine = -(*gamma);
                cosine = alpha;
            }
/* Computing MAX */
            r__1 = dabs(sine), r__2 = dabs(cosine);
            s1 = dmax(r__1,r__2);
            *s = sine / s1;
            *c__ = cosine / s1;
            tmp = sqrt(*s * *s + *c__ * *c__);
            *s /= tmp;
            *c__ /= tmp;
            return 0;
        } else if (absgam <= eps * absest) {
            *s = 0.f;
            *c__ = 1.f;
            *sestpr = absgam;
            return 0;
        } else if (absalp <= eps * absest) {
            s1 = absgam;
            s2 = absest;
            if (s1 <= s2) {
                *s = 0.f;
                *c__ = 1.f;
                *sestpr = s1;
            } else {
                *s = 1.f;
                *c__ = 0.f;
                *sestpr = s2;
            }
            return 0;
        } else if (absest <= eps * absalp || absest <= eps * absgam) {
            s1 = absgam;
            s2 = absalp;
            if (s1 <= s2) {
                tmp = s1 / s2;
                *c__ = sqrt(tmp * tmp + 1.f);
                *sestpr = absest * (tmp / *c__);
                *s = -(*gamma / s2) / *c__;
                *c__ = r_sign(&c_b5, &alpha) / *c__;
            } else {
                tmp = s2 / s1;
                *s = sqrt(tmp * tmp + 1.f);
                *sestpr = absest / *s;
                *c__ = alpha / s1 / *s;
                *s = -r_sign(&c_b5, gamma) / *s;
            }
            return 0;
        } else {

/*           normal case */

            zeta1 = alpha / absest;
            zeta2 = *gamma / absest;

/* Computing MAX */
            r__3 = zeta1 * zeta1 + 1.f + (r__1 = zeta1 * zeta2, dabs(r__1)), 
                    r__4 = (r__2 = zeta1 * zeta2, dabs(r__2)) + zeta2 * zeta2;
            norma = dmax(r__3,r__4);

/*           See if root is closer to zero or to ONE */

            test = (zeta1 - zeta2) * 2.f * (zeta1 + zeta2) + 1.f;
            if (test >= 0.f) {

/*              root is close to zero, compute directly */

                b = (zeta1 * zeta1 + zeta2 * zeta2 + 1.f) * .5f;
                *c__ = zeta2 * zeta2;
                t = *c__ / (b + sqrt((r__1 = b * b - *c__, dabs(r__1))));
                sine = zeta1 / (1.f - t);
                cosine = -zeta2 / t;
                *sestpr = sqrt(t + eps * 4.f * eps * norma) * absest;
            } else {

/*              root is closer to ONE, shift by that amount */

                b = (zeta2 * zeta2 + zeta1 * zeta1 - 1.f) * .5f;
                *c__ = zeta1 * zeta1;
                if (b >= 0.f) {
                    t = -(*c__) / (b + sqrt(b * b + *c__));
                } else {
                    t = b - sqrt(b * b + *c__);
                }
                sine = -zeta1 / t;
                cosine = -zeta2 / (t + 1.f);
                *sestpr = sqrt(t + 1.f + eps * 4.f * eps * norma) * absest;
            }
            tmp = sqrt(sine * sine + cosine * cosine);
            *s = sine / tmp;
            *c__ = cosine / tmp;
            return 0;

        }
    }
    return 0;

/*     End of SLAIC1 */

} /* slaic1_ */