#include "clapack.h"

/* Table of constant values */

static integer c__0 = 0;
static integer c__1 = 1;
static real c_b32 = 1.f;

/* Subroutine */ int ssterf_(integer *n, real *d__, real *e, integer *info)
{
    /* System generated locals */
    integer i__1;
    real r__1, r__2, r__3;

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

    /* Local variables */
    real c__;
    integer i__, l, m;
    real p, r__, s;
    integer l1;
    real bb, rt1, rt2, eps, rte;
    integer lsv;
    real eps2, oldc;
    integer lend, jtot;
    extern /* Subroutine */ int slae2_(real *, real *, real *, real *, real *)
	    ;
    real gamma, alpha, sigma, anorm;
    extern doublereal slapy2_(real *, real *);
    integer iscale;
    real oldgam;
    extern doublereal slamch_(char *);
    real safmin;
    extern /* Subroutine */ int xerbla_(char *, integer *);
    real safmax;
    extern /* Subroutine */ int slascl_(char *, integer *, integer *, real *, 
	    real *, integer *, integer *, real *, integer *, integer *);
    integer lendsv;
    real ssfmin;
    integer nmaxit;
    real ssfmax;
    extern doublereal slanst_(char *, integer *, real *, real *);
    extern /* Subroutine */ int slasrt_(char *, integer *, real *, integer *);


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

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

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

/*  SSTERF computes all eigenvalues of a symmetric tridiagonal matrix */
/*  using the Pal-Walker-Kahan variant of the QL or QR algorithm. */

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

/*  N       (input) INTEGER */
/*          The order of the matrix.  N >= 0. */

/*  D       (input/output) REAL array, dimension (N) */
/*          On entry, the n diagonal elements of the tridiagonal matrix. */
/*          On exit, if INFO = 0, the eigenvalues in ascending order. */

/*  E       (input/output) REAL array, dimension (N-1) */
/*          On entry, the (n-1) subdiagonal elements of the tridiagonal */
/*          matrix. */
/*          On exit, E has been destroyed. */

/*  INFO    (output) INTEGER */
/*          = 0:  successful exit */
/*          < 0:  if INFO = -i, the i-th argument had an illegal value */
/*          > 0:  the algorithm failed to find all of the eigenvalues in */
/*                a total of 30*N iterations; if INFO = i, then i */
/*                elements of E have not converged to zero. */

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

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

/*     Test the input parameters. */

    /* Parameter adjustments */
    --e;
    --d__;

    /* Function Body */
    *info = 0;

/*     Quick return if possible */

    if (*n < 0) {
	*info = -1;
	i__1 = -(*info);
	xerbla_("SSTERF", &i__1);
	return 0;
    }
    if (*n <= 1) {
	return 0;
    }

/*     Determine the unit roundoff for this environment. */

    eps = slamch_("E");
/* Computing 2nd power */
    r__1 = eps;
    eps2 = r__1 * r__1;
    safmin = slamch_("S");
    safmax = 1.f / safmin;
    ssfmax = sqrt(safmax) / 3.f;
    ssfmin = sqrt(safmin) / eps2;

/*     Compute the eigenvalues of the tridiagonal matrix. */

    nmaxit = *n * 30;
    sigma = 0.f;
    jtot = 0;

/*     Determine where the matrix splits and choose QL or QR iteration */
/*     for each block, according to whether top or bottom diagonal */
/*     element is smaller. */

    l1 = 1;

L10:
    if (l1 > *n) {
	goto L170;
    }
    if (l1 > 1) {
	e[l1 - 1] = 0.f;
    }
    i__1 = *n - 1;
    for (m = l1; m <= i__1; ++m) {
	if ((r__3 = e[m], dabs(r__3)) <= sqrt((r__1 = d__[m], dabs(r__1))) * 
		sqrt((r__2 = d__[m + 1], dabs(r__2))) * eps) {
	    e[m] = 0.f;
	    goto L30;
	}
/* L20: */
    }
    m = *n;

L30:
    l = l1;
    lsv = l;
    lend = m;
    lendsv = lend;
    l1 = m + 1;
    if (lend == l) {
	goto L10;
    }

/*     Scale submatrix in rows and columns L to LEND */

    i__1 = lend - l + 1;
    anorm = slanst_("I", &i__1, &d__[l], &e[l]);
    iscale = 0;
    if (anorm > ssfmax) {
	iscale = 1;
	i__1 = lend - l + 1;
	slascl_("G", &c__0, &c__0, &anorm, &ssfmax, &i__1, &c__1, &d__[l], n, 
		info);
	i__1 = lend - l;
	slascl_("G", &c__0, &c__0, &anorm, &ssfmax, &i__1, &c__1, &e[l], n, 
		info);
    } else if (anorm < ssfmin) {
	iscale = 2;
	i__1 = lend - l + 1;
	slascl_("G", &c__0, &c__0, &anorm, &ssfmin, &i__1, &c__1, &d__[l], n, 
		info);
	i__1 = lend - l;
	slascl_("G", &c__0, &c__0, &anorm, &ssfmin, &i__1, &c__1, &e[l], n, 
		info);
    }

    i__1 = lend - 1;
    for (i__ = l; i__ <= i__1; ++i__) {
/* Computing 2nd power */
	r__1 = e[i__];
	e[i__] = r__1 * r__1;
/* L40: */
    }

/*     Choose between QL and QR iteration */

    if ((r__1 = d__[lend], dabs(r__1)) < (r__2 = d__[l], dabs(r__2))) {
	lend = lsv;
	l = lendsv;
    }

    if (lend >= l) {

/*        QL Iteration */

/*        Look for small subdiagonal element. */

L50:
	if (l != lend) {
	    i__1 = lend - 1;
	    for (m = l; m <= i__1; ++m) {
		if ((r__2 = e[m], dabs(r__2)) <= eps2 * (r__1 = d__[m] * d__[
			m + 1], dabs(r__1))) {
		    goto L70;
		}
/* L60: */
	    }
	}
	m = lend;

L70:
	if (m < lend) {
	    e[m] = 0.f;
	}
	p = d__[l];
	if (m == l) {
	    goto L90;
	}

/*        If remaining matrix is 2 by 2, use SLAE2 to compute its */
/*        eigenvalues. */

	if (m == l + 1) {
	    rte = sqrt(e[l]);
	    slae2_(&d__[l], &rte, &d__[l + 1], &rt1, &rt2);
	    d__[l] = rt1;
	    d__[l + 1] = rt2;
	    e[l] = 0.f;
	    l += 2;
	    if (l <= lend) {
		goto L50;
	    }
	    goto L150;
	}

	if (jtot == nmaxit) {
	    goto L150;
	}
	++jtot;

/*        Form shift. */

	rte = sqrt(e[l]);
	sigma = (d__[l + 1] - p) / (rte * 2.f);
	r__ = slapy2_(&sigma, &c_b32);
	sigma = p - rte / (sigma + r_sign(&r__, &sigma));

	c__ = 1.f;
	s = 0.f;
	gamma = d__[m] - sigma;
	p = gamma * gamma;

/*        Inner loop */

	i__1 = l;
	for (i__ = m - 1; i__ >= i__1; --i__) {
	    bb = e[i__];
	    r__ = p + bb;
	    if (i__ != m - 1) {
		e[i__ + 1] = s * r__;
	    }
	    oldc = c__;
	    c__ = p / r__;
	    s = bb / r__;
	    oldgam = gamma;
	    alpha = d__[i__];
	    gamma = c__ * (alpha - sigma) - s * oldgam;
	    d__[i__ + 1] = oldgam + (alpha - gamma);
	    if (c__ != 0.f) {
		p = gamma * gamma / c__;
	    } else {
		p = oldc * bb;
	    }
/* L80: */
	}

	e[l] = s * p;
	d__[l] = sigma + gamma;
	goto L50;

/*        Eigenvalue found. */

L90:
	d__[l] = p;

	++l;
	if (l <= lend) {
	    goto L50;
	}
	goto L150;

    } else {

/*        QR Iteration */

/*        Look for small superdiagonal element. */

L100:
	i__1 = lend + 1;
	for (m = l; m >= i__1; --m) {
	    if ((r__2 = e[m - 1], dabs(r__2)) <= eps2 * (r__1 = d__[m] * d__[
		    m - 1], dabs(r__1))) {
		goto L120;
	    }
/* L110: */
	}
	m = lend;

L120:
	if (m > lend) {
	    e[m - 1] = 0.f;
	}
	p = d__[l];
	if (m == l) {
	    goto L140;
	}

/*        If remaining matrix is 2 by 2, use SLAE2 to compute its */
/*        eigenvalues. */

	if (m == l - 1) {
	    rte = sqrt(e[l - 1]);
	    slae2_(&d__[l], &rte, &d__[l - 1], &rt1, &rt2);
	    d__[l] = rt1;
	    d__[l - 1] = rt2;
	    e[l - 1] = 0.f;
	    l += -2;
	    if (l >= lend) {
		goto L100;
	    }
	    goto L150;
	}

	if (jtot == nmaxit) {
	    goto L150;
	}
	++jtot;

/*        Form shift. */

	rte = sqrt(e[l - 1]);
	sigma = (d__[l - 1] - p) / (rte * 2.f);
	r__ = slapy2_(&sigma, &c_b32);
	sigma = p - rte / (sigma + r_sign(&r__, &sigma));

	c__ = 1.f;
	s = 0.f;
	gamma = d__[m] - sigma;
	p = gamma * gamma;

/*        Inner loop */

	i__1 = l - 1;
	for (i__ = m; i__ <= i__1; ++i__) {
	    bb = e[i__];
	    r__ = p + bb;
	    if (i__ != m) {
		e[i__ - 1] = s * r__;
	    }
	    oldc = c__;
	    c__ = p / r__;
	    s = bb / r__;
	    oldgam = gamma;
	    alpha = d__[i__ + 1];
	    gamma = c__ * (alpha - sigma) - s * oldgam;
	    d__[i__] = oldgam + (alpha - gamma);
	    if (c__ != 0.f) {
		p = gamma * gamma / c__;
	    } else {
		p = oldc * bb;
	    }
/* L130: */
	}

	e[l - 1] = s * p;
	d__[l] = sigma + gamma;
	goto L100;

/*        Eigenvalue found. */

L140:
	d__[l] = p;

	--l;
	if (l >= lend) {
	    goto L100;
	}
	goto L150;

    }

/*     Undo scaling if necessary */

L150:
    if (iscale == 1) {
	i__1 = lendsv - lsv + 1;
	slascl_("G", &c__0, &c__0, &ssfmax, &anorm, &i__1, &c__1, &d__[lsv], 
		n, info);
    }
    if (iscale == 2) {
	i__1 = lendsv - lsv + 1;
	slascl_("G", &c__0, &c__0, &ssfmin, &anorm, &i__1, &c__1, &d__[lsv], 
		n, info);
    }

/*     Check for no convergence to an eigenvalue after a total */
/*     of N*MAXIT iterations. */

    if (jtot < nmaxit) {
	goto L10;
    }
    i__1 = *n - 1;
    for (i__ = 1; i__ <= i__1; ++i__) {
	if (e[i__] != 0.f) {
	    ++(*info);
	}
/* L160: */
    }
    goto L180;

/*     Sort eigenvalues in increasing order. */

L170:
    slasrt_("I", n, &d__[1], info);

L180:
    return 0;

/*     End of SSTERF */

} /* ssterf_ */
