163 lines
3.7 KiB
C
163 lines
3.7 KiB
C
#include "clapack.h"
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/* Subroutine */ int slarfg_(integer *n, real *alpha, real *x, integer *incx,
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real *tau)
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{
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/* System generated locals */
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integer i__1;
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real r__1;
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/* Builtin functions */
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double r_sign(real *, real *);
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/* Local variables */
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integer j, knt;
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real beta;
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extern doublereal snrm2_(integer *, real *, integer *);
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extern /* Subroutine */ int sscal_(integer *, real *, real *, integer *);
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real xnorm;
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extern doublereal slapy2_(real *, real *), slamch_(char *);
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real safmin, rsafmn;
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/* -- LAPACK auxiliary routine (version 3.1) -- */
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/* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd.. */
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/* November 2006 */
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/* .. Scalar Arguments .. */
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/* .. */
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/* .. Array Arguments .. */
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/* .. */
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/* Purpose */
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/* ======= */
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/* SLARFG generates a real elementary reflector H of order n, such */
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/* that */
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/* H * ( alpha ) = ( beta ), H' * H = I. */
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/* ( x ) ( 0 ) */
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/* where alpha and beta are scalars, and x is an (n-1)-element real */
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/* vector. H is represented in the form */
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/* H = I - tau * ( 1 ) * ( 1 v' ) , */
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/* ( v ) */
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/* where tau is a real scalar and v is a real (n-1)-element */
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/* vector. */
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/* If the elements of x are all zero, then tau = 0 and H is taken to be */
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/* the unit matrix. */
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/* Otherwise 1 <= tau <= 2. */
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/* Arguments */
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/* ========= */
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/* N (input) INTEGER */
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/* The order of the elementary reflector. */
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/* ALPHA (input/output) REAL */
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/* On entry, the value alpha. */
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/* On exit, it is overwritten with the value beta. */
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/* X (input/output) REAL array, dimension */
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/* (1+(N-2)*abs(INCX)) */
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/* On entry, the vector x. */
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/* On exit, it is overwritten with the vector v. */
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/* INCX (input) INTEGER */
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/* The increment between elements of X. INCX > 0. */
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/* TAU (output) REAL */
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/* The value tau. */
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/* ===================================================================== */
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/* .. Parameters .. */
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/* .. */
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/* .. Local Scalars .. */
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/* .. */
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/* .. External Functions .. */
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/* .. */
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/* .. Intrinsic Functions .. */
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/* .. */
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/* .. External Subroutines .. */
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/* .. */
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/* .. Executable Statements .. */
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/* Parameter adjustments */
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--x;
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/* Function Body */
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if (*n <= 1) {
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*tau = 0.f;
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return 0;
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}
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i__1 = *n - 1;
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xnorm = snrm2_(&i__1, &x[1], incx);
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if (xnorm == 0.f) {
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/* H = I */
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*tau = 0.f;
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} else {
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/* general case */
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r__1 = slapy2_(alpha, &xnorm);
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beta = -r_sign(&r__1, alpha);
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safmin = slamch_("S") / slamch_("E");
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if (dabs(beta) < safmin) {
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/* XNORM, BETA may be inaccurate; scale X and recompute them */
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rsafmn = 1.f / safmin;
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knt = 0;
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L10:
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++knt;
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i__1 = *n - 1;
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sscal_(&i__1, &rsafmn, &x[1], incx);
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beta *= rsafmn;
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*alpha *= rsafmn;
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if (dabs(beta) < safmin) {
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goto L10;
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}
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/* New BETA is at most 1, at least SAFMIN */
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i__1 = *n - 1;
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xnorm = snrm2_(&i__1, &x[1], incx);
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r__1 = slapy2_(alpha, &xnorm);
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beta = -r_sign(&r__1, alpha);
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*tau = (beta - *alpha) / beta;
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i__1 = *n - 1;
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r__1 = 1.f / (*alpha - beta);
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sscal_(&i__1, &r__1, &x[1], incx);
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/* If ALPHA is subnormal, it may lose relative accuracy */
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*alpha = beta;
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i__1 = knt;
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for (j = 1; j <= i__1; ++j) {
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*alpha *= safmin;
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/* L20: */
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}
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} else {
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*tau = (beta - *alpha) / beta;
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i__1 = *n - 1;
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r__1 = 1.f / (*alpha - beta);
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sscal_(&i__1, &r__1, &x[1], incx);
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*alpha = beta;
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}
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}
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return 0;
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/* End of SLARFG */
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} /* slarfg_ */
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