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169 lines
4.0 KiB
169 lines
4.0 KiB
/* slarfg.f -- translated by f2c (version 20061008). |
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You must link the resulting object file with libf2c: |
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on Microsoft Windows system, link with libf2c.lib; |
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on Linux or Unix systems, link with .../path/to/libf2c.a -lm |
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or, if you install libf2c.a in a standard place, with -lf2c -lm |
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-- in that order, at the end of the command line, as in |
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cc *.o -lf2c -lm |
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Source for libf2c is in /netlib/f2c/libf2c.zip, e.g., |
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http://www.netlib.org/f2c/libf2c.zip |
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*/ |
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#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 3.2) -- */ |
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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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knt = 0; |
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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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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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} |
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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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i__1 = knt; |
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for (j = 1; j <= i__1; ++j) { |
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beta *= safmin; |
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/* L20: */ |
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} |
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*alpha = beta; |
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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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