slamc1.c

来自「NIST Handwriting OCR Testbed」· C语言 代码 · 共 220 行

/*
* ======================================================================
* NIST Guide to Available Math Software.
* Fullsource for module SSYEVX.C from package CLAPACK.
* Retrieved from NETLIB on Fri Mar 10 14:23:44 2000.
* ======================================================================
*/
#include <f2c.h>

/* Subroutine */ int slamc1_(integer *beta, integer *t, logical *rnd, logical 
	*ieee1)
{
/*  -- LAPACK auxiliary routine (version 2.0) --   
       Univ. of Tennessee, Univ. of California Berkeley, NAG Ltd.,   
       Courant Institute, Argonne National Lab, and Rice University   
       October 31, 1992   


    Purpose   
    =======   

    SLAMC1 determines the machine parameters given by BETA, T, RND, and   
    IEEE1.   

    Arguments   
    =========   

    BETA    (output) INTEGER   
            The base of the machine.   

    T       (output) INTEGER   
            The number of ( BETA ) digits in the mantissa.   

    RND     (output) LOGICAL   
            Specifies whether proper rounding  ( RND = .TRUE. )  or   
            chopping  ( RND = .FALSE. )  occurs in addition. This may not 
  
            be a reliable guide to the way in which the machine performs 
  
            its arithmetic.   

    IEEE1   (output) LOGICAL   
            Specifies whether rounding appears to be done in the IEEE   
            'round to nearest' style.   

    Further Details   
    ===============   

    The routine is based on the routine  ENVRON  by Malcolm and   
    incorporates suggestions by Gentleman and Marovich. See   

       Malcolm M. A. (1972) Algorithms to reveal properties of   
          floating-point arithmetic. Comms. of the ACM, 15, 949-951.   

       Gentleman W. M. and Marovich S. B. (1974) More on algorithms   
          that reveal properties of floating point arithmetic units.   
          Comms. of the ACM, 17, 276-277.   

   ===================================================================== 
*/
    /* Initialized data */
    static logical first = TRUE_;
    /* System generated locals */
    real r__1, r__2;
    /* Local variables */
    static logical lrnd;
    static real a, b, c, f;
    static integer lbeta;
    static real savec;
    static logical lieee1;
    static real t1, t2;
    extern doublereal slamc3_(real *, real *);
    static integer lt;
    static real one, qtr;



    if (first) {
	first = FALSE_;
	one = 1.f;

/*        LBETA,  LIEEE1,  LT and  LRND  are the  local values  of  BE
TA,   
          IEEE1, T and RND.   

          Throughout this routine  we use the function  SLAMC3  to ens
ure   
          that relevant values are  stored and not held in registers, 
 or   
          are not affected by optimizers.   

          Compute  a = 2.0**m  with the  smallest positive integer m s
uch   
          that   

             fl( a + 1.0 ) = a. */

	a = 1.f;
	c = 1.f;

/* +       WHILE( C.EQ.ONE )LOOP */
L10:
	if (c == one) {
	    a *= 2;
	    c = slamc3_(&a, &one);
	    r__1 = -(doublereal)a;
	    c = slamc3_(&c, &r__1);
	    goto L10;
	}
/* +       END WHILE   

          Now compute  b = 2.0**m  with the smallest positive integer 
m   
          such that   

             fl( a + b ) .gt. a. */

	b = 1.f;
	c = slamc3_(&a, &b);

/* +       WHILE( C.EQ.A )LOOP */
L20:
	if (c == a) {
	    b *= 2;
	    c = slamc3_(&a, &b);
	    goto L20;
	}
/* +       END WHILE   

          Now compute the base.  a and c  are neighbouring floating po
int   
          numbers  in the  interval  ( beta**t, beta**( t + 1 ) )  and
 so   
          their difference is beta. Adding 0.25 to c is to ensure that
 it   
          is truncated to beta and not ( beta - 1 ). */

	qtr = one / 4;
	savec = c;
	r__1 = -(doublereal)a;
	c = slamc3_(&c, &r__1);
	lbeta = c + qtr;

/*        Now determine whether rounding or chopping occurs,  by addin
g a   
          bit  less  than  beta/2  and a  bit  more  than  beta/2  to 
 a. */

	b = (real) lbeta;
	r__1 = b / 2;
	r__2 = -(doublereal)b / 100;
	f = slamc3_(&r__1, &r__2);
	c = slamc3_(&f, &a);
	if (c == a) {
	    lrnd = TRUE_;
	} else {
	    lrnd = FALSE_;
	}
	r__1 = b / 2;
	r__2 = b / 100;
	f = slamc3_(&r__1, &r__2);
	c = slamc3_(&f, &a);
	if (lrnd && c == a) {
	    lrnd = FALSE_;
	}

/*        Try and decide whether rounding is done in the  IEEE  'round
 to   
          nearest' style. B/2 is half a unit in the last place of the 
two   
          numbers A and SAVEC. Furthermore, A is even, i.e. has last  
bit   
          zero, and SAVEC is odd. Thus adding B/2 to A should not  cha
nge   
          A, but adding B/2 to SAVEC should change SAVEC. */

	r__1 = b / 2;
	t1 = slamc3_(&r__1, &a);
	r__1 = b / 2;
	t2 = slamc3_(&r__1, &savec);
	lieee1 = t1 == a && t2 > savec && lrnd;

/*        Now find  the  mantissa, t.  It should  be the  integer part
 of   
          log to the base beta of a,  however it is safer to determine
  t   
          by powering.  So we find t as the smallest positive integer 
for   
          which   

             fl( beta**t + 1.0 ) = 1.0. */

	lt = 0;
	a = 1.f;
	c = 1.f;

/* +       WHILE( C.EQ.ONE )LOOP */
L30:
	if (c == one) {
	    ++lt;
	    a *= lbeta;
	    c = slamc3_(&a, &one);
	    r__1 = -(doublereal)a;
	    c = slamc3_(&c, &r__1);
	    goto L30;
	}
/* +       END WHILE */

    }

    *beta = lbeta;
    *t = lt;
    *rnd = lrnd;
    *ieee1 = lieee1;
    return 0;

/*     End of SLAMC1 */

} /* slamc1_ */