s_scalbn.c

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/* @(#)s_scalbn.c 1.3 95/01/18 */
/*
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunSoft, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice 
 * is preserved.
 * ====================================================
 */

/* 
 * scalbn (double x, int n)
 * scalbn(x,n) returns x* 2**n  computed by  exponent  
 * manipulation rather than by actually performing an 
 * exponentiation or a multiplication.
 */

#include "fdlibm.h"

#ifdef __STDC__
static const double
#else
static double
#endif
two54   =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
twom54  =  5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
huge   = 1.0e+300,
tiny   = 1.0e-300;

#ifdef __STDC__
    double scalbn (double x, int n)
#else
    double scalbn (x,n)
    double x; int n;
#endif
{
    int  k,hx,lx;
    hx = __HI(x);
    lx = __LO(x);
        k = (hx&0x7ff00000)>>20;        /* extract exponent */
        if (k==0) {             /* 0 or subnormal x */
            if ((lx|(hx&0x7fffffff))==0) return x; /* +-0 */
        x *= two54; 
        hx = __HI(x);
        k = ((hx&0x7ff00000)>>20) - 54; 
            if (n< -50000) return tiny*x;   /*underflow*/
        }
        if (k==0x7ff) return x+x;       /* NaN or Inf */
        k = k+n; 
        if (k >  0x7fe) return huge*copysign(huge,x); /* overflow  */
        if (k > 0)              /* normal result */
        {__HI(x) = (hx&0x800fffff)|(k<<20); return x;}
        if (k <= -54) {
            if (n > 50000)  /* in case integer overflow in n+k */
        return huge*copysign(huge,x);   /*overflow*/
        else return tiny*copysign(tiny,x);  /*underflow*/ }
        k += 54;                /* subnormal result */
        __HI(x) = (hx&0x800fffff)|(k<<20);
        return x*twom54;
}