source: vendor/FreeBSD-msun/5.x-2005-06-xx/src/s_trunc.c

/* @(#)s_floor.c 5.1 93/09/24 */
/*
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunPro, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice
 * is preserved.
 * ====================================================
 */

#include <sys/cdefs.h>
__FBSDID("$FreeBSD: src/lib/msun/src/s_trunc.c,v 1.1 2004/06/20 09:25:43 das Exp $");

/*
 * trunc(x)
 * Return x rounded toward 0 to integral value
 * Method:
 *      Bit twiddling.
 * Exception:
 *      Inexact flag raised if x not equal to trunc(x).
 */

#include "math.h"
#include "math_private.h"

static const double huge = 1.0e300;

double
trunc(double x)
{
        int32_t i0,i1,j0;
        u_int32_t i,j;
        EXTRACT_WORDS(i0,i1,x);
        j0 = ((i0>>20)&0x7ff)-0x3ff;
        if(j0<20) {
            if(j0<0) {  /* raise inexact if x != 0 */
                if(huge+x>0.0) {/* |x|<1, so return 0*sign(x) */
                    i0 &= 0x80000000U;
                    i1 = 0;
                }
            } else {
                i = (0x000fffff)>>j0;
                if(((i0&i)|i1)==0) return x; /* x is integral */
                if(huge+x>0.0) {        /* raise inexact flag */
                    i0 &= (~i); i1=0;
                }
            }
        } else if (j0>51) {
            if(j0==0x400) return x+x;   /* inf or NaN */
            else return x;              /* x is integral */
        } else {
            i = ((u_int32_t)(0xffffffff))>>(j0-20);
            if((i1&i)==0) return x;     /* x is integral */
            if(huge+x>0.0)              /* raise inexact flag */
                i1 &= (~i);
        }
        INSERT_WORDS(x,i0,i1);
        return x;
}