/* $NetBSD: compat_modf_ieee754.c,v 1.6 2024/01/20 14:52:45 christos Exp $ */

/*
 * Copyright (c) 1994, 1995 Carnegie-Mellon University.
 * All rights reserved.
 *
 * Author: Chris G. Demetriou
 * 
 * Permission to use, copy, modify and distribute this software and
 * its documentation is hereby granted, provided that both the copyright
 * notice and this permission notice appear in all copies of the
 * software, derivative works or modified versions, and any portions
 * thereof, and that both notices appear in supporting documentation.
 * 
 * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS" 
 * CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND 
 * FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
 * 
 * Carnegie Mellon requests users of this software to return to
 *
 *  Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
 *  School of Computer Science
 *  Carnegie Mellon University
 *  Pittsburgh PA 15213-3890
 *
 * any improvements or extensions that they make and grant Carnegie the
 * rights to redistribute these changes.
 */

#include <sys/types.h>
#include <machine/ieee.h>
#include <errno.h>
#include <math.h>

/*
 * double modf(double val, double *iptr)
 * returns: f and i such that |f| < 1.0, (f + i) = val, and
 *      sign(f) == sign(i) == sign(val).
 *
 * Beware signedness when doing subtraction, and also operand size!
 */
double
modf(double val, double *iptr)
{
        union ieee_double_u u, v;
        u_int64_t frac;

        /*
         * If input is +/-Inf or NaN, return +/-0 or NaN.
         */
        u.dblu_d = val;
        if (u.dblu_dbl.dbl_exp == DBL_EXP_INFNAN) {
                *iptr = u.dblu_d;
                return (0.0 / u.dblu_d);
        }

        /*
         * If input can't have a fractional part, return
         * (appropriately signed) zero, and make i be the input.
         */
        if ((int)u.dblu_dbl.dbl_exp - DBL_EXP_BIAS > DBL_FRACBITS - 1) {
                *iptr = u.dblu_d;
                v.dblu_d = 0.0;
                v.dblu_dbl.dbl_sign = u.dblu_dbl.dbl_sign;
                return (v.dblu_d);
        }

        /*
         * If |input| < 1.0, return it, and set i to the appropriately
         * signed zero.
         */
        if (u.dblu_dbl.dbl_exp < DBL_EXP_BIAS) {
                v.dblu_d = 0.0;
                v.dblu_dbl.dbl_sign = u.dblu_dbl.dbl_sign;
                *iptr = v.dblu_d;
                return (u.dblu_d);
        }

        /*
         * There can be a fractional part of the input.
         * If you look at the math involved for a few seconds, it's
         * plain to see that the integral part is the input, with the
         * low (DBL_FRACBITS - (exponent - DBL_EXP_BIAS)) bits zeroed,
         * the fractional part is the part with the rest of the
         * bits zeroed.  Just zeroing the high bits to get the
         * fractional part would yield a fraction in need of
         * normalization.  Therefore, we take the easy way out, and
         * just use subtraction to get the fractional part.
         */
        v.dblu_d = u.dblu_d;
        /* Zero the low bits of the fraction, the sleazy way. */
        frac = ((u_int64_t)v.dblu_dbl.dbl_frach << 32) + v.dblu_dbl.dbl_fracl;
        frac >>= DBL_FRACBITS - (u.dblu_dbl.dbl_exp - DBL_EXP_BIAS);
        frac <<= DBL_FRACBITS - (u.dblu_dbl.dbl_exp - DBL_EXP_BIAS);
        v.dblu_dbl.dbl_fracl = (unsigned int)(frac & 0xffffffffULL);
        v.dblu_dbl.dbl_frach = (unsigned int)(frac >> 32);
        *iptr = v.dblu_d;

        u.dblu_d -= v.dblu_d;
        u.dblu_dbl.dbl_sign = v.dblu_dbl.dbl_sign;
        return (u.dblu_d);
}