source: vendor/FreeBSD-msun/current/src/s_remquof.c

/* @(#)e_fmod.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.
 * ====================================================
 */

#include <sys/cdefs.h>
__FBSDID("$FreeBSD: src/lib/msun/src/s_remquof.c,v 1.1 2005/03/25 04:40:44 das Exp $");

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

static const float Zero[] = {0.0, -0.0,};

/*
 * Return the IEEE remainder and set *quo to the last n bits of the
 * quotient, rounded to the nearest integer.  We choose n=31 because
 * we wind up computing all the integer bits of the quotient anyway as
 * a side-effect of computing the remainder by the shift and subtract
 * method.  In practice, this is far more bits than are needed to use
 * remquo in reduction algorithms.
 */
float
remquof(float x, float y, int *quo)
{
        int32_t n,hx,hy,hz,ix,iy,sx,i;
        u_int32_t q,sxy;

        GET_FLOAT_WORD(hx,x);
        GET_FLOAT_WORD(hy,y);
        sxy = (hx ^ hy) & 0x80000000;
        sx = hx&0x80000000;             /* sign of x */
        hx ^=sx;                /* |x| */
        hy &= 0x7fffffff;       /* |y| */

    /* purge off exception values */
        if(hy==0||hx>=0x7f800000||hy>0x7f800000) /* y=0,NaN;or x not finite */
            return (x*y)/(x*y);
        if(hx<hy) {
            q = 0;
            goto fixup; /* |x|<|y| return x or x-y */
        } else if(hx==hy) {
            *quo = 1;
            return Zero[(u_int32_t)sx>>31];     /* |x|=|y| return x*0*/
        }

    /* determine ix = ilogb(x) */
        if(hx<0x00800000) {     /* subnormal x */
            for (ix = -126,i=(hx<<8); i>0; i<<=1) ix -=1;
        } else ix = (hx>>23)-127;

    /* determine iy = ilogb(y) */
        if(hy<0x00800000) {     /* subnormal y */
            for (iy = -126,i=(hy<<8); i>0; i<<=1) iy -=1;
        } else iy = (hy>>23)-127;

    /* set up {hx,lx}, {hy,ly} and align y to x */
        if(ix >= -126)
            hx = 0x00800000|(0x007fffff&hx);
        else {          /* subnormal x, shift x to normal */
            n = -126-ix;
            hx <<= n;
        }
        if(iy >= -126)
            hy = 0x00800000|(0x007fffff&hy);
        else {          /* subnormal y, shift y to normal */
            n = -126-iy;
            hy <<= n;
        }

    /* fix point fmod */
        n = ix - iy;
        q = 0;
        while(n--) {
            hz=hx-hy;
            if(hz<0) hx = hx << 1;
            else {hx = hz << 1; q++;}
            q <<= 1;
        }
        hz=hx-hy;
        if(hz>=0) {hx=hz;q++;}

    /* convert back to floating value and restore the sign */
        if(hx==0) {                             /* return sign(x)*0 */
            *quo = (sxy ? -q : q);
            return Zero[(u_int32_t)sx>>31];
        }
        while(hx<0x00800000) {          /* normalize x */
            hx <<= 1;
            iy -= 1;
        }
        if(iy>= -126) {         /* normalize output */
            hx = ((hx-0x00800000)|((iy+127)<<23));
        } else {                /* subnormal output */
            n = -126 - iy;
            hx >>= n;
        }
fixup:
        SET_FLOAT_WORD(x,hx);
        y = fabsf(y);
        if (y < 0x1p-125f) {
            if (x+x>y || (x+x==y && (q & 1))) {
                q++;
                x-=y;
            }
        } else if (x>0.5f*y || (x==0.5f*y && (q & 1))) {
            q++;
            x-=y;
        }
        GET_FLOAT_WORD(hx,x);
        SET_FLOAT_WORD(x,hx^sx);
        q &= 0x7fffffff;
        *quo = (sxy ? -q : q);
        return x;
}