prime/src/Runtime/e_fmod.c

/* @(#)e_fmod.c 1.2 95/01/04 */
/*
 * ====================================================
 * 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.
 * ====================================================
 */

/*
 * __ieee754_fmod(x,y)
 * Return x mod y in exact arithmetic
 * Method: shift and subtract
 */

#include "fdlibm.h"

#ifdef __STDC__
static const double one = 1.0, Zero[] = {
                                   0.0,
                                   -0.0,
};
#else
static double one = 1.0, Zero[] = {
                             0.0,
                             -0.0,
};
#endif

#ifdef __STDC__
double __ieee754_fmod(double x, double y)
#else
double __ieee754_fmod(x, y)
double x, y;
#endif
{
  _INT32 n, hx, hy, hz, ix, iy, sx, i;
  _UINT32 lx, ly, lz;

  hx = __HI(x);         /* high word of x */
  lx = __LO(x);         /* low  word of x */
  hy = __HI(y);         /* high word of y */
  ly = __LO(y);         /* low  word of y */
  sx = hx & 0x80000000; /* sign of x */
  hx ^= sx;             /* |x| */
  hy &= 0x7fffffff;     /* |y| */

  /* purge off exception values */
  if ((hy | ly) == 0 || (hx >= 0x7ff00000) ||   /* y=0,or x not finite */
      ((hy | ((ly | -ly) >> 31)) > 0x7ff00000)) /* or y is NaN */
    return (x * y) / (x * y);
  if (hx <= hy) {
    if ((hx < hy) || (lx < ly))
      return x; /* |x|<|y| return x */
    if (lx == ly)
      return Zero[(_UINT32)sx >> 31]; /* |x|=|y| return x*0*/
  }

  /* determine ix = ilogb(x) */
  if (hx < 0x00100000) { /* subnormal x */
    if (hx == 0) {
      for (ix = -1043, i = lx; i > 0; i <<= 1)
        ix -= 1;
    } else {
      for (ix = -1022, i = (hx << 11); i > 0; i <<= 1)
        ix -= 1;
    }
  } else
    ix = (hx >> 20) - 1023;

  /* determine iy = ilogb(y) */
  if (hy < 0x00100000) { /* subnormal y */
    if (hy == 0) {
      for (iy = -1043, i = ly; i > 0; i <<= 1)
        iy -= 1;
    } else {
      for (iy = -1022, i = (hy << 11); i > 0; i <<= 1)
        iy -= 1;
    }
  } else
    iy = (hy >> 20) - 1023;

  /* set up {hx,lx}, {hy,ly} and align y to x */
  if (ix >= -1022)
    hx = 0x00100000 | (0x000fffff & hx);
  else { /* subnormal x, shift x to normal */
    n = -1022 - ix;
    if (n <= 31) {
      hx = (hx << n) | (lx >> (32 - n));
      lx <<= n;
    } else {
      hx = lx << (n - 32);
      lx = 0;
    }
  }
  if (iy >= -1022)
    hy = 0x00100000 | (0x000fffff & hy);
  else { /* subnormal y, shift y to normal */
    n = -1022 - iy;
    if (n <= 31) {
      hy = (hy << n) | (ly >> (32 - n));
      ly <<= n;
    } else {
      hy = ly << (n - 32);
      ly = 0;
    }
  }

  /* fix point fmod */
  n = ix - iy;
  while (n--) {
    hz = hx - hy;
    lz = lx - ly;
    if (lx < ly)
      hz -= 1;
    if (hz < 0) {
      hx = hx + hx + (lx >> 31);
      lx = lx + lx;
    } else {
      if ((hz | lz) == 0)               /* return sign(x)*0 */
        return Zero[(_UINT32)sx >> 31];
      hx = hz + hz + (lz >> 31);
      lx = lz + lz;
    }
  }
  hz = hx - hy;
  lz = lx - ly;
  if (lx < ly)
    hz -= 1;
  if (hz >= 0) {
    hx = hz;
    lx = lz;
  }

  /* convert back to floating value and restore the sign */
  if ((hx | lx) == 0)               /* return sign(x)*0 */
    return Zero[(_UINT32)sx >> 31];
  while (hx < 0x00100000) {         /* normalize x */
    hx = hx + hx + (lx >> 31);
    lx = lx + lx;
    iy -= 1;
  }
  if (iy >= -1022) { /* normalize output */
    hx = ((hx - 0x00100000) | ((iy + 1023) << 20));
    __HI(x) = hx | sx;
    __LO(x) = lx;
  } else { /* subnormal output */
    n = -1022 - iy;
    if (n <= 20) {
      lx = (lx >> n) | ((_UINT32)hx << (32 - n));
      hx >>= n;
    } else if (n <= 31) {
      lx = (hx << (32 - n)) | (lx >> n);
      hx = sx;
    } else {
      lx = hx >> (n - 32);
      hx = sx;
    }
    __HI(x) = hx | sx;
    __LO(x) = lx;
    x *= one; /* create necessary signal */
  }
  return x; /* exact output */
}