fusl/src/math/remquo.c

#include <math.h>
#include <stdint.h>

double remquo(double x, double y, int* quo) {
  union {
    double f;
    uint64_t i;
  } ux = {x}, uy = {y};
  int ex = ux.i >> 52 & 0x7ff;
  int ey = uy.i >> 52 & 0x7ff;
  int sx = ux.i >> 63;
  int sy = uy.i >> 63;
  uint32_t q;
  uint64_t i;
  uint64_t uxi = ux.i;

  *quo = 0;
  if (uy.i << 1 == 0 || isnan(y) || ex == 0x7ff)
    return (x * y) / (x * y);
  if (ux.i << 1 == 0)
    return x;

  /* normalize x and y */
  if (!ex) {
    for (i = uxi << 12; i >> 63 == 0; ex--, i <<= 1)
      ;
    uxi <<= -ex + 1;
  } else {
    uxi &= -1ULL >> 12;
    uxi |= 1ULL << 52;
  }
  if (!ey) {
    for (i = uy.i << 12; i >> 63 == 0; ey--, i <<= 1)
      ;
    uy.i <<= -ey + 1;
  } else {
    uy.i &= -1ULL >> 12;
    uy.i |= 1ULL << 52;
  }

  q = 0;
  if (ex < ey) {
    if (ex + 1 == ey)
      goto end;
    return x;
  }

  /* x mod y */
  for (; ex > ey; ex--) {
    i = uxi - uy.i;
    if (i >> 63 == 0) {
      uxi = i;
      q++;
    }
    uxi <<= 1;
    q <<= 1;
  }
  i = uxi - uy.i;
  if (i >> 63 == 0) {
    uxi = i;
    q++;
  }
  if (uxi == 0)
    ex = -60;
  else
    for (; uxi >> 52 == 0; uxi <<= 1, ex--)
      ;
end:
  /* scale result and decide between |x| and |x|-|y| */
  if (ex > 0) {
    uxi -= 1ULL << 52;
    uxi |= (uint64_t)ex << 52;
  } else {
    uxi >>= -ex + 1;
  }
  ux.i = uxi;
  x = ux.f;
  if (sy)
    y = -y;
  if (ex == ey || (ex + 1 == ey && (2 * x > y || (2 * x == y && q % 2)))) {
    x -= y;
    q++;
  }
  q &= 0x7fffffff;
  *quo = sx ^ sy ? -(int)q : (int)q;
  return sx ? -x : x;
}