fusl/src/math/floor.c

#include "libm.h"

#if FLT_EVAL_METHOD == 0 || FLT_EVAL_METHOD == 1
#define EPS DBL_EPSILON
#elif FLT_EVAL_METHOD == 2
#define EPS LDBL_EPSILON
#endif
static const double_t toint = 1 / EPS;

double floor(double x) {
  union {
    double f;
    uint64_t i;
  } u = {x};
  int e = u.i >> 52 & 0x7ff;
  double_t y;

  if (e >= 0x3ff + 52 || x == 0)
    return x;
  /* y = int(x) - x, where int(x) is an integer neighbor of x */
  if (u.i >> 63)
    y = x - toint + toint - x;
  else
    y = x + toint - toint - x;
  /* special case because of non-nearest rounding modes */
  if (e <= 0x3ff - 1) {
    FORCE_EVAL(y);
    return u.i >> 63 ? -1 : 0;
  }
  if (y > 0)
    return x + y - 1;
  return x + y;
}