libc-top-half/musl/src/math/modf.c


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#include "libm.h"

double modf(double x, double *iptr)
{
	union {double f; uint64_t i;} u = {x};
	uint64_t mask;
	int e = (int)(u.i>>52 & 0x7ff) - 0x3ff;

	/* no fractional part */
	if (e >= 52) {
		*iptr = x;
		if (e == 0x400 && u.i<<12 != 0) /* nan */
			return x;
		u.i &= 1ULL<<63;
		return u.f;
	}

	/* no integral part*/
	if (e < 0) {
		u.i &= 1ULL<<63;
		*iptr = u.f;
		return x;
	}

	mask = -1ULL>>12>>e;
	if ((u.i & mask) == 0) {
		*iptr = x;
		u.i &= 1ULL<<63;
		return u.f;
	}
	u.i &= ~mask;
	*iptr = u.f;
	return x - u.f;
}