xref: /freebsd-src/lib/libc/gdtoa/_hdtoa.c (revision 559a218c9b257775fb249b67945fe4a05b7a6b9f)

/*-
 * SPDX-License-Identifier: BSD-2-Clause
 *
 * Copyright (c) 2004-2008 David Schultz <das@FreeBSD.ORG>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <float.h>
#include <limits.h>
#include <math.h>

#include "../stdio/floatio.h"
#include "fpmath.h"
#include "gdtoaimp.h"

/* Strings values used by dtoa() */
#define	INFSTR	"Infinity"
#define	NANSTR	"NaN"

#define	DBL_ADJ	(DBL_MAX_EXP - 2)
#define	SIGFIGS	((DBL_MANT_DIG + 3) / 4 + 1)

static const float one[] = { 1.0f, -1.0f };

/*
 * This procedure converts a double-precision number in IEEE format
 * into a string of hexadecimal digits and an exponent of 2.  Its
 * behavior is bug-for-bug compatible with dtoa() in mode 2, with the
 * following exceptions:
 *
 * - An ndigits < 0 causes it to use as many digits as necessary to
 *   represent the number exactly.
 * - The additional xdigs argument should point to either the string
 *   "0123456789ABCDEF" or the string "0123456789abcdef", depending on
 *   which case is desired.
 * - This routine does not repeat dtoa's mistake of setting decpt
 *   to 9999 in the case of an infinity or NaN.  INT_MAX is used
 *   for this purpose instead.
 *
 * Note that the C99 standard does not specify what the leading digit
 * should be for non-zero numbers.  For instance, 0x1.3p3 is the same
 * as 0x2.6p2 is the same as 0x4.cp3.  This implementation always makes
 * the leading digit a 1. This ensures that the exponent printed is the
 * actual base-2 exponent, i.e., ilogb(d).
 *
 * Inputs:	d, xdigs, ndigits
 * Outputs:	decpt, sign, rve
 */
char *
__hdtoa(double d, const char *xdigs, int ndigits, int *decpt, int *sign,
    char **rve)
{
	union IEEEd2bits u;
	char *s, *s0;
	int bufsize;
	uint32_t manh, manl;

	u.d = d;
	*sign = u.bits.sign;

	switch (fpclassify(d)) {
	case FP_NORMAL:
		*decpt = u.bits.exp - DBL_ADJ;
		break;
	case FP_ZERO:
		*decpt = 1;
		return (nrv_alloc("0", rve, 1));
	case FP_SUBNORMAL:
		u.d *= 0x1p514;
		*decpt = u.bits.exp - (514 + DBL_ADJ);
		break;
	case FP_INFINITE:
		*decpt = INT_MAX;
		return (nrv_alloc(INFSTR, rve, sizeof(INFSTR) - 1));
	default:	/* FP_NAN or unrecognized */
		*decpt = INT_MAX;
		return (nrv_alloc(NANSTR, rve, sizeof(NANSTR) - 1));
	}

	/* FP_NORMAL or FP_SUBNORMAL */

	if (ndigits == 0)		/* dtoa() compatibility */
		ndigits = 1;

	/*
	 * If ndigits < 0, we are expected to auto-size, so we allocate
	 * enough space for all the digits.
	 */
	bufsize = (ndigits > 0) ? ndigits : SIGFIGS;
	s0 = rv_alloc(bufsize);

	/* Round to the desired number of digits. */
	if (SIGFIGS > ndigits && ndigits > 0) {
		float redux = one[u.bits.sign];
		int offset = 4 * ndigits + DBL_MAX_EXP - 4 - DBL_MANT_DIG;
		u.bits.exp = offset;
		u.d += redux;
		u.d -= redux;
		*decpt += u.bits.exp - offset;
	}

	manh = u.bits.manh;
	manl = u.bits.manl;
	*s0 = '1';
	for (s = s0 + 1; s < s0 + bufsize; s++) {
		*s = xdigs[(manh >> (DBL_MANH_SIZE - 4)) & 0xf];
		manh = (manh << 4) | (manl >> (DBL_MANL_SIZE - 4));
		manl <<= 4;
	}

	/* If ndigits < 0, we are expected to auto-size the precision. */
	if (ndigits < 0) {
		for (ndigits = SIGFIGS; s0[ndigits - 1] == '0'; ndigits--)
			;
	}

	s = s0 + ndigits;
	*s = '\0';
	if (rve != NULL)
		*rve = s;
	return (s0);
}