uclibc-ng/libc/misc/internals/dtostr.c

/*
 * Copyright (C) 2000 Manuel Novoa III
 *
 * Function:  const char *__dtostr(double x)
 *
 * This was written for uClibc to give it the ability to at least output
 * floating point values in exponential form.  No formatting is done.
 * No trailing 0's are removed.  Number of digits generated is not
 * runtime selectable.  It does however handle +/- infinity and nan on i386.
 *
 * The goal was usable floating point %e-type output in minimal size.  For
 * me, "gcc -c -Os -fomit-frame-pointer dtostr.c && size dtostr.o" gives
 *   text    data     bss    dec     hex filename
 *   535       9      26     570     23a dtostr.o    (WANT_EXP_FORM = 1)
 *   530       9      26     565     235 dtostr.o    (WANT_EXP_FORM = 0)
 *
 * Output is of the form [-][#].######{e|E}{+|-}##.  Choices of upper or
 * lower case exponent character, and initial digit/initial decimal forms
 * are compile-time options.  Number of digits generated is also selected
 * at compile time (MAX_DIGITS).
 *
 * Notes:
 *
 * The primary objective of this implementation was minimal size while
 * maintaining reasonable accuracy.  It should also be fairly portable,
 * as not assumptions are made about the bit-layout of doubles.
 *
 * It should be too difficult to convert this to handle long doubles on i386.
 * For information, see the comments below.
 *
 * There are 2 compile-time options below, as well as some tuning parameters.
 *
 * TODO: 
 *   long double and/or float version?  (note: for float can trim code some).
 */

/*****************************************************************************/
/*                            OPTIONS                                        */
/*****************************************************************************/

/*
 * Set this if you want output results with 1 digit before the decimal point.
 * If this is 0, all digits follow a leading decimal point.
 */
#define WANT_EXP_FORM   1

/*
 * Set if you want exponent character 'E' rather than 'e'.
 */
#define EXP_UPPERCASE   1

/*****************************************************************************/
/* Don't change anything that follows unless you know what you're doing.     */
/*****************************************************************************/

/*
 * Configuration for the scaling power table.  Ignoring denormals, you
 * should have 2**EXP_TABLE_SIZE >= MAX_DBL_EXP >= 2**(EXP_TABLE_SIZE-1).
 * The minimum for standard C is 6.  For IEEE 8bit doubles, 9 suffices.
 */
#define EXP_TABLE_SIZE       9

/* 
 * Set this to the maximum number of digits you want converted.
 * Conversion is done in blocks of DIGITS_PER_BLOCK (9 by default) digits.
 * 17 digits suffices to uniquely determine a double on i386.
 */
#define MAX_DIGITS          17

/*
 * This is really only used to check for infinities.  The macro produces
 * smaller code for i386 and, since this is tested before any floating point
 * calculations, it doesn't appear to suffer from the excess precision problem
 * caused by the FPU that strtod had.  If it causes problems, call the function
 * and compile zoicheck.c with -ffloat-store.
 */
#if 1
#define _zero_or_inf_check(x) ( x == (x/4) )
#else
extern int _zero_or_inf_check(double x);
#endif

/*
 * Fairly portable nan check.  Bitwise for i386 generated larger code.
 * If you have a better version, comment this out.
 */
#define isnan(x) (x != x)

/*****************************************************************************/
/* Don't change anything that follows peroid!!!  ;-)                         */
/*****************************************************************************/

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

/*
 * Set things up for the scaling power table.
 */

#if EXP_TABLE_SIZE < 6
#error EXP_TABLE_SIZE should be at least 6 to comply with standards
#endif

#define EXP_TABLE_MAX      (1U<<(EXP_TABLE_SIZE-1))

/*
 * Only bother checking if this is too small.
 * Throw in some play for denormals ( roughly O(-324) vs O(-307) on i386 ).
 */

#if (3+DBL_DIG-DBL_MIN_10_EXP)/2 > EXP_TABLE_MAX
#error larger EXP_TABLE_SIZE needed
#endif

/*
 * With 32 bit ints, we can get 9 digits per block.
 */
#define DIGITS_PER_BLOCK     9

#if (INT_MAX >> 30)
#define DIGIT_BLOCK_TYPE     int
#elif (LONG_MAX >> 30)
#define DIGIT_BLOCK_TYPE     long
#else
#error need at least 32 bit longs
#endif

/*
 * This is kind of a place-holder for LONG_DOUBLE support to show what I
 * think needs to be changed.  I haven't tried it though.  Changing this
 * from 3 to 4 and converting double to long double should work on i386.
 * DON'T FORGET to increase EXP_TABLE_SIZE and MAX_DIGITS.
 * DON'T FORGET the "larger EXP_TABLE_SIZE needed" check above.
 */
#define MAX_EXP_DIGITS 3

/*****************************************************************************/

#define NUM_DIGIT_BLOCKS   ((MAX_DIGITS+DIGITS_PER_BLOCK-1)/DIGITS_PER_BLOCK)

static char infstr[] = " inf";	/* save space for a - sign */
static char nanstr[] = "nan";

/* extra space for '-', '.', 'e+###', and nul */
/*static char buf[ 5 + MAX_EXP_DIGITS + NUM_DIGIT_BLOCKS * DIGITS_PER_BLOCK];*/
#define BUF_SIZE  ( 5 + MAX_EXP_DIGITS + NUM_DIGIT_BLOCKS * DIGITS_PER_BLOCK )
/*****************************************************************************/

const char *__dtostr(char *buf, double x)
{
	double exp_table[EXP_TABLE_SIZE];
	double p10;
	DIGIT_BLOCK_TYPE digit_block; /* int of at least 32 bits */
	int i, j;
	int exp, exp_neg;
	int negative = 0;
	char *pos;

	if (isnan(x)) {				/* nan check */
		return nanstr;
	}

	if (x == 0) {				/* handle 0 now to avoid false positive */
		exp = 0;				/* with inf test, and to avoid scaling  */
		goto GENERATE_DIGITS;	/* note: time vs space tradeoff */
	}

	if (x < 0) {				/* convert negatives to positives */
		negative = 1;
		x = -x;
	}

	if (_zero_or_inf_check(x)) { /* must be inf since zero handled above */
		pos = infstr + 1;
		goto DO_SIGN;
	}

	/* need to build the scaling table */
	for (i = 0, p10 = 10 ; i < EXP_TABLE_SIZE ; i++) {
		exp_table[i] = p10;
		p10 *= p10;
	}

	exp_neg = 0;
	if (x < 1e8) {				/* do we need to scale up or down? */
		exp_neg = 1;
	}

#if WANT_EXP_FORM
	exp = DIGITS_PER_BLOCK - 1;
#else
	exp = DIGITS_PER_BLOCK;
#endif

	i = EXP_TABLE_SIZE;
	j = EXP_TABLE_MAX;
	while ( i-- ) {				/* scale x such that 1e8 <= x < 1e9 */
		if (exp_neg) {
			if (x * exp_table[i] < 1e9) {
				x *= exp_table[i];
				exp -= j;
			}
		} else {
			if (x / exp_table[i] >= 1e8) {
				x /= exp_table[i];
				exp += j;
			}
		}
		j >>= 1;
	}

 GENERATE_DIGITS:
	pos = buf - BUF_SIZE + 1 + DIGITS_PER_BLOCK + 1; /* leave space for '.' and - */

	for (i = 0 ; i < NUM_DIGIT_BLOCKS ; ++i ) {
		digit_block = (int) x;
		x = (x - digit_block) * 1e9;
		for (j = 0 ; j < DIGITS_PER_BLOCK ; j++) {
			*--pos = '0' + (digit_block % 10);
			digit_block /= 10;
		}
		pos += (2*DIGITS_PER_BLOCK);
	}
	pos -= (DIGITS_PER_BLOCK*(NUM_DIGIT_BLOCKS+1))-MAX_DIGITS;

	/* start generating the exponent */
#if EXP_UPPERCASE
	*pos = 'E';
#else
	*pos = 'e';
#endif
	*++pos = '+';
	if (exp < 0) {
		*pos = '-';
		exp = -exp;
	}
	pos += 3;					/* WARNING: Assumes max exp < 1000!!! */
	if (exp >= 100) {
		++pos;
#if MAX_EXP_DIGITS > 4
#error need to modify exponent string generation code
#elif MAX_EXP_DIGITS > 3
		if (exp >= 1000) {		/* WARNING: hasn't been checked */
			++pos;				/*    but should work */
		}
#endif
	}
	*pos = '\0';
		
	for (j = 0 ; (j < 2) || exp ; j++) { /* standard says at least 2 digits */
		*--pos = '0' + (exp % 10);
		exp /= 10;
	}

	/* insert the decimal point */
	pos = buf - BUF_SIZE + 1;

#if WANT_EXP_FORM
	*pos = *(pos+1);
	*(pos+1) = '.';
#else
	*pos = '.';
#endif

 DO_SIGN:
	if (negative) {
		*--pos = '-';
	}

	return pos;
}