fltfmt.c

这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

#include <u.h>
#include <libc.h>
#include <ctype.h>
#include "fmtdef.h"

enum
{
	FDIGIT	= 30,
	FDEFLT	= 6,
	NSIGNIF	= 17,
	NEXP10	= 308,
};

static int
xadd(char *a, int n, int v)
{
	char *b;
	int c;

	if(n < 0 || n >= NSIGNIF)
		return 0;
	for(b = a+n; b >= a; b--) {
		c = *b + v;
		if(c <= '9') {
			*b = c;
			return 0;
		}
		*b = '0';
		v = 1;
	}
	*a = '1';	// overflow adding
	return 1;
}

static int
xsub(char *a, int n, int v)
{
	char *b;
	int c;

	for(b = a+n; b >= a; b--) {
		c = *b - v;
		if(c >= '0') {
			*b = c;
			return 0;
		}
		*b = '9';
		v = 1;
	}
	*a = '9';	// underflow subtracting
	return 1;
}

static void
xdtoa(Fmt *fmt, char *s2, double f)
{
	char s1[NSIGNIF+10];
	double g, h;
	int e, d, i, n;
	int c1, c2, c3, c4, ucase, sign, chr, prec;

	prec = FDEFLT;
	if(fmt->flags & FmtPrec)
		prec = fmt->prec;
	if(prec > FDIGIT)
		prec = FDIGIT;
	if(isNaN(f)) {
		strcpy(s2, "NaN");
		return;
	}
	if(isInf(f, 1)) {
		strcpy(s2, "+Inf");
		return;
	}
	if(isInf(f, -1)) {
		strcpy(s2, "-Inf");
		return;
	}
	sign = 0;
	if(f < 0) {
		f = -f;
		sign++;
	}
	ucase = 0;
	chr = fmt->r;
	if(isupper(chr)) {
		ucase = 1;
		chr = tolower(chr);
	}

	e = 0;
	g = f;
	if(g != 0) {
		frexp(f, &e);
		e = e * .301029995664;
		if(e >= -150 && e <= +150) {
			d = 0;
			h = f;
		} else {
			d = e/2;
			h = f * pow10(-d);
		}
		g = h * pow10(d-e);
		while(g < 1) {
			e--;
			g = h * pow10(d-e);
		}
		while(g >= 10) {
			e++;
			g = h * pow10(d-e);
		}
	}

	/*
	 * convert NSIGNIF digits and convert
	 * back to get accuracy.
	 */
	for(i=0; i<NSIGNIF; i++) {
		d = g;
		s1[i] = d + '0';
		g = (g - d) * 10;
	}
	s1[i] = 0;

	/*
	 * try decimal rounding to eliminate 9s
	 */
	c2 = prec + 1;
	if(chr == 'f')
		c2 += e;
	if(c2 >= NSIGNIF-2) {
		strcpy(s2, s1);
		d = e;
		s1[NSIGNIF-2] = '0';
		s1[NSIGNIF-1] = '0';
		sprint(s1+NSIGNIF, "e%d", e-NSIGNIF+1);
		g = strtod(s1, nil);
		if(g == f)
			goto found;
		if(xadd(s1, NSIGNIF-3, 1)) {
			e++;
			sprint(s1+NSIGNIF, "e%d", e-NSIGNIF+1);
		}
		g = strtod(s1, nil);
		if(g == f)
			goto found;
		strcpy(s1, s2);
		e = d;
	}

	/*
	 * convert back so s1 gets exact answer
	 */
	for(;;) {
		sprint(s1+NSIGNIF, "e%d", e-NSIGNIF+1);
		g = strtod(s1, nil);
		if(f > g) {
			if(xadd(s1, NSIGNIF-1, 1))
				e--;
			continue;
		}
		if(f < g) {
			if(xsub(s1, NSIGNIF-1, 1))
				e++;
			continue;
		}
		break;
	}

found:
	/*
	 * sign
	 */
	d = 0;
	i = 0;
	if(sign)
		s2[d++] = '-';
	else if(fmt->flags & FmtSign)
		s2[d++] = '+';
	else if(fmt->flags & FmtSpace)
		s2[d++] = ' ';

	/*
	 * copy into final place
	 * c1 digits of leading '0'
	 * c2 digits from conversion
	 * c3 digits of trailing '0'
	 * c4 digits after '.'
	 */
	c1 = 0;
	c2 = prec + 1;
	c3 = 0;
	c4 = prec;
	switch(chr) {
	default:
		if(xadd(s1, c2, 5))
			e++;
		break;
	case 'g':
		/*
		 * decide on 'e' of 'f' style convers
		 */
		if(xadd(s1, c2, 5))
			e++;
		if(e >= -5 && e <= prec) {
			c1 = -e - 1;
			c4 = prec - e;
			chr = 'h';	// flag for 'f' style
		}
		break;
	case 'f':
		if(xadd(s1, c2+e, 5))
			e++;
		c1 = -e;
		if(c1 > prec)
			c1 = c2;
		c2 += e;
		break;
	}

	/*
	 * clean up c1 c2 and c3
	 */
	if(c1 < 0)
		c1 = 0;
	if(c2 < 0)
		c2 = 0;
	if(c2 > NSIGNIF) {
		c3 = c2-NSIGNIF;
		c2 = NSIGNIF;
	}

	/*
	 * copy digits
	 */
	while(c1 > 0) {
		if(c1+c2+c3 == c4)
			s2[d++] = '.';
		s2[d++] = '0';
		c1--;
	}
	while(c2 > 0) {
		if(c2+c3 == c4)
			s2[d++] = '.';
		s2[d++] = s1[i++];
		c2--;
	}
	while(c3 > 0) {
		if(c3 == c4)
			s2[d++] = '.';
		s2[d++] = '0';
		c3--;
	}

	/*
	 * strip trailing '0' on g conv
	 */
	if(fmt->flags & FmtSharp) {
		if(0 == c4)
			s2[d++] = '.';
	} else
	if(chr == 'g' || chr == 'h') {
		for(n=d-1; n>=0; n--)
			if(s2[n] != '0')
				break;
		for(i=n; i>=0; i--)
			if(s2[i] == '.') {
				d = n;
				if(i != n)
					d++;
				break;
			}
	}
	if(chr == 'e' || chr == 'g') {
		if(ucase)
			s2[d++] = 'E';
		else
			s2[d++] = 'e';
		c1 = e;
		if(c1 < 0) {
			s2[d++] = '-';
			c1 = -c1;
		} else
			s2[d++] = '+';
		if(c1 >= 100) {
			s2[d++] = c1/100 + '0';
			c1 = c1%100;
		}
		s2[d++] = c1/10 + '0';
		s2[d++] = c1%10 + '0';
	}
	s2[d] = 0;
}

int
_floatfmt(Fmt *fmt, double f)
{
	char s[1+NEXP10+1+FDIGIT+1];

	/*
	 * The max length of a %f string is
	 *	'[+-]'+"max exponent"+'.'+"max precision"+'\0'
	 * which is 341 currently.
	 */	
	xdtoa(fmt, s, f);
	fmt->flags &= FmtWidth|FmtLeft;
	_fmtcpy(fmt, s, strlen(s), strlen(s));
	return 0;
}

int
_efgfmt(Fmt *f)
{
	double d;

	d = va_arg(f->args, double);
	return _floatfmt(f, d);
}