xref: /plan9/sys/src/ape/lib/ap/stdio/ftoa.c (revision 3e12c5d1bb89fc02707907988834ef147769ddaf)
1 #include <math.h>
2 #include <stdlib.h>
3 double pow10(int);
4 #define	NDIG	18
5 #define	NFTOA	(NDIG+4)
6 /*
7  * convert floating to ascii.  ftoa returns an integer e such that
8  * f=g*10**e, with .1<=|g|<1 (e=0 when g==0) and puts an ascii
9  * representation of g in the buffer pointed to by bp.  bp[0] will
10  * be '+' or '-', and bp[1] to bp[NFTOA-2]
11  * will be appropriate digits of g. bp[NFTOA-1] will be '\0'
12  */
ftoa(double f,char * bp)13 int ftoa(double f, char *bp){
14 	int e, e1, e2, i;
15 	double digit, g, p;
16 	if(f>=0) *bp++='+';
17 	else{
18 		f=-f;
19 		*bp++='-';
20 	}
21 	/* find e such that f==0 or 1<=f*pow10(e)<10, and set f=f*pow10(e) */
22 	if(f==0.) e=1;
23 	else{
24 		frexp(f, &e);
25 		e=-e*30103/100000;
26 		/* split in 2 pieces to guard against overflow in extreme cases */
27 		e1=e/2;
28 		e2=e-e1;
29 		p=f*pow10(e2);
30 		while((g=p*pow10(e1))<1.) e1++;
31 		while((g=p*pow10(e1))>=10.) --e1;
32 		e=e1+e2;
33 		f=g;
34 	}
35 	for(i=0;i!=NDIG;i++){
36 		f=modf(f, &digit)*10.;
37 		*bp++=digit+'0';
38 	}
39 	*bp='\0';
40 	return 1-e;
41 }
42