ref: ff2ebe6cfdacd890a74a6d2f9b0acd062d2633b7
dir: /sys/src/ape/lib/ap/stdio/ftoa.c/
#include <math.h>
#include <stdlib.h>
double pow10(int);
#define NDIG 18
#define NFTOA (NDIG+4)
/*
* convert floating to ascii. ftoa returns an integer e such that
* f=g*10**e, with .1<=|g|<1 (e=0 when g==0) and puts an ascii
* representation of g in the buffer pointed to by bp. bp[0] will
* be '+' or '-', and bp[1] to bp[NFTOA-2]
* will be appropriate digits of g. bp[NFTOA-1] will be '\0'
*/
int ftoa(double f, char *bp){
int e, e1, e2, i;
double digit, g, p;
if(f>=0) *bp++='+';
else{
f=-f;
*bp++='-';
}
/* find e such that f==0 or 1<=f*pow10(e)<10, and set f=f*pow10(e) */
if(f==0.) e=1;
else{
frexp(f, &e);
e=-e*30103/100000;
/* split in 2 pieces to guard against overflow in extreme cases */
e1=e/2;
e2=e-e1;
p=f*pow10(e2);
while((p*pow10(e1))<1.) e1++;
while((g=p*pow10(e1))>=10.) --e1;
e=e1+e2;
f=g;
}
for(i=0;i!=NDIG;i++){
f=modf(f, &digit)*10.;
*bp++=digit+'0';
}
*bp='\0';
return 1-e;
}