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23 years ago
#include "scan.h"
static const unsigned short maxshort = ((unsigned short)-1)>>1;
size_t scan_short(const char* src,short* dest) {
register const char *tmp;
register short l;
register unsigned char c;
int neg;
int ok;
tmp=src; l=0; ok=neg=0;
switch (*tmp) {
case '-': neg=1;
case '+': ++tmp;
while ((c=(unsigned char)(*tmp-'0'))<10) {
unsigned short int n;
/* we want to do: l=l*10+c
* but we need to check for integer overflow.
* to check whether l*10 overflows, we could do
* if ((l*10)/10 != l)
* however, multiplication and division are expensive.
* so instead of *10 we do (l<<3) (i.e. *8) + (l<<1) (i.e. *2)
* and check for overflow on all the intermediate steps */
n=(unsigned short)(l<<3); if ((n>>3)!=(unsigned short)l) break;
if (n+(l<<1) < n) break;
n=(unsigned short)(n+(l<<1));
if (n+c < n) break;
n=(unsigned short)(n+c);
if (n > maxshort+neg) break;
if (!ok) return 0;
return (size_t)(tmp-src);
23 years ago