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61 lines
1.7 KiB
C
61 lines
1.7 KiB
C
#include "scan.h"
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static const unsigned int maxint = ((unsigned int)-1)>>1;
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size_t scan_int(const char* src,int* dest) {
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register const char *tmp;
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register int l;
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register unsigned char c;
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unsigned int neg;
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int ok;
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tmp=src; l=0; ok=0; neg=0;
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switch (*tmp) {
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case '-': neg=1; /* fall through */
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case '+': ++tmp;
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}
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while ((c=(unsigned char)(*tmp-'0'))<10) {
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unsigned int n;
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#if defined(__GNUC__) && (__GNUC__ >= 5)
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if (__builtin_mul_overflow(l,10,&n) || __builtin_add_overflow(n,c,&n))
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break;
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#else
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/* we want to do: l=l*10+c
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* but we need to check for integer overflow.
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* to check whether l*10 overflows, we could do
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* if ((l*10)/10 != l)
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* however, multiplication and division are expensive.
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* so instead of *10 we do (l<<3) (i.e. *8) + (l<<1) (i.e. *2)
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* and check for overflow on all the intermediate steps */
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n=(unsigned int)l<<3; if ((n>>3)!=(unsigned int)l) break;
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if (n+((unsigned int)l<<1) < n) break;
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n+=(unsigned int)l<<1;
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if (n+c < n) break;
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n+=c;
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#endif
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if (n > maxint+neg) break;
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l=(int)n;
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++tmp;
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ok=1;
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}
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if (!ok) return 0;
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*dest=(neg?-l:l);
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return (size_t)(tmp-src);
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}
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#ifdef UNITTEST
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#include <assert.h>
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int main() {
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int i;
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assert(scan_int("1234",&i)==4 && i==1234);
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assert(scan_int("-1234",&i)==5 && i==-1234);
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assert(scan_int("+1234",&i)==5 && i==1234);
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assert(scan_int("4294967295",&i)==9 && i==429496729); // overflow
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assert(scan_int("2147483647",&i)==10 && i==2147483647); // MAX_INT
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assert(scan_int("2147483648",&i)==9 && i==214748364); // overflow
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assert(scan_int("-2147483648",&i)==11 && i==-2147483648); // MIN_INT
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assert(scan_int("-2147483649",&i)==10 && i==-214748364); // underflow
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return 0;
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}
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#endif
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