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comment non-obvious de bruijn sequence code in int parser

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Rich Felker 2011-07-25 09:21:40 -04:00
commit dd92a09eca
1 changed files with 2 additions and 0 deletions
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src/internal/intparse.c
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@ -70,6 +70,8 @@ int __intparse(struct intparse *v, const void *buf, size_t n)
for (; n && *s-'0'<10U && v->small<=SLIM; n--, s++) for (; n && *s-'0'<10U && v->small<=SLIM; n--, s++)
v->small = v->small * 10 + (*s-'0'); v->small = v->small * 10 + (*s-'0');
} else if ((b&-b) == b) { } else if ((b&-b) == b) {
/* Compute bitshift for power-of-two bases
* using a De Bruijn B(2,3) sequence. */
int bs = "\0\1\2\4\7\3\6\5"[(0x17*b)>>5&7]; int bs = "\0\1\2\4\7\3\6\5"[(0x17*b)>>5&7];
for (; n && (d=digits[*s])<b && v->small<=SLIM; n--, s++) for (; n && (d=digits[*s])<b && v->small<=SLIM; n--, s++)
v->small = (v->small<<bs) + d; v->small = (v->small<<bs) + d;