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#include <u.h>
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#include <libc.h>
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#define ULONG_MAX	4294967295UL
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ulong
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strtoul(char *nptr, char **endptr, int base)
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{
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	char *p;
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	ulong n, nn, m;
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	int c, ovfl, neg, v, ndig;
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	p = nptr;
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	neg = 0;
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	n = 0;
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	ndig = 0;
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	ovfl = 0;
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	/*
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	 * White space
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	 */
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	for(;;p++){
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		switch(*p){
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		case ' ':
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		case '\t':
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		case '\n':
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		case '\f':
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		case '\r':
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		case '\v':
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			continue;
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		}
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		break;
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	}
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	/*
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	 * Sign
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	 */
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	if(*p=='-' || *p=='+')
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		if(*p++ == '-')
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			neg = 1;
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	/*
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	 * Base
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	 */
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	if(base==0){
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		if(*p != '0')
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			base = 10;
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		else{
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			base = 8;
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			if(p[1]=='x' || p[1]=='X')
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				base = 16;
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		}
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	}
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 	if(base<2 || 36<base)
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		goto Return;
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	if(base==16 && *p=='0'){
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		if(p[1]=='x' || p[1]=='X')
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			if(('0' <= p[2] && p[2] <= '9')
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			 ||('a' <= p[2] && p[2] <= 'f')
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			 ||('A' <= p[2] && p[2] <= 'F'))
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				p += 2;
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	}
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	/*
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	 * Non-empty sequence of digits
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	 */
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	n = 0;
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	m = ULONG_MAX/base;
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	for(;; p++,ndig++){
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		c = *p;
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		v = base;
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		if('0'<=c && c<='9')
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			v = c - '0';
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		else if('a'<=c && c<='z')
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			v = c - 'a' + 10;
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		else if('A'<=c && c<='Z')
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			v = c - 'A' + 10;
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		if(v >= base)
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			break;
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		if(n > m)
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			ovfl = 1;
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		nn = n*base + v;
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		if(nn < n)
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			ovfl = 1;
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		n = nn;
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	}
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    Return:
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	if(ndig == 0)
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		p = nptr;
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	if(endptr)
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		*endptr = p;
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	if(ovfl)
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		return ULONG_MAX;
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	if(neg)
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		return -n;
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	return n;
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}