#include <stdlib.h>
#include <stdbool.h>
#include <ctype.h>
#include <errno.h>

unsigned long long int strtoull(char const * restrict ptr,
	char ** restrict endptr, int base)
{
	/* Save the value of ptr in endptr now in case the format is invalid */
	if(endptr) *endptr = (char *)ptr;

	/* Skip initial whitespace */
	while(isspace(*ptr)) ptr++;

	/* Accept a sign character */
	bool negative = false;
	if(*ptr == '-') negative = true;
	if(*ptr == '-' || *ptr == '+') ptr++;

	unsigned long long x = 0;
	bool valid = false;

	/* Read prefixes and determine base */
	if((base == 0 || base == 16) && ptr[0]=='0' && tolower(ptr[1])=='x') {
		ptr += 2;
		base = 16;
	}
	else if(base == 0 && ptr[0] == '0') {
		ptr++;
		base = 8;
	}
	else if(base == 0) {
		base = 10;
	}

	/* Read digits */
	while(1) {
		int v = -1;
		if(isdigit(*ptr)) v = *ptr - '0';
		if(islower(*ptr)) v = *ptr - 'a' + 10;
		if(v == -1 || v >= base) break;

		/* The value is valid as long as there is at least one digit */
		valid = true;

		/* (x = base*x + v) but with overflow checks */
		if(__builtin_umulll_overflow(x, base, &x))
			errno = ERANGE;
		if(__builtin_uaddll_overflow(x, v, &x))
			errno = ERANGE;

		ptr++;
	}

	if(negative) {
		if(x != 0) errno = ERANGE;
		x = -x;
	}

	if(endptr && valid) *endptr = (char *)ptr;
	return x;
}