main.c

#include <stdio.h>
#include <assert.h>
#include <stdlib.h>

#define xor_to(x) _xor_to((x),0,0)
long long _xor_to(long long to,long long cur,long long acc)
{
	cur+=1;
	if(cur > to)
	{
		return acc;
	}

	acc^=cur;

	return _xor_to(to,cur,acc);
}

int main(int argc,char **argv)
{
	assert(argc == 2);

	long long i=atoll(argv[1]);

	printf("%llx\n",xor_to(i));

	return 0;
}