#include <boost/preprocessor/arithmetic/dec.hpp>
#include <boost/preprocessor/arithmetic/div.hpp>
#include <boost/preprocessor/control/while.hpp>
#include <boost/preprocessor/tuple/elem.hpp>
#define PRED(d, data) BOOST_PP_TUPLE_ELEM(2, 0, data)
#define OP(d, data) /* ............... */ \
( \
BOOST_PP_DEC( \
BOOST_PP_TUPLE_ELEM(2, 0, data) \
), \
BOOST_PP_DIV_D( \
d, \
BOOST_PP_TUPLE_ELEM(2, 1, data), \
2 \
) \
) \
/**/
// halve 'x' 'n' times
#define HALVE(x, n) BOOST_PP_TUPLE_ELEM(2, 1, BOOST_PP_WHILE(PRED, OP, (n, x)))
HALVE(8, 2) // expands to 2
HALVE(16, 1) // expands to 8