inline unsigned int count_leading_zeros(N n) {
#if defined(__GNUC__) || defined(__clang__)
// GCC’s `__builtin_clz` returns an undefined value when called with 0
// as argument.
// [<http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html>]
if (n == 0) {
// Return the quantity of zero bits in `n` rather than
// returning that undefined value.
return static_cast<unsigned int>(bit_width(n));
}
#endif
// Dispatch, on the static type of `n`, to one of the
// `count_leading_zeros_impl` functions.
return count_leading_zeros_impl(n, bit_width(n));
// The second argument to `count_leading_zeros_impl` specifies the
// width, in bits, of `n`.
//
// This is necessary because `n` may be widened (or, alas, shrunk),
// and thus the information of `n`’s true width may be lost.
//
// At least, this *was* necessary before there were so many overloads
// of `count_leading_zeros_impl`, but I’ve kept it anyway as an extra
// precautionary measure, that will (I hope) be optimized out.
//
// To be clear, `n` would only be shrunk in cases noted above as
// having an undefined result.
}
static constexpr value_type mod_shift( value_type x )
{
return x & ( 1 << ( bit_width() - 1 ) )
? ( x << 1 ) ^ Polynomial
: x << 1
;
}
static constexpr value_type initial_remainder( std::size_t n )
{
return n << ( bit_width() - 8 );
}
