c - Variadic macro trick

Question

What's the trick to create a variadic macro FOO(a1, a2, a3,..., an) such that it expands to FOOn(a1, a2, a3,..., an) for values of n in whatever preselected bounded range you choose? That is, FOO(a) should expand to FOO1(a), FOO(a, b, c) to FOO3(a, b, c), etc. I know there's a standard trick but I can't seem to find it.

Please feel free to mark this question as a duplicate and close it if there's another question with the answer. I suspect there is but I couldn't find it.

Answer 1

#define VA_NARGS_IMPL(_1, _2, _3, _4, _5, N, ...) N
#define VA_NARGS(...) VA_NARGS_IMPL(__VA_ARGS__, 5, 4, 3, 2, 1)

#define FOO_IMPL2(count, ...) FOO ## count (__VA_ARGS__)
#define FOO_IMPL(count, ...) FOO_IMPL2(count, __VA_ARGS__) 
#define FOO(...) FOO_IMPL(VA_NARGS(__VA_ARGS__), __VA_ARGS__)

FOO(a)
FOO(a, b)
FOO(a, b, c)

The invocations are replaced by:

FOO1 (a)
FOO2 (a, b)
FOO3 (a, b, c)