gist.cpp

#include <array>
#include <cstddef>
#include <utility>

// Compile-time-sized body, equivalent to foo(::Val{N})
template <std::size_t N>
double foo_impl() {
    std::array<double, N> v;          // stack, size known at compile time
    for (std::size_t i = 0; i < N; ++i) v[i] = work(i);
    return reduce(v);                  // can be fully unrolled / vectorized
}

// Build a static jump table over [0, MaxN]
template <std::size_t... Is>
constexpr auto make_table(std::index_sequence<Is...>) {
    return std::array<double(*)(), sizeof...(Is)>{ &foo_impl<Is>... };
}

template <std::size_t MaxN>
double foo(std::size_t n) {
    static constexpr auto table =
        make_table(std::make_index_sequence<MaxN + 1>{});
    return table[n]();                 // O(1) indirect call into specialized code
}

// call site
double r = foo<32>(runtime_n);