constexpr hash function

Question

Here's a constexpr hash function, that will pack a string into the largest unsigned integral type available. So, what do you think?

#include <climits>
#include <cstdint>
#include <utility>
#include <iostream>
namespace detail
{
template <typename T, std::size_t ...I>
constexpr T hash(char const* const s, std::size_t const N,
 std::index_sequence<I...>) noexcept
{
 return ((T(s[I < N ? I : 0]) << ((I < N ? I : 0) * CHAR_BIT)) | ...);
}
}
template <typename T = std::uintmax_t>
constexpr T hash(char const* const s, std::size_t const N) noexcept
{
 return detail::hash<T>(s, N, std::make_index_sequence<sizeof(T)>());
}
template <typename T = std::uintmax_t, std::size_t N>
constexpr T hash(char const(&s)[N]) noexcept
{
 return hash<T>(s, N - 1);
}
int main()
{
 std::cout << (hash("a") == 'a') << std::endl;
 return 0;
}

https://wandbox.org/permlink/KbPiWJc434xYLL3q

Answer 1

Your code is too complex. With C++17, you can write more complex constexpr functions, so you don't need the variadic template tricks:

template <typename T = std::uintmax_t, std::size_t N>
constexpr T hash(char const(&s)[N]) noexcept
{
 T val{};
 for (size_t i = 0; i < N; ++i)
 val |= s[i] << (i * CHAR_BIT);
 return val;
}

Apart from that, this is a terrible hash function! The output is highly correlated to the input. It will also only hash up to sizeof(T) characters, so long strings with a common prefix might all get the same hash value.

• user1095108: take one more look at the code, what does std::make_index_sequence<sizeof(T)>() do?
• G. Sliepen: Ah, missed that!
• user1095108: The whole idea was to implement a perfect hash for small strings. We are going 128-bit, so it might not be such a bad idea. Even now, it's possible to enable a 128-bit uint by using -std=gnu++17 That's 16 chars.
• G. Sliepen: Can you clarify in your question what the use case is of this hash function? Because I don't see the point in what basically is a memcpy() from a string into a suitably large int. I wouldn't call it hashing.
• G. Sliepen: How oes this help you avoid a chain of if-comparisons? Please give a concrete example of how you intend to use this hash function.