Use a templated constexpr at runtime

There is this crc32 implementation that I like: CygnusX1 CRC32

It works well at compile time:

ctcrc32("StackOverflow");

But is it possible to use it at runtime:

void myfunction(const std::string& str)
{
  uint32_t hash = ctcrc32(str);
  // ...
}

So far I had to rewrite another (runtime) function but would prefer to use just one.

EDIT

I did tried with

ctcrc32(str.c_str()) 

But it doesn't work (** mismatched types ‘const char [len]’ and ‘const char*’ **). It seems to require a compile-time length.

Here is the implementation:

namespace detail {
// CRC32 Table (zlib polynomial)
static constexpr uint32_t crc_table[256] = { 0x00000000L, 0x77073096L, ... }

template<size_t idx>
constexpr uint32_t combine_crc32(const char * str, uint32_t part) {
  return (part >> 8) ^ crc_table[(part ^ str[idx]) & 0x000000FF];
}

template<size_t idx>
constexpr uint32_t crc32(const char * str) {
  return combine_crc32<idx>(str, crc32<idx - 1>(str));
}

// This is the stop-recursion function
template<>
constexpr uint32_t crc32<size_t(-1)>(const char * str) {
  return 0xFFFFFFFF;
}

} //namespace detail

template <size_t len>
constexpr uint32_t ctcrc32(const char (&str)[len]) {
  return detail::crc32<len - 2>(str) ^ 0xFFFFFFFF;
}

You cannot use it with a std::string without rewriting it. If you look at the main function:

template <size_t len>
constexpr uint32_t ctcrc32(const char (&str)[len]) {
  return detail::crc32<len - 2>(str) ^ 0xFFFFFFFF;
}

...you see that it needs the length of the string at compile time because it uses it as a template parameter (detail::crc32<len - 2>).

ctcrc32 will only works with character arrays whose size is known at compile time (they don't have to be const or constexpr, but the size must be known).

I wrote an answer based on the original implementation to the linked question that allows both compile-time and runtime strings:

https://stackoverflow.com/a/48924267/2666289