FNV1a c++11 constexpr compile time hash functions, 32 and 64 bit

hash_fnv1a.h

#pragma once
#include <stdint.h>

//fnv1a 32 and 64 bit hash functions
// key is the data to hash, len is the size of the data (or how much of it to hash against)
// code license: public domain or equivalent
// post: https://notes.underscorediscovery.com/constexpr-fnv1a/

inline const uint32_t hash_32_fnv1a(const void* key, const uint32_t len) {

    const char* data = (char*)key;
    uint32_t hash = 0x811c9dc5;
    uint32_t prime = 0x1000193;

    for(int i = 0; i < len; ++i) {
        uint8_t value = data[i];
        hash = hash ^ value;
        hash *= prime;
    }

    return hash;

} //hash_32_fnv1a

inline const uint64_t hash_64_fnv1a(const void* key, const uint64_t len) {
    
    const char* data = (char*)key;
    uint64_t hash = 0xcbf29ce484222325;
    uint64_t prime = 0x100000001b3;
    
    for(int i = 0; i < len; ++i) {
        uint8_t value = data[i];
        hash = hash ^ value;
        hash *= prime;
    }
    
    return hash;

} //hash_64_fnv1a

hash_fnv1a_constexpr.h

#pragma once
#include <stdint.h>

  // FNV1a c++11 constexpr compile time hash functions, 32 and 64 bit
  // str should be a null terminated string literal, value should be left out 
  // e.g hash_32_fnv1a_const("example")
  // code license: public domain or equivalent
  // post: https://notes.underscorediscovery.com/constexpr-fnv1a/

  constexpr uint32_t val_32_const = 0x811c9dc5;
  constexpr uint32_t prime_32_const = 0x1000193;
  constexpr uint64_t val_64_const = 0xcbf29ce484222325;
  constexpr uint64_t prime_64_const = 0x100000001b3;

  inline constexpr uint32_t hash_32_fnv1a_const(const char* const str, const uint32_t value = val_32_const) noexcept {
      return (str[0] == '\0') ? value : hash_32_fnv1a_const(&str[1], (value ^ uint32_t((uint8_t)str[0])) * prime_32_const);
  }

  inline constexpr uint64_t hash_64_fnv1a_const(const char* const str, const uint64_t value = val_64_const) noexcept {
      return (str[0] == '\0') ? value : hash_64_fnv1a_const(&str[1], (value ^ uint64_t((uint8_t)str[0])) * prime_64_const);
  }

Is there any license on this code? I would love to use this in a little graphics demo that I am building. It's for a non-commercial purpose, just for fun and learning :).

How would one use this to hash wstrings?
With std::wstring text = L"Hello There", I get different results from hash_64_fnv1a_const(L"Hello There") and hash_64_fnv1a(text.c_str(), string.size() * 2 + 1).

@epicabsol
c_str() adds a \0 byte at the end.

Actually, this code still only delivers the same result when the string is only using 7-bit ASCII when the system has a signed char (as is default on gcc/clang/msvc). The original non-constexpr code solves this issue by using uint8_t. So there needs to be an added cast, to the the constexpr code to:

inline constexpr uint32_t hash_32_fnv1a_const(const char* const str, const uint32_t value = val_32_const) noexcept {
    return (str[0] == '\0') ? value : hash_32_fnv1a_const(&str[1], (value ^ uint32_t((uint8_t)str[0])) * prime_32_const);
}

inline constexpr uint64_t hash_64_fnv1a_const(const char* const str, const uint64_t value = val_64_const) noexcept {
    return (str[0] == '\0') ? value : hash_64_fnv1a_const(&str[1], (value ^ uint64_t((uint8_t)str[0])) * prime_64_const);
}

Now the results are the same for the full range of char even on a signed char system.

(edit: original was updated)

I think we can also get rid of the recursion too!

constexpr std::uint32_t hash_32_fnv1a_const2(const char* str, std::uint32_t value = val_32_const) noexcept {
	for (; *str != '\0'; ++str) {
		value = (value ^ static_cast<std::uint32_t>(static_cast<uint8_t>(*str))) * prime_32_const;
	}
	return value;
}

constexpr std::uint64_t hash_64_fnv1a_const2(const char* str, std::uint64_t value = val_64_const) noexcept {
	for (; *str != '\0'; ++str) {
		value = (value ^ static_cast<std::uint64_t>(static_cast<uint8_t>(*str))) * prime_64_const;
	}
	return value;
}