test.cpp

// This code is released under the terms of the "CC0" license.  Full terms and conditions
// can be found at: http://creativecommons.org/publicdomain/zero/1.0/

#include <iostream>

// NOTE: hashing algorithm used is FNV-1a

// FNV-1a constants
static constexpr unsigned long long basis = 14695981039346656037ULL;
static constexpr unsigned long long prime = 1099511628211ULL;

// compile-time hash helper function
constexpr unsigned long long hash_one(char c, const char* remain, unsigned long long value)
{
	return c == 0 ? value : hash_one(remain[0], remain + 1, (value ^ c) * prime);
}

// compile-time hash
constexpr unsigned long long hash(const char* str)
{
	return hash_one(str[0], str + 1, basis);
}

// run-time hash
unsigned long long hash_rt(const char* str)
{
	unsigned long long hash = basis;
	while (*str != 0) {
		hash ^= str[0];
		hash *= prime;
		++str;
	}
	return hash;
}

// ensure that the constexpr value is really compile-time constant
template <unsigned long long NUM>
struct test_const
{
	static const unsigned long long value = NUM;
};

// test
int main()
{
	constexpr unsigned long long value = test_const<hash("hello, world!")>::value;
	unsigned long long value_rt = hash_rt("hello, world!");

	std::cout << "hash =    " << value << std::endl;
	std::cout << "hash_rt = " << value_rt << std::endl;

	return value == value_rt ? 0 : 127;
}