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#include <cstdint>
#include <string_view>
namespace fnv1b {
/* FNV-1b hash function with extra rotation
* https://en.wikipedia.org/wiki/Fowler–Noll–Vo_hash_function
*/
template<typename T = std::size_t>
inline T hash(const char* str, std::size_t size) noexcept;
template<typename T>
inline constexpr T ror(T x, unsigned N) noexcept {
return (x << (sizeof(T)*8 - N)) | ((x) >> (N));
}
template<>
inline constexpr uint32_t hash<uint32_t>(const char* str, std::size_t size) noexcept {
constexpr uint32_t val32 = 0x811c9dc5;
constexpr uint32_t prime32 = 16777619;
uint32_t result = val32;
while(size--) {
result ^= uint32_t(*str);
result = ror(result, (8+(0xF&(*str++)))); /* added rotation 8-23 bits */
result *= prime32;
}
return result;
}
template<>
inline constexpr uint64_t hash<uint64_t>(const char* str, std::size_t size) noexcept {
constexpr uint64_t val64 = 0xcbf29ce484222325;
constexpr uint64_t prime64 = 1099511628211ULL;
uint64_t result = val64;
while(size--) {
result ^= uint64_t(*str);
result = ror(result, (8+(*str++)%48)); /* added rotation 8-55 bits */
result *= prime64;
}
return result;
}
template<typename T = std::size_t>
inline constexpr T hash(const std::string_view str) noexcept {
return hash<T>(str.data(), str.size());
}
}
constexpr uint32_t operator "" _h(const char* str, size_t size) noexcept {
return fnv1b::hash<uint32_t>(str, size);
}
#include <iostream>
#include <string>
using namespace std;
int main() {
auto s = "hello";
switch(fnv1b::hash<uint32_t>(string_view(s))) {
case "world"_h: cout << "World"; break;
case "hello"_h: cout << "Hello"; break;
}
cout << endl;
}
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