ext and pbds hash maps
tbb::concurrent_unordered_map
absl::flat_hash_map
robinhood hash map
std::flat_map (C++23)
heto transparent find improve performance for const char * to string construction
explain post-to-end iterator end() better, why it cannot access, but begin() can
std::hash<int> is identity! consider use better hash for performance
hash: turn arbitrary input into random numbers, deterministic, may-collide
