Should I cache the hash code of an STL string used as a hash key?

Question

I've doing some performance analysis on the software I develop, and I've found that lookups on a global dictionary of URL's takes about 10% of the application's "load" phase time. The dictionary is implemented as a C++ STL std::map, which has O(lg n) lookups. I'm going to move it to a hash_map, which has roughly fixed time lookups. The stl string class doesn't have a hash code property, and it certainly doesn't cache a hash code. That means that each lookup requires re-generating the hash code.

I'm skeptical that caching the hash code is worth the effort. It would mean changing many lines of code to use a new string class with a cached hash code property. Given that the current implementation does log(n) full string comparisons on every lookup, I think reducing it to basically one string traversal (by the hash function) per lookup is a big win.

Does anyone have experience with caching string hash codes? Has it ever proven worth the effort?

Answer 1

One word of warning.

While a hash map can have fixed time lookups, it also can end up having O(N) lookups. While it's not a common case, it does happen.

So while you always have to pay for the O(log N) time in a map, you are also guaranteed that it will not be worse.

Answer 2

I don't have experience with caching hash codes, but I've done some work recently converting std::map to std::tr1::unordered_map. Two thoughts come to mind. First, try profiling that relatively simple change first, because it sometimes makes things worse, depending on what your code is doing. It might give you enough speedup on its own before you try optimizing further. Secondly, what does your profiler say about the other 90% of your initialization time? Even if you optimized the global dictionary stuff down to 0 time, you will at most improve performance by 10%.

Answer 3

You'll of course need to profile to check your results. Change to a hash map, and then see where most of your time is spent. Unless you're hashing keys left and right, I doubt most of your time will be spent there. Hashing is intended to be a fast operation, otherwise a hash map would have no advantages over an ordered container.

The compiler itself will know if a string hasn't been changed, and can probably cache the result for you (within the same scope). That said, you don't want to inherit from std::string; STL classes weren't made for that.

Rather, make a std::pair and pass that around:

std::pair<const std::string, const size_t> string_hash_pair;

You'd then need to overload the (going by Boost here, not TR1; I don't know how similar they are) hash_value function for your type, in the same namespace as the pair is defined:

size_t hash_value(const string_hash_pair& pPair)
{
    return pPair.second; // don't actually hash
}

And that's it. Note that in the pair, both string and size_t are immutable. This is because if the string changes, your hash is wrong. So we make it const, and we may as well make the hash const too.

You'll want a helper function:

string_hash_pair make_string_hash(const std::string& pStr)
{
    return std::make_pair(pStr, boost::hash_value(pStr));
}

Now if you're going to be using a string for look-ups, just make a pair out of it and you get constant-time hashing.

That said, I really doubt this much work is necessary. Hashing functions really are trivial, usually. Also, don't make your own. Use a pre-existing tried-and-tested hash; it's quite easy to make a crappy hash.

Answer 4

When you compare the hash map to the map, also try a Trie, or related data structure (whatever you can get off the shelf):

http://stackoverflow.com/questions/1036504/trie-implementation

Unfortunately you may then spend a lot of time worrying about cache-friendliness. In that respect a Trie is similar to the tree you already have, and a hash map will probably be better-behaved than a naively-allocated tree.

Also, I'm a little confused by the question. If you're looking up the same string object multiple times, such that caching its hash value is worthwhile, shouldn't you just be caching the result of the lookup? The whole point of a hash table is that different objects which are value-equal hash to the same value. If you aren't computing the same hash several times from distinct strings containing the same characters, then your hash table probably isn't doing its job.

If you mean caching the values of the keys already in the hash table, that's up to the hash table.