iPad上的MonoTouch：如何使文本搜索更快？

Question

I need to do text search based on user input in a relative large list (about 37K lines with 50 to 100 chars each line). The search is done after entering each character and the result is shown in a UITableView . This is my current code:

if (input.Any(x => Char.IsUpper(x)))
    return _list.Where(x => x.Desc.Contains(input));
else
    return _list.Where(x => x.Desc.ToLower().Contains(input));

It performs okay on a MacBook running simulator, but too slow on iPad.

On interesting thing I observed is that it takes longer and longer as input grows. For example, say "examin" as input. It takes about 1 second after entering e, 2 seconds after x, 5 seconds after a, but 28 seconds after m and so on. Why that?

I hope there is a simple way to improve it.

Answer 1

Always take care to avoid memory allocations in time sensitive code.

For example we often produce code often allocates string without realizing it, eg

x => x.Desc.ToLower().Contains(input)

That will allocate a string to return from ToLower . From your description this will occurs many time. You can easily avoid this by using:

x = x.Desc.IndexOf ("s", StringComparison.OrdinalIgnoreCase) != -1

note: just select the StringComparison.*IgnoreCase that match your need.

Also LINQ is nice but it hides allocations in many cases - maybe not in your case but measuring is key to get things faster. In that case using another algorithm (like suggested in another answer) could give you much better results (but keep in mind the allocations ;-)

UPDATE:

Mono's Contains(string) will call, after a few checks, the following:

CultureInfo.CurrentCulture.CompareInfo.IndexOf (this, value, 0, length, CompareOptions.Ordinal);

which, with your ToLower requirement that using StringComparison.OrdinalIgnoreCase is the perfect (ie identical) match for your existing code (it did not do any culture specific comparison).

Answer 2

Generally I've found that contains operations are not preferable for search, so I'd recommend you take a look at the Mastering Core Data Session (login required ) video on the WWDC 2010 page (around the 10 min mark). Apple knows that 'contains' is terrible w/ SQLite on mobile devices, you can essentially do what Apple does to sort of "hack" FTS on the version of SQLite they ship.

Essentially they do prefix matching by creating a table like:

[[ pk_id || input || normalized_input ]]

Where input and normalized_input are both indexed explicitly. Then they prefix match against the normalized value. So for instance if a user is searching for 'snuggles' and so far they've typed in 'snu' the prefix matching query would look like:

normalized_input >= 'snu' and normalized_input < 'snt'

Not sure if this translates given your use case, but I thought it was worth mentioning. Hope it's helpful!

Answer 3

You need to use a trie. See http://en.wikipedia.org/wiki/Trie