Question

I am implementing a sort of autocomplete for an iOS app. The data I am using for the autocomplete values is a comma-separated text file with about 100,000 strings. This is what I am doing now:

1. Read the text file, and create an NSArray with 100,000 NSString.
2. As the user types, do [array containsObject:text]

Surely there is a better/faster way to do this lookup. Any thoughts?

Answer 1

Absolutely, there is! It’s not “in Objective-C” though: most likely, you would need to code it yourself.

The idea is to convert your list of string to a suffix tree, a data structure that lets you search by prefix very quickly. Searching for possible completions in a suffix tree are very fast, but the structure itself is not easy to build. A quick search on the internet revealed that there is no readily available implementation in Objective C, but you may be able to port an implementation in another language, use a C implementation, or even write your own if you are not particularly pressed for time.

Perhaps an easier approach would be to sort your strings alphabetically, and run a binary search on the prefix that has been entered so far. Though not as efficient as a suffix tree, the sorted array approach will be acceptable for 100K strings, because you get to the right spot in under seventeen checks.