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I have a large list of strings with associated values. I need to do a fast lookup into the list by the string prefix, but also by the string's ordinal within the list. For example,

string,value,ordinal
--------------------
alice, 10, 0
bob,   99, 1
carol, 32, 2

search_by_prefix("bo") -> [("bob", 99),]
search_by_ordinal(2) -> ("carol", 32)

I'm looking for a good compressed data structure.

A simple sorted string table (SST) does the job, but it's not well-compressed. You can do key prefix compression, but it doesn't buy you much.

I'd prefer to do a finite state transducer (FST) because it's very compact and lookups are fast. The problem is that I can't find a simple explanation of how to construct one, and it also isn't clear how to do a by-ordinal lookup.

Suggestions?

This data structure can be read-only. No need for inserts or deletes.

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A reasonable approach is to build two data structures, one optimized for prefix-based search and one optimized for ordinal-based lookup.

For prefix-based search, a trie is the natural data structure. It might not be very compressed, either, due to number of pointers it requires. A Patricia tree may be somewhat better but it still requires many pointers. It's possible you might be able to come up with a way to lay out nodes in memory so that most pointers can be replaced by small indices.

For ordinal-based search, a simple array of pointers should suffice.

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