This SQLite extension creates a SQL function called okapi_bm25 that returns the Okapi BM25 ranking for results of a full-text search. Okapi BM25 is a modern ranking function that calculates a score for each result based on its relevance to the search query. This extension only works with MATCH queries on FTS4 tables. The matchinfo function must be called with 'pcnalx' as the second argument. This argument defines the structure of the data given to the okapi_bm25 function, which accepts the data in only one form. If the matchinfo function is called with a different second argument, the extension may provide incorrect results or fail to work entirely.