sphinxsearchHow can I create a custom ranker for SphinxSearch?

Creating a custom ranker for SphinxSearch requires some understanding of how the ranking works.

First, you need to understand the different ranking algorithms that SphinxSearch provides. These include BM25, TF-IDF, Word Count, and Proximity. You can read more about them here.

Once you have decided which algorithm you want to use, you can create a custom ranker by writing a UDF (User Defined Function). This is done in C++ and requires some knowledge of the language. Here is an example of a simple UDF that uses BM25 to rank results:

#include <sphinxudf.h>

using namespace Sphinx;

int custom_ranker(UDF_INIT *initid, UDF_ARGS *args, char *error,
                  char *result, unsigned long *length, char *is_null, char *is_error)
{
    // Get the arguments
    float rank = *((float *) args->args[0]);
    float bm25 = *((float *) args->args[1]);

    // Calculate the rank
    float custom_rank = rank * bm25;

    // Return the result
    *((float *) result) = custom_rank;
    *length = sizeof(float);
    return 0;
}

Once the UDF is written, you can register it with SphinxSearch by adding it to the udf_functions array in the sphinx.conf file.

udf_functions = (
    custom_ranker,
    "float",
    "float,float",
    "custom_ranker"
)

Finally, you can use the custom ranker with the @ranker directive in the search query.

SELECT * FROM index WHERE MATCH('query') OPTION ranker=custom_ranker;

This will use the custom ranker to rank the results.

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