[MariaDB developers] SOUNDEX sources

I'm developing a project which uses soundex matching (alongside other options more sophisticated than that). This is custom software with a mariad back end. MariaDB soundex built-in produces codes of apparently arbitary length; the usual is 4 characters. I've come to the conclusion that the best way forward is to mimic Maria's soundex in my code. I cannot find a description of the algorithm Maria uses though: the only source code I can find is in mf_soundex.c, and that is explicitly limited to 4 characters. But I have installed triggers which generate codes such as A4152636125262 Where is the soundex function found? Anyone know?