nx_bm25f.nx

nx_bm25f.nx -- BM25F: fielded BM25 (Robertson/Zaragoza "Simple BM25 extension to multiple weighted fields", CIKM'04 -- cited sovereign-fetched lib_bm25.txt: "BM25F defines each type of field as a stream, applying a per-stream weighting"). The honest relevance rung above plain BM25: a query term in the TITLE field should outrank the same term buried in the BODY. The fix is exactly what nx_bm25's own header predicted -- "a thin extension via a weight table". Two fields here (title, body); adding more (url/anchor/tags) = more rows in the weight table, no formula change (Cardinal 11/22). SIMPLE BM25F (the correct form -- length-normalize + weight PER FIELD, then saturate ONCE over the combined pseudo-frequency, so a title hit isn't double-counted): pseudo_tf(t,d) = SUM_f w_f * tf(t,f,d) / ( 1 - b + b * dl_f/avgdl_f ) score(d,q) = SUM_t IDF(t) * pseudo_tf*(k1+1) / ( k1 + pseudo_tf ) Integer/fixed-point throughout (reuses nx_bm25's bm_idf_micro keystone), x1e6 scale like bm_score. license_tier: ORIGINAL