[CaffeinatedCoder]

Refs #70 (closed; shipped in 5.3.0 by #177).

Problem

Full-text search with BM25 ranking shipped in 5.3.0, but only CHANGELOG.md mentions it. The README has no section for it, so HasFullTextIndex, FullTextMatch and FullTextSearchAsync are undocumented.

Change

README.md only:

• One line under Key Features → Advanced Indexing: Full-Text Search (BM25).
• A Full-Text Search (v5.3.0) section after the geospatial one, in the same shape:
  • HasFullTextIndex(x => x.Body) and its default name idx_fts_{Property};
  • LINQ filtering with FullTextMatch;
  • ranked search with FullTextSearchAsync(indexName, query, k, matchAllTerms) returning FullTextSearchResult<T> (Document, Score);
  • CreateFullTextIndexAsync(x => ...) at runtime;
  • the DynamicCollection form: CreateFullTextIndexAsync("field") (default name idx_fts_{field}) and FullTextSearchAsync.
• A short note on ranking and limitations, each point taken from the code or its doc comments:
  • BM25 with k1 = 1.2, b = 0.75 (FullTextIndex), and scores compare only within one search (FullTextSearchResult);
  • when FullTextMatch uses the index (IndexOptimizer, and the planner tests in FullTextSearchTests);
  • whole-term matching with no segmentation of CJK scripts (FullTextAnalyzer);
  • BLQL has no full-text operator (BlqlFilterParser);
  • write cost and the compatibility rule for BLite < 5.3 (doc comments on HasFullTextIndex and CreateFullTextIndexAsync).

No other files touched. There is no roadmap row or per-version table for full-text to update, and docs/ has no per-feature guides, so there is no new guide.

Tests

The README samples were compiled and run in a scratch console app (net10.0, referencing BLite.Core, BLite.Bson and the source generator as an analyzer, as tests/BLite.Shared does). The app contained both code blocks verbatim and an Article model with a generated DocumentDbContext:

• typed: FullTextMatch("quick fox") returned the two documents containing both terms. FullTextSearchAsync(..., k: 10) returned them ranked by score. With matchAllTerms: false it also returned a document containing only fox, with a lower score.
• "Café, au lait!".FullTextMatch("cafe LAIT") is true, and the same filter in LINQ returned that document.
• CreateFullTextIndexAsync(x => x.Title) created idx_fts_Title next to idx_fts_Body.
• dynamic: CreateFullTextIndexAsync("text") created idx_fts_text, and FullTextSearchAsync with TryGetString("title", ...) printed the matching note.

No compiler warnings came from the sample code.

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