BM25: Boost file name matches at root

With this change we prioritize file name matches at the root of the
repository. This is based on the intuition that more important files
tend to be closer to the root.

We also change the parameter b in the BM25 scoring function from 0.75 to
0.3 to reduce the impact of the document length on the final score. This
is based on experiments that showed that our current scoring overly
penalizes long but important documents.

For example, we consider documents such as a README.md or CHANGELOG at
the root of the repository of high quality. However, these documents
also tend to be relatively long and are thus penalized.

Test plan:
Updated unit test

build/scoring_test.go

@@ -106,8 +106,8 @@ func TestBM25(t *testing.T) {
 			fileName: "a/b/c/config.go",
 			query:    &query.Substring{Pattern: "config.go"},
 			language: "Go",
-			// bm25-score: 0.60 <- sum-termFrequencyScore: 5.00, length-ratio: 0.00
-			wantScore: 0.60,
+			// bm25-score: 0.45 <- sum-termFrequencyScore: 2.00, length-ratio: 0.00
+			wantScore: 0.45,
 		},
 	}
 

eval.go

@@ -330,7 +330,7 @@ nextFileMatch:
 			// document frequencies. Since we don't store document frequencies in the index,
 			// we have to defer the calculation of the final BM25 score to after the whole
 			// shard has been processed.
-			tf = calculateTermFrequency(finalCands, df)
+			tf = calculateTermFrequency(&fileMatch, finalCands, df)
 		} else {
 			// Use the standard, non-experimental scoring method by default
 			d.scoreFile(&fileMatch, nextDoc, mt, known, opts)

score.go

@@ -108,18 +108,32 @@ func (d *indexData) scoreFile(fileMatch *FileMatch, doc uint32, mt matchTree, kn
 	}
 }
 
+func isAtRoot(path string) bool {
+	return strings.IndexRune(path, '/') == -1
+}
+
 // calculateTermFrequency computes the term frequency for the file match.
 //
 // Filename matches count more than content matches. This mimics a common text
 // search strategy where you 'boost' matches on document titles.
-func calculateTermFrequency(cands []*candidateMatch, df termDocumentFrequency) map[string]int {
+func calculateTermFrequency(fm *FileMatch, cands []*candidateMatch, df termDocumentFrequency) map[string]int {
 	// Treat each candidate match as a term and compute the frequencies. For now, ignore case
 	// sensitivity and treat filenames and symbols the same as content.
 	termFreqs := map[string]int{}
+
+	boostFileName := 2
+
+	var evaluated bool
 	for _, cand := range cands {
 		term := string(cand.substrLowered)
 		if cand.fileName {
-			termFreqs[term] += 5
+			if !evaluated {
+				if isAtRoot(fm.FileName) {
+					boostFileName = 5
+				}
+				evaluated = true
+			}
+			termFreqs[term] += boostFileName
 		} else {
 			termFreqs[term]++
 		}
@@ -155,8 +169,18 @@ type termFrequency struct {
 // This keeps things simple for now, since BM25 is not normalized and can be
 // tricky to combine with other scoring signals.
 func (d *indexData) scoreFilesUsingBM25(fileMatches []FileMatch, tfs []termFrequency, df termDocumentFrequency, opts *SearchOptions) {
-	// Use standard parameter defaults (used in Lucene and academic papers)
-	k, b := 1.2, 0.75
+	// k determines how quickly the TF score saturates with increasing term
+	// frequencies and b ∈ [0,1] determines how much the score is down-weighted for
+	// longer documents.
+	//
+	// The standard parameter values, used in Lucene and academic papers, are k=1.2
+	// and b=0.75. However, there is some evidence that other values might work
+	// better depending on the characteristics of the corpus.
+	//
+	// In our experiments we found that smaller values of b work well for our use
+	// case. This means we don't penalize long files as much. b=0.3 is at the lower
+	// end of the spectrum of values that are reported in the literature.
+	k, b := 1.2, 0.3
 
 	averageFileLength := float64(d.boundaries[d.numDocs()]) / float64(d.numDocs())
 	// This is very unlikely, but explicitly guard against division by zero.

score_test.go

@@ -37,7 +37,7 @@ func TestCalculateTermFrequency(t *testing.T) {
 		t.Run("", func(t *testing.T) {
 			fm := FileMatch{}
 			df := make(termDocumentFrequency)
-			tf := calculateTermFrequency(c.cands, df)
+			tf := calculateTermFrequency(&fm, c.cands, df)
 
 			if !maps.Equal(df, c.wantDF) {
 				t.Errorf("got %v, want %v", df, c.wantDF)