From c06509afe410df1425566d1ec8762024d3feee1d Mon Sep 17 00:00:00 2001
From: Mathias <mthbqv@gmail.com>
Date: Tue, 20 Oct 2026 05:58:14 +0200
Subject: [PATCH] feat(wing): BM25 scorer as the independent cross-check (#205
 S1)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
---
 ingestion/internal/wing/bm25scorer.go      | 92 ++++++++++++++++++++++
 ingestion/internal/wing/bm25scorer_test.go | 45 +++++++++++
 2 files changed, 137 insertions(+)
 create mode 100644 ingestion/internal/wing/bm25scorer.go
 create mode 100644 ingestion/internal/wing/bm25scorer_test.go

diff --git a/ingestion/internal/wing/bm25scorer.go b/ingestion/internal/wing/bm25scorer.go
new file mode 100644
index 0000000..74cd037
--- /dev/null
+++ b/ingestion/internal/wing/bm25scorer.go
@@ -0,0 +1,92 @@
+package wing
+
+import (
+	"sort"
+	"strings"
+	"unicode"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/search"
+)
+
+const (
+	maxQueryTerms = 12
+	minTermLen    = 5
+	hitsPerWing   = 3
+)
+
+// BM25Scorer scores a note against each wing by running the brain's own BM25
+// search scoped to that wing. It is deliberately a different mechanism from
+// the LLM, so agreement between the two is evidence and not an echo.
+type BM25Scorer struct {
+	brainDir string
+	wings    []string
+	exclude  string
+}
+
+// NewBM25Scorer scores against wings under brainDir. exclude is a brain-
+// relative path ignored in results (the backtest sample itself); "" for none.
+func NewBM25Scorer(brainDir string, wings []string, exclude string) *BM25Scorer {
+	return &BM25Scorer{brainDir: brainDir, wings: wings, exclude: exclude}
+}
+
+// Score implements Scorer. A wing whose search fails scores 0 rather than
+// failing the note: the Resolver treats "no clear opinion" as an Exception.
+func (b *BM25Scorer) Score(text string) map[string]float64 {
+	query := strings.Join(topTerms(text), " ")
+	out := map[string]float64{}
+	if query == "" {
+		return out
+	}
+	for _, w := range b.wings {
+		hits, err := search.Query(b.brainDir, search.QueryOptions{Query: query, Wing: w, Limit: hitsPerWing + 1})
+		if err != nil {
+			continue
+		}
+		var sum float64
+		n := 0
+		for _, h := range hits {
+			if b.exclude != "" && h.Path == b.exclude {
+				continue
+			}
+			if n == hitsPerWing {
+				break
+			}
+			sum += float64(h.Score)
+			n++
+		}
+		if sum > 0 {
+			out[w] = sum
+		}
+	}
+	return out
+}
+
+// topTerms picks the most frequent longer words, ties broken alphabetically
+// so the query is deterministic.
+func topTerms(text string) []string {
+	if len(text) > maxNoteChars {
+		text = text[:maxNoteChars]
+	}
+	counts := map[string]int{}
+	for _, f := range strings.FieldsFunc(strings.ToLower(text), func(r rune) bool {
+		return !unicode.IsLetter(r) && !unicode.IsDigit(r)
+	}) {
+		if len([]rune(f)) >= minTermLen {
+			counts[f]++
+		}
+	}
+	terms := make([]string, 0, len(counts))
+	for t := range counts {
+		terms = append(terms, t)
+	}
+	sort.Slice(terms, func(i, j int) bool {
+		if counts[terms[i]] != counts[terms[j]] {
+			return counts[terms[i]] > counts[terms[j]]
+		}
+		return terms[i] < terms[j]
+	})
+	if len(terms) > maxQueryTerms {
+		terms = terms[:maxQueryTerms]
+	}
+	return terms
+}
diff --git a/ingestion/internal/wing/bm25scorer_test.go b/ingestion/internal/wing/bm25scorer_test.go
new file mode 100644
index 0000000..b648ca3
--- /dev/null
+++ b/ingestion/internal/wing/bm25scorer_test.go
@@ -0,0 +1,45 @@
+package wing_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/wing"
+)
+
+func writePage(t *testing.T, dir, rel, body string) {
+	t.Helper()
+	full := filepath.Join(dir, rel)
+	require.NoError(t, os.MkdirAll(filepath.Dir(full), 0o755))
+	require.NoError(t, os.WriteFile(full, []byte(body), 0o644))
+}
+
+func scorerBrain(t *testing.T) string {
+	dir := t.TempDir()
+	writePage(t, dir, "wiki/infra/facts/k3s.md", "---\ntitle: K3s cluster\n---\nkubernetes cluster nodes scheduling pods deployment\n")
+	writePage(t, dir, "wiki/finance/facts/invoice.md", "---\ntitle: Invoices\n---\ninvoice payment bookkeeping ledger reconciliation\n")
+	return dir
+}
+
+func TestBM25Scorer_RanksTheWingWhosePagesMatch(t *testing.T) {
+	s := wing.NewBM25Scorer(scorerBrain(t), []string{"infra", "finance"}, "")
+	got := s.Score("notes about kubernetes cluster scheduling and pods")
+	assert.Greater(t, got["infra"], got["finance"])
+	assert.Greater(t, got["infra"], 0.0)
+}
+
+func TestBM25Scorer_ExcludesThePageBeingClassified(t *testing.T) {
+	// A page must not vouch for its own wing when it is the backtest sample.
+	s := wing.NewBM25Scorer(scorerBrain(t), []string{"infra", "finance"}, "wiki/infra/facts/k3s.md")
+	got := s.Score("kubernetes cluster nodes scheduling pods deployment")
+	assert.Zero(t, got["infra"], "the excluded page is the only infra match")
+}
+
+func TestBM25Scorer_NoUsableTermsScoresNothing(t *testing.T) {
+	s := wing.NewBM25Scorer(scorerBrain(t), []string{"infra", "finance"}, "")
+	assert.Empty(t, s.Score("a an to of"))
+}