feat: add exact vector search

internal/vector/exact.go

@@ -0,0 +1,59 @@
+package vector
+
+import (
+	"math"
+	"sort"
+)
+
+type Item struct {
+	ThreadID int64
+	Vector   []float64
+}
+
+type Neighbor struct {
+	ThreadID int64   `json:"thread_id"`
+	Score    float64 `json:"score"`
+}
+
+func Query(items []Item, query []float64, limit int, excludeThreadID int64) []Neighbor {
+	if limit <= 0 {
+		limit = 20
+	}
+	out := make([]Neighbor, 0, len(items))
+	for _, item := range items {
+		if item.ThreadID == excludeThreadID {
+			continue
+		}
+		score := Cosine(query, item.Vector)
+		if score <= 0 {
+			continue
+		}
+		out = append(out, Neighbor{ThreadID: item.ThreadID, Score: score})
+	}
+	sort.SliceStable(out, func(i, j int) bool {
+		if out[i].Score == out[j].Score {
+			return out[i].ThreadID < out[j].ThreadID
+		}
+		return out[i].Score > out[j].Score
+	})
+	if len(out) > limit {
+		out = out[:limit]
+	}
+	return out
+}
+
+func Cosine(left, right []float64) float64 {
+	if len(left) == 0 || len(left) != len(right) {
+		return 0
+	}
+	var dot, leftMag, rightMag float64
+	for index := range left {
+		dot += left[index] * right[index]
+		leftMag += left[index] * left[index]
+		rightMag += right[index] * right[index]
+	}
+	if leftMag == 0 || rightMag == 0 {
+		return 0
+	}
+	return dot / (math.Sqrt(leftMag) * math.Sqrt(rightMag))
+}

internal/vector/exact_test.go

@@ -0,0 +1,26 @@
+package vector
+
+import "testing"
+
+func TestCosine(t *testing.T) {
+	if got := Cosine([]float64{1, 0}, []float64{1, 0}); got != 1 {
+		t.Fatalf("cosine same: got %f want 1", got)
+	}
+	if got := Cosine([]float64{1, 0}, []float64{0, 1}); got != 0 {
+		t.Fatalf("cosine orthogonal: got %f want 0", got)
+	}
+}
+
+func TestQuerySortsByScore(t *testing.T) {
+	got := Query([]Item{
+		{ThreadID: 1, Vector: []float64{1, 0}},
+		{ThreadID: 2, Vector: []float64{0.5, 0.5}},
+		{ThreadID: 3, Vector: []float64{0, 1}},
+	}, []float64{1, 0}, 2, 0)
+	if len(got) != 2 {
+		t.Fatalf("neighbors: got %d want 2", len(got))
+	}
+	if got[0].ThreadID != 1 || got[1].ThreadID != 2 {
+		t.Fatalf("order: %#v", got)
+	}
+}