HNSW

Fix algorithm

TestHNSW/HNSWDemo/Program.cs

@@ -84,6 +84,7 @@ namespace HNSWDemo
 
             var sw = new Stopwatch();
             var world = new SmallWorld<float[]>(NSWOptions<float[]>.Create(6, 4, 120, 120, CosineDistance.ForUnits));
+
             for (int i = 0; i < (count / batchSize); i++)
             {
                 var batch = samples.Skip(i * batchSize).Take(batchSize).ToArray();
@@ -101,7 +102,7 @@ namespace HNSWDemo
 
             //HNSWFilter filter = new HNSWFilter(ids => ids.Where(id => { var p = _database[id]; return p.Age > 45 && p.Gender == Gender.Feemale; }));
 
-/*var fackupCount = 0;
+            /*var fackupCount = 0;
                         foreach (var v in vectors)
                         {
                             var result = world.Search(v, 10, filter);                

ZeroLevel.HNSW/Layer.cs

@@ -13,6 +13,11 @@ namespace ZeroLevel.HNSW
         private readonly VectorSet<TItem> _vectors;
         private CompactBiDirectionalLinksSet _links = new CompactBiDirectionalLinksSet();
 
+        /// <summary>
+        /// Count nodes at layer
+        /// </summary>
+        public int Count => (_links.Count >> 1);
+
         public Layer(NSWOptions<TItem> options, VectorSet<TItem> vectors)
         {
             _options = options;
@@ -50,148 +55,107 @@ namespace ZeroLevel.HNSW
             }
         }
 
-        public int GetEntryPointFor(int q)
-        {
-            var randomLinkId = DefaultRandomGenerator.Instance.Next(0, _links.Count);
-            var entryId = _links[randomLinkId].Item1;
-            var v = new VisitedBitSet(_vectors._set.Count, _options.M);
-            // v ← ep // set of visited elements
-            var (ep, ed) = DFS_SearchMinFrom(entryId, q, v);
-            return ep;
-        }
-
-        private (int, float) DFS_SearchMinFrom(int entryId, int id, VisitedBitSet visited)
-        {
-            visited.Add(entryId);
-            int candidate = entryId;
-            var candidateDistance = _options.Distance(_vectors[entryId], _vectors[id]);
-            int counter = 0;
-            do
-            {
-                var (mid, dist) = GetMinNearest(visited, entryId, candidate, candidateDistance);
-                if (dist > candidateDistance)
-                {
-                    break;
-                }
-                candidate = mid;
-                candidateDistance = dist;
-
-                counter++;
-            } while (counter < _options.EFConstruction);
-            return (candidate, candidateDistance);
-        }
-
-        private (int, float) GetMinNearest(VisitedBitSet visited, int entryId, int id, float entryDistance)
-        {
-            var minId = entryId;
-            var minDist = entryDistance;
-            foreach (var candidate in _links.FindLinksForId(entryId).Select(l => l.Item2))
-            {
-                if (visited.Contains(candidate) == false)
-                {
-                    var dist = _options.Distance(_vectors[candidate], _vectors[id]);
-                    if (dist < minDist)
-                    {
-                        minDist = dist;
-                        minId = candidate;
-                    }
-                    visited.Add(candidate);
-                }
-            }
-            return (minId, minDist);
-        }
-
         #region Implementation of https://arxiv.org/ftp/arxiv/papers/1603/1603.09320.pdf
-
         /// <summary>
         /// Algorithm 2
         /// </summary>
         /// <param name="q">query element</param>
         /// <param name="ep">enter points ep</param>
         /// <returns>Output: ef closest neighbors to q</returns>
-        public IDictionary<int, float> SEARCH_LAYER(int q, int ep, int ef)
+        public void RunKnnAtLayer(int entryPointId, Func<int, float> targetCosts, IDictionary<int, float> W, int ef)
-        {
+        {
-            var v = new VisitedBitSet(_vectors._set.Count, _options.M);
+            /*
+             * v ← ep // set of visited elements
+             * C ← ep // set of candidates
+             * W ← ep // dynamic list of found nearest neighbors
+             * while │C│ > 0
+             *   c ← extract nearest element from C to q
+             *   f ← get furthest element from W to q
+             *   if distance(c, q) > distance(f, q)
+             *     break // all elements in W are evaluated
+             *   for each e ∈ neighbourhood(c) at layer lc // update C and W
+             *     if e ∉ v
+             *       v ← v ⋃ e
+             *       f ← get furthest element from W to q
+             *       if distance(e, q) < distance(f, q) or │W│ < ef
+             *         C ← C ⋃ e
+             *         W ← W ⋃ e
+             *         if │W│ > ef
+             *           remove furthest element from W to q
+             * return W
+             */
+
+            var v = new VisitedBitSet(_vectors.Count, _options.M);
             // v ← ep // set of visited elements
-            v.Add(ep);
+            v.Add(entryPointId);
             // C ← ep // set of candidates
             var C = new Dictionary<int, float>();
-            C.Add(ep, _options.Distance(_vectors[ep], _vectors[q]));
+            C.Add(entryPointId, targetCosts(entryPointId));
             // W ← ep // dynamic list of found nearest neighbors
-            var W = new Dictionary<int, float>();
+            W.Add(entryPointId, C[entryPointId]);
-            W.Add(ep, C[ep]);
+
-            // while │C│ > 0
+
+            // run bfs
             while (C.Count > 0)
             {
-                // c ← extract nearest element from C to q
+                // get next candidate to check and expand
-                var nearest = W.OrderBy(p => p.Value).First();
+                var toExpand = popCandidate();
-                var c = nearest.Key;
+                var farthestResult = fartherFromResult();
-                var md = nearest.Value;
+                if (toExpand.Item2 > farthestResult.Item2)
-                // var (c, md) = GetMinimalDistanceIndex(C, q);
-                C.Remove(c);
-                // f ← get furthest element from W to q
-                var f = W.OrderBy(p => p.Value).First().Key;
-                //var f = GetMaximalDistanceIndex(W, q);
-                // if distance(c, q) > distance(f, q)
-                if (_options.Distance(_vectors[c], _vectors[q]) > _options.Distance(_vectors[f], _vectors[q]))
                 {
-                    // break // all elements in W are evaluated
+                    // the closest candidate is farther than farthest result
                     break;
                 }
-                // for each e ∈ neighbourhood(c) at layer lc // update C and W
+
-                foreach (var l in _links.FindLinksForId(c))
+                // expand candidate
+                var neighboursIds = GetNeighbors(toExpand.Item1).ToArray();
+                for (int i = 0; i < neighboursIds.Length; ++i)
                 {
-                    var e = l.Item2;
+                    int neighbourId = neighboursIds[i];
-                    // if e ∉ v
+                    if (!v.Contains(neighbourId))
-                    if (v.Contains(e) == false)
                     {
-                        // v ← v ⋃ e
+                        // enqueue perspective neighbours to expansion list
-                        v.Add(e);
+                        farthestResult = fartherFromResult();
-                        // f ← get furthest element from W to q
+
-                        f = W.OrderByDescending(p => p.Value).First().Key;
+                        var neighbourDistance = targetCosts(neighbourId);
-                        //f = GetMaximalDistanceIndex(W, q);
+                        if (W.Count < ef || neighbourDistance < farthestResult.Item2)
-                        // if distance(e, q) < distance(f, q) or │W│ < ef
-                        var ed = _options.Distance(_vectors[e], _vectors[q]);
-                        if (ed > _options.Distance(_vectors[f], _vectors[q])
-                            || W.Count < ef)
                         {
-                            // C ← C ⋃ e
+                            C.Add(neighbourId, neighbourDistance);
-                            C.Add(e, ed);
+                            W.Add(neighbourId, neighbourDistance);
-                            // W ← W ⋃ e
-                            W.Add(e, ed);
-                            // if │W│ > ef
                             if (W.Count > ef)
                             {
-                                // remove furthest element from W to q
+                                fartherPopFromResult();
-                                f = W.OrderByDescending(p => p.Value).First().Key;
-                                //f = GetMaximalDistanceIndex(W, q);
-                                W.Remove(f);
                             }
                         }
+                        v.Add(neighbourId);
                     }
                 }
             }
-            //  return W
+            C.Clear();
-            return W;
+            v.Clear();
         }
 
         /// <summary>
         /// Algorithm 3
         /// </summary>
-        /// <param name="q">base element</param>
+        public IDictionary<int, float> SELECT_NEIGHBORS_SIMPLE(Func<int, float> distance, IDictionary<int, float> candidates, int M)
-        /// <param name="C">candidate elements</param>
+        {
-        /// <returns>Output: M nearest elements to q</returns>
+            var bestN = M;
-        public IDictionary<int, float> SELECT_NEIGHBORS_SIMPLE(int q, IDictionary<int, float> C)
+            var W = new Dictionary<int, float>(candidates);
+            if (W.Count > bestN)
             {
-            if (C.Count <= _options.M)
+                var popFarther = new Action(() => { var pair = W.OrderByDescending(e => e.Value).First(); W.Remove(pair.Key); });
+                while (W.Count > bestN)
                 {
-                return new Dictionary<int, float>(C);
+                    popFarther();
+                }
             }
-            var output = new Dictionary<int, float>();
             // return M nearest elements from C to q
-            return new Dictionary<int, float>(C.OrderBy(p => p.Value).Take(_options.M));
+            return W;
         }
 
+
+
         /// <summary>
         /// Algorithm 4
         /// </summary>
@@ -200,41 +164,56 @@ namespace ZeroLevel.HNSW
         /// <param name="extendCandidates">flag indicating whether or not to extend candidate list</param>
         /// <param name="keepPrunedConnections">flag indicating whether or not to add discarded elements</param>
         /// <returns>Output: M elements selected by the heuristic</returns>
-        public IDictionary<int, float> SELECT_NEIGHBORS_HEURISTIC(int q, IDictionary<int, float> C, bool extendCandidates, bool keepPrunedConnections)
+        public IDictionary<int, float> SELECT_NEIGHBORS_HEURISTIC(Func<int, float> distance, IDictionary<int, float> candidates, int M, bool extendCandidates, bool keepPrunedConnections)
         {
             // R ← ∅
             var R = new Dictionary<int, float>();
             // W ← C // working queue for the candidates
-            var W = new List<int>(C.Select(p => p.Key));
+            var W = new Dictionary<int, float>(candidates);
             // if extendCandidates // extend candidates by their neighbors
             if (extendCandidates)
             {
+                var extendBuffer = new HashSet<int>();
                 // for each e ∈ C
-                foreach (var e in C)
+                foreach (var e in W)
                 {
+                    var neighbors = GetNeighbors(e.Key);
                     // for each e_adj ∈ neighbourhood(e) at layer lc
-                    foreach (var l in _links.FindLinksForId(e.Key))
+                    foreach (var e_adj in neighbors)
                     {
-                        var e_adj = l.Item2;
                         // if eadj ∉ W
-                        if (W.Contains(e_adj) == false)
+                        if (extendBuffer.Contains(e_adj) == false)
                         {
-                            // W ← W ⋃ eadj
+                            extendBuffer.Add(e_adj);
-                            W.Add(e_adj);
                         }
                     }
                 }
+                // W ← W ⋃ eadj
+                foreach (var id in extendBuffer)
+                {
+                    W.Add(id, distance(id));
+                }
             }
+
             //  Wd ← ∅ // queue for the discarded candidates
             var Wd = new Dictionary<int, float>();
+
+
+            var popCandidate = new Func<(int, float)>(() => { var pair = W.OrderBy(e => e.Value).First(); W.Remove(pair.Key); return (pair.Key, pair.Value); });
+            var fartherFromResult = new Func<(int, float)>(() => { if (R.Count == 0) return (-1, 0f); var pair = R.OrderByDescending(e => e.Value).First(); return (pair.Key, pair.Value); });
+            var popNearestDiscarded = new Func<(int, float)>(() => { var pair = Wd.OrderBy(e => e.Value).First(); W.Remove(pair.Key); return (pair.Key, pair.Value); });
+
+
             // while │W│ > 0 and │R│< M
-            while (W.Count > 0 && R.Count < _options.M)
+            while (W.Count > 0 && R.Count < M)
             {
                 // e ← extract nearest element from W to q
-                var (e, ed) = GetMinimalDistanceIndex(W, q);
+                var (e, ed) = popCandidate();
-                W.Remove(e);
+                var (fe, fd) = fartherFromResult();
+
                 // if e is closer to q compared to any element from R
-                if (ed < R.Min(pair => pair.Value))
+                if (R.Count == 0 ||
+                    ed < fd)
                 {
                     // R ← R ⋃ e
                     R.Add(e, ed);
@@ -248,37 +227,20 @@ namespace ZeroLevel.HNSW
                 if (keepPrunedConnections)
                 {
                     // while │Wd│> 0 and │R│< M
-                    while (Wd.Count > 0 && R.Count < _options.M)
+                    while (Wd.Count > 0 && R.Count < M)
                     {
                         // R ← R ⋃ extract nearest element from Wd to q
-                        var nearest = Wd.Aggregate((l, r) => l.Value < r.Value ? l : r);
+                        var nearest = popNearestDiscarded();
-                        Wd.Remove(nearest.Key);
+                        R.Add(nearest.Item1, nearest.Item2);
-                        R.Add(nearest.Key, nearest.Value);
                     }
                 }
             }
             //  return R
             return R;
         }
-
-
         #endregion
 
 
-        private (int, float) GetMinimalDistanceIndex(IList<int> self, int q)
+        private IEnumerable<int> GetNeighbors(int id) => _links.FindLinksForId(id).Select(d => d.Item2);
-        {
-            float min = _options.Distance(_vectors[self[0]], _vectors[q]);
-            int minIndex = 0;
-            for (int i = 1; i < self.Count; ++i)
-            {
-                var dist = _options.Distance(_vectors[self[i]], _vectors[q]);
-                if (dist < min)
-                {
-                    min = self[i];
-                    minIndex = i;
-                }
-            }
-            return (minIndex, min);
-        }
     }
 }

ZeroLevel.HNSW/Services/VectorSet.cs

@@ -1,15 +1,16 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Threading;
 
 namespace ZeroLevel.HNSW
 {
     public class VectorSet<T>
     {
-        public IList<T> _set = new List<T>();
+        private List<T> _set = new List<T>();
+        private SpinLock _lock = new SpinLock();
 
         public T this[int index] => _set[index];
-
+        public int Count => _set.Count;
-        SpinLock _lock = new SpinLock();
 
         public int Append(T vector)
         {
@@ -27,5 +28,30 @@ namespace ZeroLevel.HNSW
                 if (gotLock) _lock.Exit();
             }
         }
+
+        public int[] Append(IEnumerable<T> vectors)
+        {
+            bool gotLock = false;
+            int startIndex, endIndex;
+            gotLock = false;
+            try
+            {
+                _lock.Enter(ref gotLock);
+                startIndex = _set.Count;
+                _set.AddRange(vectors);
+                endIndex = _set.Count;                
+            }
+            finally
+            {
+                // Only give up the lock if you actually acquired it
+                if (gotLock) _lock.Exit();
+            }
+            var ids = new int[endIndex - startIndex];
+            for (int i = startIndex, j = 0; i < endIndex; i++, j++)
+            {
+                ids[j] = i;
+            }
+            return ids;
+        }
     }
 }

ZeroLevel.HNSW/SmallWorld.cs

@@ -1,9 +1,38 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading;
 
 namespace ZeroLevel.HNSW
 {
+    public class ProbabilityLayerNumberGenerator
+    {
+        private const float DIVIDER = 4.362f;
+        private readonly float[] _probabilities;
+
+        public ProbabilityLayerNumberGenerator(int maxLayers, int M)
+        {
+            _probabilities = new float[maxLayers];
+            var probability = 1.0f / DIVIDER;
+            for (int i = 0; i < maxLayers; i++)
+            {
+                _probabilities[i] = probability;
+                probability /= DIVIDER;
+            }
+        }
+
+        public int GetRandomLayer()
+        {
+            var probability = DefaultRandomGenerator.Instance.NextFloat();
+            for (int i = 0; i < _probabilities.Length; i++)
+            {
+                if (probability > _probabilities[i])
+                    return i;
+            }
+            return 0;
+        }
+    }
+
     public class SmallWorld<TItem>
     {
         private readonly NSWOptions<TItem> _options;
@@ -12,12 +41,17 @@ namespace ZeroLevel.HNSW
 
         private Layer<TItem> EnterPointsLayer => _layers[_layers.Length - 1];
         private Layer<TItem> LastLayer => _layers[0];
+        private int EntryPoint = -1;
+        private int MaxLayer = -1;
+        private readonly ProbabilityLayerNumberGenerator _layerLevelGenerator;
+        private ReaderWriterLockSlim _lockGraph = new ReaderWriterLockSlim();
 
         public SmallWorld(NSWOptions<TItem> options)
         {
             _options = options;
             _vectors = new VectorSet<TItem>();
             _layers = new Layer<TItem>[_options.LayersCount];
+            _layerLevelGenerator = new ProbabilityLayerNumberGenerator(_options.LayersCount, _options.M);
             for (int i = 0; i < _options.LayersCount; i++)
             {
                 _layers[i] = new Layer<TItem>(_options, _vectors);
@@ -31,93 +65,174 @@ namespace ZeroLevel.HNSW
 
         public int[] AddItems(IEnumerable<TItem> vectors)
         {
-            var insert = vectors.ToArray();
+            _lockGraph.EnterWriteLock();
-            var ids = new int[insert.Length];
+            try
-            for (int i = 0; i < insert.Length; i++)
+            {
+                var ids = _vectors.Append(vectors);
+                for (int i = 0; i < ids.Length; i++)
                 {
-                var item = insert[i];
+                    INSERT(ids[i]);
-                ids[i] = Insert(item);
                 }
                 return ids;
             }
+            finally
+            {
+                _lockGraph.ExitWriteLock();
+            }
+        }
 
-        public int Insert(TItem item)
+        public void TestLevelGenerator()
+        {
+            var levels = new Dictionary<int, float>();
+            for (int i = 0; i < 10000; i++)
+            {
+                var level = _layerLevelGenerator.GetRandomLayer();
+                if (levels.ContainsKey(level) == false)
+                {
+                    levels.Add(level, 1);
+                }
+                else
+                {
+                    levels[level] += 1.0f;
+                }
+            }
+            foreach (var pair in levels.OrderBy(l => l.Key))
             {
-            var id = _vectors.Append(item);
+                Console.WriteLine($"[{pair.Key}]: {pair.Value / 100.0f}% ({pair.Value})");
-            INSERT(id);
+            }
-            return id;
         }
 
         #region https://arxiv.org/ftp/arxiv/papers/1603/1603.09320.pdf
         /// <summary>
         /// Algorithm 1
         /// </summary>
-        /// <param name="q">new element</param>
         public void INSERT(int q)
         {
+            var distance = new Func<int, float>(candidate => _options.Distance(_vectors[q], _vectors[candidate]));
+
             // W ← ∅ // list for the currently found nearest elements
-            IDictionary<int, float> W;
+            IDictionary<int, float> W = new Dictionary<int, float>();
             // ep ← get enter point for hnsw
-            var ep = EnterPointsLayer.GetEntryPointFor(q);
+            var ep = EntryPoint == -1 ? 0 : EntryPoint;
+            var epDist = 0.0f;
             // L ← level of ep // top layer for hnsw
-            var L = _layers.Length - 1;
+            var L = MaxLayer;
             // l ← ⌊-ln(unif(0..1))∙mL⌋ // new element’s level            
-            int l = DefaultRandomGenerator.Instance.Next(0, _options.LayersCount - 1);
+            int l = _layerLevelGenerator.GetRandomLayer();
+            if (L == -1)
+            {
+                L = l;
+                MaxLayer = l;
+            }
             // for lc ← L … l+1
-            for (int lc = L; lc > l; lc--)
+            // Проход с верхнего уровня до уровня где появляется элемент, для нахождения точки входа
+            for (int lc = L; lc > l; --lc)
             {
                 // W ← SEARCH-LAYER(q, ep, ef = 1, lc)
-                W = _layers[lc].SEARCH_LAYER(q, ep, 1);
+                _layers[lc].RunKnnAtLayer(ep, distance, W, 1);
                 // ep ← get the nearest element from W to q
-                ep = W.OrderBy(p => p.Value).First().Key;
+                var nearest = W.OrderBy(p => p.Value).First();
+                ep = nearest.Key;
+                epDist = nearest.Value;
+                W.Clear();
             }
             //for lc ← min(L, l) … 0
-            for (int lc = Math.Min(L, l); lc >= 0; lc--)
+            // connecting new node to the small world
+            for (int lc = Math.Min(L, l); lc >= 0; --lc)
             {
                 // W ← SEARCH - LAYER(q, ep, efConstruction, lc)
-                W = _layers[lc].SEARCH_LAYER(q, ep, _options.EFConstruction);
+                _layers[lc].RunKnnAtLayer(ep, distance, W, _options.EFConstruction);
                 // neighbors ← SELECT-NEIGHBORS(q, W, M, lc) // alg. 3 or alg. 4
-                var neighbors = _layers[lc].SELECT_NEIGHBORS_SIMPLE(q, W);
+                var neighbors = SelectBestForConnecting(lc, distance, W);;
                 // add bidirectionall connectionts from neighbors to q at layer lc
                 // for each e ∈ neighbors // shrink connections if needed
                 foreach (var e in neighbors)
                 {
                     // eConn ← neighbourhood(e) at layer lc
                     _layers[lc].AddBidirectionallConnectionts(q, e.Key, e.Value);
+                    // if distance from newNode to newNeighbour is better than to bestPeer => update bestPeer
+                    if (e.Value < epDist)
+                    {
+                        ep = e.Key;
+                        epDist = e.Value;
+                    }
                 }
                 // ep ← W
                 ep = W.OrderBy(p => p.Value).First().Key;
+                W.Clear();
             }
             // if l > L
+            if (l > L)
+            {
                 // set enter point for hnsw to q
+                L = l;
+                MaxLayer = l;
+                EntryPoint = ep;
+            }
+        }
+
+        /// <summary>
+        /// Get maximum allowed connections for the given level.
+        /// </summary>
+        /// <remarks>
+        /// Article: Section 4.1:
+        /// "Selection of the Mmax0 (the maximum number of connections that an element can have in the zero layer) also
+        /// has a strong influence on the search performance, especially in case of high quality(high recall) search.
+        /// Simulations show that setting Mmax0 to M(this corresponds to kNN graphs on each layer if the neighbors
+        /// selection heuristic is not used) leads to a very strong performance penalty at high recall.
+        /// Simulations also suggest that 2∙M is a good choice for Mmax0;
+        /// setting the parameter higher leads to performance degradation and excessive memory usage."
+        /// </remarks>
+        /// <param name="layer">The level of the layer.</param>
+        /// <returns>The maximum number of connections.</returns>
+        internal int GetM(int layer)
+        {
+            return layer == 0 ? 2 * _options.M : _options.M;
+        }
+
+        private IDictionary<int, float> SelectBestForConnecting(int layer, Func<int, float> distance, IDictionary<int, float> candidates)
+        {
+            return _layers[layer].SELECT_NEIGHBORS_SIMPLE(distance, candidates, GetM(layer));
         }
 
         /// <summary>
         /// Algorithm 5
         /// </summary>
-        /// <param name="q">query element</param>
+        internal IEnumerable<(int, float)> KNearest(TItem q, int k)
-        /// <param name="K">number of nearest neighbors to return</param>
+        {
-        /// <returns>: K nearest elements to q</returns>
+            _lockGraph.EnterReadLock();
-        public IList<int> K_NN_SEARCH(int q, int K)
+            try
             {
+                if (_vectors.Count == 0)
+                {
+                    return Enumerable.Empty<(int, float)>();
+                }
+                var distance = new Func<int, float>(candidate => _options.Distance(q, _vectors[candidate]));
+
                 // W ← ∅ // set for the current nearest elements
-            IDictionary<int, float> W;
+                var W = new Dictionary<int, float>(k + 1);
                 // ep ← get enter point for hnsw
-            var ep = EnterPointsLayer.GetEntryPointFor(q);
+                var ep = EntryPoint;
                 // L ← level of ep // top layer for hnsw
-            var L = _options.LayersCount - 1;
+                var L = MaxLayer;
                 // for lc ← L … 1
-            for (var lc = L; lc > 0; lc--)
+                for (int layer = L; layer > 0; --layer)
                 {
                     // W ← SEARCH-LAYER(q, ep, ef = 1, lc)
-                W = _layers[lc].SEARCH_LAYER(q, ep, 1);
+                    _layers[layer].RunKnnAtLayer(ep, distance, W, 1);
                     // ep ← get nearest element from W to q
                     ep = W.OrderBy(p => p.Value).First().Key;
+                    W.Clear();
                 }
                 // W ← SEARCH-LAYER(q, ep, ef, lc =0)
-            W = LastLayer.SEARCH_LAYER(q, ep, _options.EF);
+                _layers[0].RunKnnAtLayer(ep, distance, W, k);
                 // return K nearest elements from W to q
-            return W.OrderBy(p => p.Value).Take(K).Select(p => p.Key).ToList();
+                return W.Select(p => (p.Key, p.Value));
+            }
+            finally
+            {
+                _lockGraph.ExitReadLock();
+            }
         }
         #endregion
     }