Better work per thread calculation.

src/annoylib.h

@@ -972,19 +972,12 @@ template<typename S, typename T, typename Distance, typename Random>
 
     _n_nodes = _n_items;
 
-    std::mutex _nodes_mutex;
+    std::mutex nodes_mutex;
     ThreadBarrier barrier(n_threads);
     vector<std::thread> threads(n_threads);
-    int work_per_thread = (int)floor(q / (double)n_threads);
-    int work_remainder = q % n_threads;
     for (int i = 0; i < n_threads; i++) {
-      int trees_per_thread = -1;
-      if (q > -1) {
-        // First thread picks up the remainder of the work
-        trees_per_thread = i == 0 ? work_per_thread + work_remainder : work_per_thread;
-      }
-
-      threads[i] = std::thread(&AnnoyIndex<S, T, D, Random>::_thread_build, this, trees_per_thread, i, std::ref(barrier), std::ref(_nodes_mutex));
+      int trees_per_thread = q == -1 ? -1 : (int)floor((q + i) / n_threads);
+      threads[i] = std::thread(&AnnoyIndex<S, T, D, Random>::_thread_build, this, trees_per_thread, i, std::ref(barrier), std::ref(nodes_mutex));
     }
 
     for (auto& thread : threads) {
@@ -1206,7 +1199,7 @@ template<typename S, typename T, typename Distance, typename Random>
     return get_node_ptr<S, Node>(_nodes, _s, i);
   }
 
-  void _thread_build(int q, int thread_idx, ThreadBarrier& barrier, std::mutex& _nodes_mutex) {
+  void _thread_build(int q, int thread_idx, ThreadBarrier& barrier, std::mutex& nodes_mutex) {
     Random _random;
     // Each thread needs its own seed, otherwise each thread would be building the same tree(s)
     int seed = _is_seeded ? _seed + thread_idx : thread_idx;
@@ -1250,7 +1243,7 @@ template<typename S, typename T, typename Distance, typename Random>
     // Wait for all threads to finish before we can start inserting tree nodes into global _nodes array
     barrier.wait();
 
-    _nodes_mutex.lock();
+    nodes_mutex.lock();
     // When a thread wants to insert local tree nodes into global _nodes it has to stop pretending that there is
     // going to be only one tree. Each thread has to update all split nodes children that are pointing to other split nodes
     // because their indices will change once inserted into global _nodes.
@@ -1283,7 +1276,7 @@ template<typename S, typename T, typename Distance, typename Random>
       thread_roots[tree_idx] += split_nodes_offset;
     }
     _roots.insert(_roots.end(), thread_roots.begin(), thread_roots.end());
-    _nodes_mutex.unlock();
+    nodes_mutex.unlock();
   }
 
   S _make_tree(const vector<S >& indices, vector<Node* >& split_nodes, bool is_root, Random& _random) {