Fix shadowed variable in faiss/impl/HNSW.cpp (facebookresearch#3961)

Summary:
Pull Request resolved: facebookresearch#3961

Our upcoming compiler upgrade will require us not to have shadowed variables. Such variables have a _high_ bug rate and reduce readability, so we would like to avoid them even if the compiler was not forcing us to do so.

This codemod attempts to fix an instance of a shadowed variable. Please review with care: if it's failed the result will be a silent bug.

**What's a shadowed variable?**

Shadowed variables are variables in an inner scope with the same name as another variable in an outer scope. Having the same name for both variables might be semantically correct, but it can make the code confusing to read! It can also hide subtle bugs.

This diff fixes such an issue by renaming the variable.

 - If you approve of this diff, please use the "Accept & Ship" button :-)

Reviewed By: meyering

Differential Revision: D64398743

fbshipit-source-id: 3ec24a1655133ee0d3b94a55e38857ffa8853268

Author: r-barnes

faiss/impl/HNSW.cpp

@@ -166,10 +166,10 @@ void HNSW::print_neighbor_stats(int level) const {
 }
 
 void HNSW::fill_with_random_links(size_t n) {
-    int max_level = prepare_level_tab(n);
+    int max_level_2 = prepare_level_tab(n);
     RandomGenerator rng2(456);
 
-    for (int level = max_level - 1; level >= 0; --level) {
+    for (int level = max_level_2 - 1; level >= 0; --level) {
         std::vector<int> elts;
         for (int i = 0; i < n; i++) {
             if (levels[i] > level) {
@@ -210,16 +210,16 @@ int HNSW::prepare_level_tab(size_t n, bool preset_levels) {
         }
     }
 
-    int max_level = 0;
+    int max_level_2 = 0;
     for (int i = 0; i < n; i++) {
         int pt_level = levels[i + n0] - 1;
-        if (pt_level > max_level)
-            max_level = pt_level;
+        if (pt_level > max_level_2)
+            max_level_2 = pt_level;
         offsets.push_back(offsets.back() + cum_nb_neighbors(pt_level + 1));
     }
     neighbors.resize(offsets.back(), -1);
 
-    return max_level;
+    return max_level_2;
 }
 
 /** Enumerate vertices from nearest to farthest from query, keep a
@@ -493,17 +493,17 @@ void HNSW::add_links_starting_from(
 
     ::faiss::shrink_neighbor_list(ptdis, link_targets, M, keep_max_size_level0);
 
-    std::vector<storage_idx_t> neighbors;
-    neighbors.reserve(link_targets.size());
+    std::vector<storage_idx_t> neighbors_2;
+    neighbors_2.reserve(link_targets.size());
     while (!link_targets.empty()) {
         storage_idx_t other_id = link_targets.top().id;
         add_link(*this, ptdis, pt_id, other_id, level, keep_max_size_level0);
-        neighbors.push_back(other_id);
+        neighbors_2.push_back(other_id);
         link_targets.pop();
     }
 
     omp_unset_lock(&locks[pt_id]);
-    for (storage_idx_t other_id : neighbors) {
+    for (storage_idx_t other_id : neighbors_2) {
         omp_set_lock(&locks[other_id]);
         add_link(*this, ptdis, other_id, pt_id, level, keep_max_size_level0);
         omp_unset_lock(&locks[other_id]);