diff --git a/src/compression.cpp b/src/compression.cpp
index 464c16b..a1f0dae 100644
--- a/src/compression.cpp
+++ b/src/compression.cpp
@@ -126,6 +126,16 @@ double calculate_reduction(double n, std::vector<double> &unit1, std::vector<dou
     return n_total / n * m_total;
 }
 
+typedef std::pair<std::string, std::string> t_neighbor;
+
+t_neighbor neighbor_make_pair(std::string a, std::string b)
+{
+    if (a < b)
+        return t_neighbor(a, b);
+    else
+        return t_neighbor(b, a);
+}
+
 // [[Rcpp::export]]
 List compress_compute_cpp(
     std::string neighbors_option,
@@ -151,20 +161,21 @@ List compress_compute_cpp(
     double n_total, m_total;
     std::tie(n_total, m_total) = calculate_m(data);
 
-    // prepare neighbors data structure (list of sets with 2 elements each)
-    std::map<std::set<std::string>, double> neighbors;
+    // prepare neighbors data structure (list of pairs, where order is unimportant)
+    std::map<t_neighbor, double> neighbors;
     if (neighbors_option == "all")
     {
         for (int row = 0; row < unit_names.size(); row++)
         {
             for (int col = row + 1; col < unit_names.size(); col++)
             {
-                neighbors[{unit_names[row], unit_names[col]}] = 0;
+                neighbors[neighbor_make_pair(unit_names[row], unit_names[col])] = 0;
             }
         }
     }
     else if (neighbors_option == "local")
     {
+
         auto ls = calculate_ls(data);
         for (int i = 0; i < ls.size(); i++)
         {
@@ -173,7 +184,7 @@ List compress_compute_cpp(
                  j++)
             {
                 if (i != j)
-                    neighbors[{ls[i].first, ls[j].first}] = 0;
+                    neighbors[neighbor_make_pair(ls[i].first, ls[j].first)] = 0;
             }
         }
     }
@@ -184,7 +195,7 @@ List compress_compute_cpp(
             std::string unit1 = Rcpp::as<std::string>(m_neighbors(i, 0));
             std::string unit2 = Rcpp::as<std::string>(m_neighbors(i, 1));
             if (unit1 != unit2)
-                neighbors[{unit1, unit2}] = 0;
+                neighbors[neighbor_make_pair(unit1, unit2)] = 0;
         }
     }
 
@@ -193,10 +204,7 @@ List compress_compute_cpp(
     // might do a lot of duplicate calculations)
     for (const auto &[key, reduction] : neighbors)
     {
-        auto iter = key.begin();
-        const std::string unit1 = *iter;
-        const std::string unit2 = *next(iter);
-        neighbors[{unit1, unit2}] = calculate_reduction(n_total, data[unit1], data[unit2]);
+        neighbors[neighbor_make_pair(key.first, key.second)] = calculate_reduction(n_total, data[key.first], data[key.second]);
     }
 
     // determine maximum number of iterations
@@ -225,7 +233,7 @@ List compress_compute_cpp(
 
         // find smallest reduction
         double min_reduction = 10000;
-        std::set<std::string> min_key;
+        t_neighbor min_key;
         for (const auto &[key, reduction] : neighbors)
         {
             if (reduction < min_reduction)
@@ -237,48 +245,36 @@ List compress_compute_cpp(
             }
         }
 
-        auto iter = min_key.begin();
-        const std::string unit_keep = *iter;
-        const std::string unit_delete = *next(iter);
+        const std::string unit_keep = min_key.first;
+        const std::string unit_delete = min_key.second;
         // add counts of 'delete' to 'keep', delete unit
         for (int i = 0; i < n_groups; i++)
             data[unit_keep][i] += data[unit_delete][i];
         data.erase(unit_delete);
 
         // update neighbors
-        std::vector<std::set<std::string>> delete_neighbors;
-        std::map<std::set<std::string>, double> new_neighbors;
+        neighbors.erase(min_key);
+
+        std::vector<t_neighbor> delete_neighbors;
+        std::map<t_neighbor, double> new_neighbors;
         for (const auto &[key, reduction] : neighbors)
         {
-            const bool delete_found = key.find(unit_delete) != key.end();
-            const bool keep_found = key.find(unit_keep) != key.end();
-            if (keep_found && delete_found)
+            // update pairs if deleted unit is involved
+            if (key.first == unit_delete || key.second == unit_delete)
             {
-                // remove the neighbor pair
+                // this is a pair some_unit - deleted_unit -- replace deleted_unit with unit_keep
                 delete_neighbors.push_back(key);
+                std::string some_unit = (key.first == unit_delete) ? key.second : key.first;
+                new_neighbors[neighbor_make_pair(unit_keep, some_unit)] =
+                    calculate_reduction(n_total, data[unit_keep], data[some_unit]);
             }
-            else if (delete_found)
+            // recalculate reduction if kept unit is involved
+            else if (key.first == unit_keep || key.second == unit_keep)
             {
-                // replace deleted unit with new unit
-                delete_neighbors.push_back(key);
-                std::set<std::string> new_key(key);
-                new_key.erase(unit_delete);
-                new_key.insert(unit_keep);
-
-                const auto iter = new_key.begin();
-                const std::string unit1 = *iter;
-                const std::string unit2 = *next(iter);
-                new_neighbors[{unit1, unit2}] = calculate_reduction(n_total, data[unit1], data[unit2]);
-            }
-            else if (keep_found)
-            {
-                // recalculate if updated unit is involved
-                const auto iter = key.begin();
-                const std::string unit1 = *iter;
-                const std::string unit2 = *next(iter);
-                new_neighbors[{unit1, unit2}] = calculate_reduction(n_total, data[unit1], data[unit2]);
+                new_neighbors[key] = calculate_reduction(n_total, data[key.first], data[key.second]);
             }
         }
+
         // delete neighbors that involve the old unit
         for (int i = 0; i < delete_neighbors.size(); i++)
             neighbors.erase(delete_neighbors[i]);