refactor: Track BnB selection by index

This is a performance optimization - rather than track all visited values
in a bool vector, track the selected index in a vector. This results in a
complexity reduction of O(utxo_size) to O(selection_size).

Author: Empact

src/wallet/coinselection.cpp

@@ -66,9 +66,7 @@ std::optional<SelectionResult> SelectCoinsBnB(std::vector<OutputGroup>& utxo_poo
 {
     SelectionResult result(selection_target);
     CAmount curr_value = 0;
-
-    std::vector<bool> curr_selection; // select the utxo at this index
-    curr_selection.reserve(utxo_pool.size());
+    std::vector<size_t> curr_selection; // selected utxo indexes
 
     // Calculate curr_available_value
     CAmount curr_available_value = 0;
@@ -85,11 +83,11 @@ std::optional<SelectionResult> SelectCoinsBnB(std::vector<OutputGroup>& utxo_poo
     std::sort(utxo_pool.begin(), utxo_pool.end(), descending);
 
     CAmount curr_waste = 0;
-    std::vector<bool> best_selection;
+    std::vector<size_t> best_selection;
     CAmount best_waste = MAX_MONEY;
 
     // Depth First search loop for choosing the UTXOs
-    for (size_t i = 0; i < TOTAL_TRIES; ++i) {
+    for (size_t i = 0, utxo_pool_index = 0; i < TOTAL_TRIES; ++i, ++utxo_pool_index) {
         // Conditions for starting a backtrack
         bool backtrack = false;
         if (curr_value + curr_available_value < selection_target ||                // Cannot possibly reach target with the amount remaining in the curr_available_value.
@@ -104,7 +102,6 @@ std::optional<SelectionResult> SelectCoinsBnB(std::vector<OutputGroup>& utxo_poo
             // explore any more UTXOs to avoid burning money like that.
             if (curr_waste <= best_waste) {
                 best_selection = curr_selection;
-                best_selection.resize(utxo_pool.size());
                 best_waste = curr_waste;
                 if (best_waste == 0) {
                     break;
@@ -116,36 +113,37 @@ std::optional<SelectionResult> SelectCoinsBnB(std::vector<OutputGroup>& utxo_poo
 
         // Backtracking, moving backwards
         if (backtrack) {
-            // Walk backwards to find the last included UTXO that still needs to have its omission branch traversed.
-            while (!curr_selection.empty() && !curr_selection.back()) {
-                curr_selection.pop_back();
-                curr_available_value += utxo_pool.at(curr_selection.size()).GetSelectionAmount();
-            }
-
             if (curr_selection.empty()) { // We have walked back to the first utxo and no branch is untraversed. All solutions searched
                 break;
             }
 
+            // Walk backwards to find the last included UTXO that still needs to have its omission branch traversed.
+            for (--utxo_pool_index; utxo_pool_index > curr_selection.back(); --utxo_pool_index) {
+                curr_available_value += utxo_pool.at(utxo_pool_index).GetSelectionAmount();
+            }
+
             // Output was included on previous iterations, try excluding now.
-            curr_selection.back() = false;
-            OutputGroup& utxo = utxo_pool.at(curr_selection.size() - 1);
+            assert(utxo_pool_index == curr_selection.back());
+            OutputGroup& utxo = utxo_pool.at(utxo_pool_index);
             curr_value -= utxo.GetSelectionAmount();
             curr_waste -= utxo.fee - utxo.long_term_fee;
+            curr_selection.pop_back();
         } else { // Moving forwards, continuing down this branch
-            OutputGroup& utxo = utxo_pool.at(curr_selection.size());
+            OutputGroup& utxo = utxo_pool.at(utxo_pool_index);
 
             // Remove this utxo from the curr_available_value utxo amount
             curr_available_value -= utxo.GetSelectionAmount();
 
             // Avoid searching a branch if the previous UTXO has the same value and same waste and was excluded. Since the ratio of fee to
             // long term fee is the same, we only need to check if one of those values match in order to know that the waste is the same.
-            if (!curr_selection.empty() && !curr_selection.back() &&
-                utxo.GetSelectionAmount() == utxo_pool.at(curr_selection.size() - 1).GetSelectionAmount() &&
-                utxo.fee == utxo_pool.at(curr_selection.size() - 1).fee) {
-                curr_selection.push_back(false);
-            } else {
+            if (curr_selection.empty() ||
+                // The previous index is included and therefore not relevant for exclusion shortcut
+                (utxo_pool_index - 1) == curr_selection.back() ||
+                utxo.GetSelectionAmount() != utxo_pool.at(utxo_pool_index - 1).GetSelectionAmount() ||
+                utxo.fee != utxo_pool.at(utxo_pool_index - 1).fee)
+            {
                 // Inclusion branch first (Largest First Exploration)
-                curr_selection.push_back(true);
+                curr_selection.push_back(utxo_pool_index);
                 curr_value += utxo.GetSelectionAmount();
                 curr_waste += utxo.fee - utxo.long_term_fee;
             }
@@ -158,10 +156,8 @@ std::optional<SelectionResult> SelectCoinsBnB(std::vector<OutputGroup>& utxo_poo
     }
 
     // Set output set
-    for (size_t i = 0; i < best_selection.size(); ++i) {
-        if (best_selection.at(i)) {
-            result.AddInput(utxo_pool.at(i));
-        }
+    for (const size_t& i : best_selection) {
+        result.AddInput(utxo_pool.at(i));
     }
 
     return result;