Deduplicate shift

src/wallet/coinselection.cpp

@@ -301,8 +301,8 @@ util::Result<SelectionResult> CoinGrinder(std::vector<OutputGroup>& utxo_pool, c
         curr_amount += utxo.GetSelectionAmount();
         curr_weight += utxo.m_weight;
         curr_selection.push_back(next_utxo);
-        ++curr_try;
         ++next_utxo;
+        ++curr_try;
 
         // EVALUATE current selection, default to exploring the inclusion branch further, else do exactly one SHIFT or CUT.
         if (curr_amount + lookahead[curr_selection.back()] < selection_target + change_target) {
@@ -340,42 +340,45 @@ util::Result<SelectionResult> CoinGrinder(std::vector<OutputGroup>& utxo_pool, c
         }
 
         if (should_cut) {
-            // Redirect to exploring Omission branch of second to last selected UTXO (i.e. deselect last selected UTXO, then SHIFT)
+
+            // Neither adding to the current selection nor exploring the omission branch of the last selected UTXO can
+            // find any solutions. Redirect to exploring Omission branch of the penultimate selected UTXO (i.e. deselect
+            // last selected UTXO, then SHIFT)
+
             should_cut = false;
             deselect_last();
             should_shift  = true;
         }
 
-        if (should_shift) {
-            // Set `next_utxo` to one after last selected, then deselect last selected UTXO
-            if (curr_selection.empty()) {
-                // Exhausted search space before running into attempt limit
-                result.SetAlgoCompleted(true);
-                break;
-            }
-            next_utxo = curr_selection.back() + 1;
-            deselect_last();
-            should_shift  = false;
-        }
-
-        // Find next undecided UTXO that does not produce equivalent prefix
-        while (next_utxo > 0
-                && (curr_selection.empty() || curr_selection.back() != next_utxo - 1)
-                && utxo_pool[next_utxo - 1].GetSelectionAmount() == utxo_pool[next_utxo].GetSelectionAmount()
-                && utxo_pool[next_utxo - 1].fee == utxo_pool[next_utxo].fee) {
-            if (next_utxo < utxo_pool.size() - 1) {
-                // Skip if previous UTXO is equivalent and unselected
-                ++next_utxo;
-            } else {
-                // "Skipping" end of branch: SHIFT instead
+        while (!is_done && should_shift) {
+            if (should_shift) {
+                // Set `next_utxo` to one after last selected, then deselect last selected UTXO
                 if (curr_selection.empty()) {
-                    // Exhausted search space before running into limit
+                    // Exhausted search space before running into attempt limit
                     is_done = true;
                     result.SetAlgoCompleted(true);
                     break;
                 }
                 next_utxo = curr_selection.back() + 1;
                 deselect_last();
+                should_shift  = false;
+            }
+
+            // After SHIFTing to an omission branch, the `next_utxo` might have the same value and same weight as the
+            // UTXO we just omitted (i.e. it is a duplicate). If so, selecting `next_utxo` would produce an equivalent
+            // selection as one we previously evaluated. In that case, increment `next_utxo` until we find a UTXO with a
+            // differing amount or weight.
+            while (!should_shift && next_utxo > 0
+                    && (curr_selection.empty() || curr_selection.back() != next_utxo - 1)
+                    && utxo_pool[next_utxo - 1].GetSelectionAmount() == utxo_pool[next_utxo].GetSelectionAmount()
+                    && utxo_pool[next_utxo - 1].fee == utxo_pool[next_utxo].fee) {
+                if (next_utxo < utxo_pool.size() - 1) {
+                    // Skip clone: previous UTXO is equivalent and unselected
+                    ++next_utxo;
+                } else {
+                    // Reached end of UTXO pool skipping clones: SHIFT instead
+                    should_shift = true;
+                }
             }
         }
     }