opt: Skip over combinations of sand

If you have a lot of small value UTXOs, they may in sum add up to a
sufficient lookahead, but we can gauge from the amount of funds we are
still missing, the maximal amount in the tail, and the minimal weight in
the tail whether any sufficient combination would exceed the best weight
and in that case cut the subtree.

src/wallet/coinselection.cpp

@@ -13,6 +13,7 @@
 #include <util/check.h>
 #include <util/moneystr.h>
 
+#include <cmath>
 #include <numeric>
 #include <optional>
 #include <queue>
@@ -335,7 +336,11 @@ util::Result<SelectionResult> CoinGrinder(std::vector<OutputGroup>& utxo_pool, c
                 best_selection = curr_selection;
                 best_selection_weight = curr_weight;
                 best_selection_amount = curr_amount;
+                std::cout << "NEW BEST!" << std::endl;
             }
+        } else if (!best_selection.empty() && curr_weight + min_tail_weight[curr_selection.back()] * std::ceil((selection_target + change_target - curr_amount) / utxo_pool[curr_selection.back()].GetSelectionAmount()) > best_selection_weight) {
+            // Compare minimal tail weight and last selected amount with the amount missing to gauge whether a better weight is still possible.
+            should_cut = true;
         }
 
         if (curr_try >= TOTAL_TRIES) {

src/wallet/test/coinselector_tests.cpp

@@ -1178,7 +1178,7 @@ BOOST_AUTO_TEST_CASE(coin_grinder_tests)
         });
         BOOST_CHECK(res);
         // If this takes more attempts, the implementation has regressed
-        size_t expected_attempts = 184;
+        size_t expected_attempts = 37;
         BOOST_CHECK_MESSAGE(res->GetSelectionsEvaluated() == expected_attempts, "Expected " + std::to_string(expected_attempts) + " attempts, but got " + std::to_string(res->GetSelectionsEvaluated()));
     }
 
@@ -1247,7 +1247,7 @@ BOOST_AUTO_TEST_CASE(coin_grinder_tests)
 
     {
         // #################################################################################################################
-        // 5) Lots of tiny UTXOs of different amounts quickly exhausts the search attempts
+        // 5) Test that lots of tiny UTXOs can be skipped even if there are enough funds in lookahead
         // #################################################################################################################
         SelectionResult expected_result(CAmount(0), SelectionAlgorithm::CG);
         CAmount target =  1.9L * COIN;
@@ -1264,11 +1264,11 @@ BOOST_AUTO_TEST_CASE(coin_grinder_tests)
             return available_coins;
         });
         expected_result.Clear();
-        add_coin(1.8 * COIN, 1, expected_result);
+        add_coin(1 * COIN, 1, expected_result);
         add_coin(1 * COIN, 2, expected_result);
         BOOST_CHECK(EquivalentResult(expected_result, *res));
         // If this takes more attempts, the implementation has regressed
-        size_t expected_attempts = 100000;
+        size_t expected_attempts = 7;
         BOOST_CHECK_MESSAGE(res->GetSelectionsEvaluated() == expected_attempts, "Expected " + std::to_string(expected_attempts) + " attempts, but got " + std::to_string(res->GetSelectionsEvaluated()));
     }