Merge bitcoin/bitcoin#22938: test: Add remaining scenarios of 0 waste, in wallet waste_test

efcaefc7b5ffe0495e7c809032342ee5ca4841be test: Add remaining scenarios of 0 waste (rajarshimaitra)

Pull request description:

  As per the [review club](https://bitcoincore.reviews/22009) discussion on #22009 , it was observed that there were other two fee scenarios in which selection waste could be zero.

  These are:
   - (LTF - Fee) == Change Cost
   - (LTF - Fee) == Excess

  Even though these are obvious by the definition of waste metric, adding tests for them can be helpful in explaining its behavior
  to new readers of the code base, along with pinning the behavior for future.

  This PR adds those two cases to waste calculation unit test.

  Also let me know if I am missing more scenarios.

ACKs for top commit:
  jonatack:
    Tested re-ACK efcaefc7b5ffe0495e7c809032342ee5ca4841be
  achow101:
    ACK efcaefc7b5ffe0495e7c809032342ee5ca4841be
  meshcollider:
    ACK efcaefc7b5ffe0495e7c809032342ee5ca4841be

Tree-SHA512: 13fe3e2c0ea7bb58d34e16c32908b84705130dec16382ff941e5e60ca5b379f9c5811b33f36c4c72d7a98cfbb6af2f196d0a69e96989afa4b9e49893eaadd7cb

1 files changed, 16 insertions, 3 deletions

diff --git a/src/wallet/test/coinselector_tests.cpp b/src/wallet/test/coinselector_tests.cpp
index 5d51809241..40d9e90d56 100644
--- a/src/wallet/test/coinselector_tests.cpp
+++ b/src/wallet/test/coinselector_tests.cpp
@@ -724,12 +724,25 @@ BOOST_AUTO_TEST_CASE(waste_test)
     BOOST_CHECK_LT(waste_nochange2, waste_nochange1);
     selection.clear();
 
-    // 0 Waste only when fee == long term fee, no change, and no excess
+    // No Waste when fee == long_term_fee, no change, and no excess
     add_coin(1 * COIN, 1, selection, fee, fee);
     add_coin(2 * COIN, 2, selection, fee, fee);
-    const CAmount exact_target = in_amt - 2 * fee;
-    BOOST_CHECK_EQUAL(0, GetSelectionWaste(selection, 0, exact_target));
+    const CAmount exact_target{in_amt - fee * 2};
+    BOOST_CHECK_EQUAL(0, GetSelectionWaste(selection, /* change_cost */ 0, exact_target));
+    selection.clear();
 
+    // No Waste when (fee - long_term_fee) == (-cost_of_change), and no excess
+    const CAmount new_change_cost{fee_diff * 2};
+    add_coin(1 * COIN, 1, selection, fee, fee + fee_diff);
+    add_coin(2 * COIN, 2, selection, fee, fee + fee_diff);
+    BOOST_CHECK_EQUAL(0, GetSelectionWaste(selection, new_change_cost, target));
+    selection.clear();
+
+    // No Waste when (fee - long_term_fee) == (-excess), no change cost
+    const CAmount new_target{in_amt - fee * 2 - fee_diff * 2};
+    add_coin(1 * COIN, 1, selection, fee, fee + fee_diff);
+    add_coin(2 * COIN, 2, selection, fee, fee + fee_diff);
+    BOOST_CHECK_EQUAL(0, GetSelectionWaste(selection, /* change cost */ 0, new_target));
 }
 
 BOOST_AUTO_TEST_SUITE_END()