1 files changed, 39 insertions, 7 deletions

diff --git a/src/wallet/coinselection.h b/src/wallet/coinselection.h
index e257d3f7c7..761c2be0b3 100644
--- a/src/wallet/coinselection.h
+++ b/src/wallet/coinselection.h
@@ -123,6 +123,10 @@ struct CoinSelectionParams {
     /** Mininmum change to target in Knapsack solver: select coins to cover the payment and
      * at least this value of change. */
     CAmount m_min_change_target{0};
+    /** Minimum amount for creating a change output.
+     * If change budget is smaller than min_change then we forgo creation of change output.
+     */
+    CAmount min_viable_change{0};
     /** Cost of creating the change output. */
     CAmount m_change_fee{0};
     /** Cost of creating the change output + cost of spending the change output in the future. */
@@ -252,6 +256,7 @@ struct OutputGroup
 
 /** Choose a random change target for each transaction to make it harder to fingerprint the Core
  * wallet based on the change output values of transactions it creates.
+ * Change target covers at least change fees and adds a random value on top of it.
  * The random value is between 50ksat and min(2 * payment_value, 1milsat)
  * When payment_value <= 25ksat, the value is just 50ksat.
  *
@@ -261,8 +266,9 @@ struct OutputGroup
  * coins selected are just sufficient to cover the payment amount ("unnecessary input" heuristic).
  *
  * @param[in]   payment_value   Average payment value of the transaction output(s).
+ * @param[in]   change_fee      Fee for creating a change output.
  */
-[[nodiscard]] CAmount GenerateChangeTarget(CAmount payment_value, FastRandomContext& rng);
+[[nodiscard]] CAmount GenerateChangeTarget(const CAmount payment_value, const CAmount change_fee, FastRandomContext& rng);
 
 enum class SelectionAlgorithm : uint8_t
 {
@@ -279,17 +285,16 @@ struct SelectionResult
 private:
     /** Set of inputs selected by the algorithm to use in the transaction */
     std::set<COutput> m_selected_inputs;
+    /** The target the algorithm selected for. Equal to the recipient amount plus non-input fees */
+    CAmount m_target;
+    /** The algorithm used to produce this result */
+    SelectionAlgorithm m_algo;
     /** Whether the input values for calculations should be the effective value (true) or normal value (false) */
     bool m_use_effective{false};
     /** The computed waste */
     std::optional<CAmount> m_waste;
 
 public:
-    /** The target the algorithm selected for. Note that this may not be equal to the recipient amount as it can include non-input fees */
-    const CAmount m_target;
-    /** The algorithm used to produce this result */
-    const SelectionAlgorithm m_algo;
-
     explicit SelectionResult(const CAmount target, SelectionAlgorithm algo)
         : m_target(target), m_algo(algo) {}
 
@@ -298,20 +303,47 @@ public:
     /** Get the sum of the input values */
     [[nodiscard]] CAmount GetSelectedValue() const;
 
+    [[nodiscard]] CAmount GetSelectedEffectiveValue() const;
+
     void Clear();
 
     void AddInput(const OutputGroup& group);
 
     /** Calculates and stores the waste for this selection via GetSelectionWaste */
-    void ComputeAndSetWaste(CAmount change_cost);
+    void ComputeAndSetWaste(const CAmount min_viable_change, const CAmount change_cost, const CAmount change_fee);
     [[nodiscard]] CAmount GetWaste() const;
 
+    void Merge(const SelectionResult& other);
+
     /** Get m_selected_inputs */
     const std::set<COutput>& GetInputSet() const;
     /** Get the vector of COutputs that will be used to fill in a CTransaction's vin */
     std::vector<COutput> GetShuffledInputVector() const;
 
     bool operator<(SelectionResult other) const;
+
+    /** Get the amount for the change output after paying needed fees.
+     *
+     * The change amount is not 100% precise due to discrepancies in fee calculation.
+     * The final change amount (if any) should be corrected after calculating the final tx fees.
+     * When there is a discrepancy, most of the time the final change would be slightly bigger than estimated.
+     *
+     * Following are the possible factors of discrepancy:
+     *  + non-input fees always include segwit flags
+     *  + input fee estimation always include segwit stack size
+     *  + input fees are rounded individually and not collectively, which leads to small rounding errors
+     *  - input counter size is always assumed to be 1vbyte
+     *
+     * @param[in]  min_viable_change  Minimum amount for change output, if change would be less then we forgo change
+     * @param[in]  change_fee         Fees to include change output in the tx
+     * @returns Amount for change output, 0 when there is no change.
+     *
+     */
+    CAmount GetChange(const CAmount min_viable_change, const CAmount change_fee) const;
+
+    CAmount GetTarget() const { return m_target; }
+
+    SelectionAlgorithm GetAlgo() const { return m_algo; }
 };
 
 std::optional<SelectionResult> SelectCoinsBnB(std::vector<OutputGroup>& utxo_pool, const CAmount& selection_target, const CAmount& cost_of_change);