wallet: add SelectionResult::GetChange

src/wallet/coinselection.cpp

@@ -421,6 +421,11 @@ CAmount SelectionResult::GetSelectedValue() const
     return std::accumulate(m_selected_inputs.cbegin(), m_selected_inputs.cend(), CAmount{0}, [](CAmount sum, const auto& coin) { return sum + coin.txout.nValue; });
 }
 
+CAmount SelectionResult::GetSelectedEffectiveValue() const
+{
+    return std::accumulate(m_selected_inputs.cbegin(), m_selected_inputs.cend(), CAmount{0}, [](CAmount sum, const auto& coin) { return sum + coin.GetEffectiveValue(); });
+}
+
 void SelectionResult::Clear()
 {
     m_selected_inputs.clear();
@@ -480,4 +485,24 @@ std::string GetAlgorithmName(const SelectionAlgorithm algo)
     }
     assert(false);
 }
+
+CAmount SelectionResult::GetChange(const CAmount min_viable_change, const CAmount change_fee) const
+{
+    // change = SUM(inputs) - SUM(outputs) - fees
+    // 1) With SFFO we don't pay any fees
+    // 2) Otherwise we pay all the fees:
+    //  - input fees are covered by GetSelectedEffectiveValue()
+    //  - non_input_fee is included in m_target
+    //  - change_fee
+    const CAmount change = m_use_effective
+                           ? GetSelectedEffectiveValue() - m_target - change_fee
+                           : GetSelectedValue() - m_target;
+
+    if (change < min_viable_change) {
+        return 0;
+    }
+
+    return change;
+}
+
 } // namespace wallet

src/wallet/coinselection.h

@@ -303,6 +303,8 @@ public:
     /** Get the sum of the input values */
     [[nodiscard]] CAmount GetSelectedValue() const;
 
+    [[nodiscard]] CAmount GetSelectedEffectiveValue() const;
+
     void Clear();
 
     void AddInput(const OutputGroup& group);
@@ -320,6 +322,25 @@ public:
 
     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; }