adding new CalculateFee method

tx.go

@@ -260,6 +260,19 @@ func (tx *Tx) ChangeToOutput(index uint, f []*Fee) error {
 	return nil
 }
 
+// CalculateFee will return the amount of fees the current transaction will
+// require.
+func (tx *Tx) CalculateFee(f []*Fee) (uint64, error) {
+	total := tx.GetTotalInputSatoshis() - tx.GetTotalOutputSatoshis()
+	sats, _, err := tx.change(nil, f, false)
+	if err != nil {
+		return 0, err
+	}
+	return total - sats, nil
+}
+
+// change will return the amount of satoshis to add to an input after fees are removed.
+// True will be returned if change has been added.
 func (tx *Tx) change(s *bscript.Script, f []*Fee, newOutput bool) (uint64, bool, error) {
 	inputAmount := tx.GetTotalInputSatoshis()
 	outputAmount := tx.GetTotalOutputSatoshis()

tx_test.go

@@ -851,3 +851,50 @@ func TestTx_ChangeToOutput(t *testing.T) {
 		})
 	}
 }
+
+func TestTx_CalculateChange(t *testing.T) {
+	tests := map[string]struct {
+		tx      *bt.Tx
+		fees    []*bt.Fee
+		expFees uint64
+		err     error
+	}{
+		"Transaction with one input one output should return 96": {
+			tx: func() *bt.Tx {
+				tx := bt.NewTx()
+				assert.NoError(t, tx.From(
+					"07912972e42095fe58daaf09161c5a5da57be47c2054dc2aaa52b30fefa1940b",
+					0,
+					"76a914af2590a45ae401651fdbdf59a76ad43d1862534088ac",
+					1000))
+				assert.NoError(t, tx.PayTo("mxAoAyZFXX6LZBWhoam3vjm6xt9NxPQ15f", 500))
+				return tx
+			}(),
+			fees:    bt.DefaultFees(),
+			expFees: 96,
+		}, "Transaction with one input 4 outputs should return 147": {
+			tx: func() *bt.Tx {
+				tx := bt.NewTx()
+				assert.NoError(t, tx.From(
+					"07912972e42095fe58daaf09161c5a5da57be47c2054dc2aaa52b30fefa1940b",
+					0,
+					"76a914af2590a45ae401651fdbdf59a76ad43d1862534088ac",
+					2500))
+				assert.NoError(t, tx.PayTo("mxAoAyZFXX6LZBWhoam3vjm6xt9NxPQ15f", 500))
+				assert.NoError(t, tx.PayTo("mxAoAyZFXX6LZBWhoam3vjm6xt9NxPQ15f", 500))
+				assert.NoError(t, tx.PayTo("mxAoAyZFXX6LZBWhoam3vjm6xt9NxPQ15f", 500))
+				assert.NoError(t, tx.PayTo("mxAoAyZFXX6LZBWhoam3vjm6xt9NxPQ15f", 500))
+				return tx
+			}(),
+			fees:    bt.DefaultFees(),
+			expFees: 147,
+		},
+	}
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			fee, err := test.tx.CalculateFee(test.fees)
+			assert.Equal(t, test.err, err)
+			assert.Equal(t, test.expFees, fee)
+		})
+	}
+}