sweepbatcher: consolidate identical change pkscripts

batch separate change outputs with identical pkscripts
are tallied up and consolidated to a single change output.

Author: hieblmi

sweepbatcher/greedy_batch_selection.go

@@ -210,11 +210,21 @@ func estimateBatchWeight(batch *batch) (feeDetails, error) {
 			err)
 	}
 
-	// Add change output weights.
+	// Add change output weights. Change outputs with identical pkscript
+	// will be consolidated into a single output.
+	changeOutputs := make(map[string]struct{})
 	for _, s := range batch.sweeps {
-		if s.change != nil {
-			weight.AddOutput(s.change.PkScript)
+		if s.change == nil {
+			continue
 		}
+
+		pkScriptString := string(s.change.PkScript)
+		if _, has := changeOutputs[pkScriptString]; has {
+			continue
+		}
+
+		weight.AddOutput(s.change.PkScript)
+		changeOutputs[pkScriptString] = struct{}{}
 	}
 
 	// Add inputs.

sweepbatcher/sweep_batch.go

@@ -1260,10 +1260,22 @@ func constructUnsignedTx(sweeps []sweep, address btcutil.Address,
 		LockTime: uint32(currentHeight),
 	}
 
-	var changeOutputs []*wire.TxOut
-	for _, sweep := range sweeps {
-		if sweep.change != nil {
-			changeOutputs = append(changeOutputs, sweep.change)
+	// Consolidate change outputs with identical pkscript.
+	changeOutputs := make(map[string]*wire.TxOut)
+	for _, s := range sweeps {
+		if s.change == nil {
+			continue
+		}
+
+		stringPkScript := string(s.change.PkScript)
+		if _, has := changeOutputs[stringPkScript]; has {
+			changeOutputs[stringPkScript].Value += s.change.Value
+			continue
+		}
+
+		changeOutputs[stringPkScript] = &wire.TxOut{
+			Value:    s.change.Value,
+			PkScript: s.change.PkScript,
 		}
 	}
 

sweepbatcher/sweep_batch_test.go

@@ -395,6 +395,57 @@ func TestConstructUnsignedTx(t *testing.T) {
 			wantFee:          1248,
 		},
 
+		{
+			name: "all sweeps identical change pkscripts",
+			sweeps: []sweep{
+				{
+					outpoint: op1,
+					value:    1_000_000,
+				},
+				{
+					outpoint: op2,
+					value:    2_000_000,
+					change:   change1,
+				},
+				{
+					outpoint: op3,
+					value:    3_000_000,
+					change:   change1,
+				},
+			},
+			address:       p2trAddress,
+			currentHeight: 800_000,
+			feeRate:       1000,
+			wantTx: &wire.MsgTx{
+				Version:  2,
+				LockTime: 800_000,
+				TxIn: []*wire.TxIn{
+					{
+						PreviousOutPoint: op1,
+					},
+					{
+						PreviousOutPoint: op2,
+					},
+					{
+						PreviousOutPoint: op3,
+					},
+				},
+				TxOut: []*wire.TxOut{
+					{
+						Value:    5_798_924,
+						PkScript: p2trPkScript,
+					},
+					{
+						Value:    2 * change1.Value,
+						PkScript: change1.PkScript,
+					},
+				},
+			},
+			wantWeight:       1076,
+			wantFeeForWeight: 1076,
+			wantFee:          1076,
+		},
+
 		{
 			name: "change exceeds input value",
 			sweeps: []sweep{