sweep: extract positive input set struct

A refactoring that introduces no functional changes. This prepares for
the addition of wallet utxos to push the sweep tx above the dust limit.

It also enabled access to input-specific sweep parameters during tx
generation. This will be used in later commits to control the sweep
process.

sweep/tx_input_set_test.go

@@ -0,0 +1,62 @@
+package sweep
+
+import (
+	"testing"
+
+	"github.com/btcsuite/btcutil"
+	"github.com/lightningnetwork/lnd/input"
+)
+
+// TestTxInputSet tests adding various sized inputs to the set.
+func TestTxInputSet(t *testing.T) {
+	const (
+		feeRate   = 1000
+		relayFee  = 300
+		maxInputs = 10
+	)
+	set := newTxInputSet(feeRate, relayFee, maxInputs)
+
+	if set.dustLimit != 537 {
+		t.Fatalf("incorrect dust limit")
+	}
+
+	// Create a 300 sat input. The fee to sweep this input to a P2WKH output
+	// is 439 sats. That means that this input yields -139 sats and we
+	// expect it not to be added.
+	if set.add(createP2WKHInput(300)) {
+		t.Fatal("expected add of negatively yielding input to fail")
+	}
+
+	// A 700 sat input should be accepted into the set, because it yields
+	// positively.
+	if !set.add(createP2WKHInput(700)) {
+		t.Fatal("expected add of positively yielding input to succeed")
+	}
+
+	// The tx output should now be 700-439 = 261 sats. The dust limit isn't
+	// reached yet.
+	if set.outputValue != 261 {
+		t.Fatal("unexpected output value")
+	}
+	if set.dustLimitReached() {
+		t.Fatal("expected dust limit not yet to be reached")
+	}
+
+	// Add a 1000 sat input. This increases the tx fee to 712 sats. The tx
+	// output should now be 1000+700 - 712 = 988 sats.
+	if !set.add(createP2WKHInput(1000)) {
+		t.Fatal("expected add of positively yielding input to succeed")
+	}
+	if set.outputValue != 988 {
+		t.Fatal("unexpected output value")
+	}
+	if !set.dustLimitReached() {
+		t.Fatal("expected dust limit to be reached")
+	}
+}
+
+// createP2WKHInput returns a P2WKH test input with the specified amount.
+func createP2WKHInput(amt btcutil.Amount) input.Input {
+	input := createTestInput(int64(amt), input.WitnessKeyHash)
+	return &input
+}