wallet/size: add EstimateVirtualSize

This commit adds a new method EstimateVirtualSize that calculates
the worst case estimate vsize for a transaction with a given set
of inputs and outputs. This method is aware of P2PKH, P2WPKH and
P2SH-P2WPKH inputs, and caulculates the transaction vsize with
the witness data included.

wallet/internal/txsizes/size.go

@@ -5,6 +5,7 @@
 package txsizes
 
 import (
+	"github.com/roasbeef/btcd/blockchain"
 	"github.com/roasbeef/btcd/wire"
 
 	h "github.com/roasbeef/btcwallet/internal/helpers"
@@ -52,7 +53,69 @@ const (
 	//   - 25 bytes P2PKH output script
 	P2PKHOutputSize = 8 + 1 + P2PKHPkScriptSize
 
-	// TODO(roasbeef): add estimates for nested p2wkh and p2pkh
+	// P2WPKHPkScriptSize is the size of a transaction output script that
+	// pays to a witness pubkey hash. It is calculated as:
+	//
+	//   - OP_0
+	//   - OP_DATA_20
+	//   - 20 bytes pubkey hash
+	P2WPKHPkScriptSize = 1 + 1 + 20
+
+	// P2WPKHOutputSize is the serialize size of a transaction output with a
+	// P2WPKH output script. It is calculated as:
+	//
+	//   - 8 bytes output value
+	//   - 1 byte compact int encoding value 22
+	//   - 22 bytes P2PKH output script
+	P2WPKHOutputSize = 8 + 1 + P2WPKHPkScriptSize
+
+	// RedeemP2WPKHScriptSize is the size of a transaction input script
+	// that spends a pay-to-witness-public-key hash (P2WPKH). The redeem
+	// script for P2WPKH spends MUST be empty.
+	RedeemP2WPKHScriptSize = 0
+
+	// RedeemP2WPKHInputSize is the worst case size of a transaction
+	// input redeeming a P2WPKH output. It is calculated as:
+	//
+	//   - 32 bytes previous tx
+	//   - 4 bytes output index
+	//   - 1 byte encoding empty redeem script
+	//   - 0 bytes redeem script
+	//   - 4 bytes sequence
+	RedeemP2WPKHInputSize = 32 + 4 + 1 + RedeemP2WPKHScriptSize + 4
+
+	// RedeemNestedP2WPKHScriptSize is the worst case size of a transaction
+	// input script that redeems a pay-to-witness-key hash nested in P2SH
+	// (P2SH-P2WPKH). It is calculated as:
+	//
+	//   - 1 byte compact int encoding value 22
+	//   - OP_0
+	//   - 1 byte compact int encoding value 20
+	//   - 20 byte key hash
+	RedeemNestedP2WPKHScriptSize = 1 + 1 + 1 + 20
+
+	// RedeemNestedP2WPKHInputSize is the worst case size of a
+	// transaction input redeeming a P2SH-P2WPKH output. It is
+	// calculated as:
+	//
+	//   - 32 bytes previous tx
+	//   - 4 bytes output index
+	//   - 1 byte compact int encoding value 23
+	//   - 23 bytes redeem script (scriptSig)
+	//   - 4 bytes sequence
+	RedeemNestedP2WPKHInputSize = 32 + 4 + 1 +
+		RedeemNestedP2WPKHScriptSize + 4
+
+	// RedeemP2WPKHInputWitnessWeight is the worst case weight of
+	// a witness for spending P2WPKH and nested P2WPKH outputs. It
+	// is calculated as:
+	//
+	//   - 1 wu compact int encoding value 2 (number of items)
+	//   - 1 wu compact int encoding value 73
+	//   - 72 wu DER signature + 1 wu sighash
+	//   - 1 wu compact int encoding value 33
+	//   - 33 wu serialized compressed pubkey
+	RedeemP2WPKHInputWitnessWeight = 1 + 1 + 73 + 1 + 33
 )
 
 // EstimateSerializeSize returns a worst case serialize size estimate for a
@@ -74,3 +137,48 @@ func EstimateSerializeSize(inputCount int, txOuts []*wire.TxOut, addChangeOutput
 		h.SumOutputSerializeSizes(txOuts) +
 		changeSize
 }
+
+// EstimateVirtualSize returns a worst case virtual size estimate for a
+// signed transaction that spends the given number of P2PKH, P2WPKH and
+// (nested) P2SH-P2WPKH outputs, and contains each transaction output
+// from txOuts. The estimate is incremented for an additional P2PKH
+// change output if addChangeOutput is true.
+func EstimateVirtualSize(numP2PKHIns, numP2WPKHIns, numNestedP2WPKHIns int,
+	txOuts []*wire.TxOut, addChangeOutput bool) int {
+	changeSize := 0
+	outputCount := len(txOuts)
+	if addChangeOutput {
+		// We are always using P2WPKH as change output.
+		changeSize = P2WPKHOutputSize
+		outputCount++
+	}
+
+	// Version 4 bytes + LockTime 4 bytes + Serialized var int size for the
+	// number of transaction inputs and outputs + size of redeem scripts +
+	// the size out the serialized outputs and change.
+	baseSize := 8 +
+		wire.VarIntSerializeSize(
+			uint64(numP2PKHIns+numP2WPKHIns+numNestedP2WPKHIns)) +
+		wire.VarIntSerializeSize(uint64(len(txOuts))) +
+		numP2PKHIns*RedeemP2PKHInputSize +
+		numP2WPKHIns*RedeemP2WPKHInputSize +
+		numNestedP2WPKHIns*RedeemNestedP2WPKHInputSize +
+		h.SumOutputSerializeSizes(txOuts) +
+		changeSize
+
+	// If this transaction has any witness inputs, we must count the
+	// witness data.
+	witnessWeight := 0
+	if numP2WPKHIns+numNestedP2WPKHIns > 0 {
+		// Additional 2 weight units for segwit marker + flag.
+		witnessWeight = 2 +
+			wire.VarIntSerializeSize(
+				uint64(numP2WPKHIns+numNestedP2WPKHIns)) +
+			numP2WPKHIns*RedeemP2WPKHInputWitnessWeight +
+			numNestedP2WPKHIns*RedeemP2WPKHInputWitnessWeight
+	}
+
+	// We add 3 to the witness weight to make sure the result is
+	// always rounded up.
+	return baseSize + (witnessWeight+3)/blockchain.WitnessScaleFactor
+}