feat(consensus): fee extraction with overflow checks

TxFee calculates pre-HF4 fees as sum(inputs) - sum(outputs) with
overflow detection. Coinbase transactions return zero fee.

Co-Authored-By: Charon <charon@lethean.io>

consensus/fee.go

@@ -0,0 +1,79 @@
+// Copyright (c) 2017-2026 Lethean (https://lt.hn)
+//
+// Licensed under the European Union Public Licence (EUPL) version 1.2.
+// SPDX-License-Identifier: EUPL-1.2
+
+package consensus
+
+import (
+	"fmt"
+	"math"
+
+	"forge.lthn.ai/core/go-blockchain/types"
+)
+
+// TxFee calculates the transaction fee for pre-HF4 (v0/v1) transactions.
+// Coinbase transactions return 0. For standard transactions, fee equals
+// the difference between total input amounts and total output amounts.
+func TxFee(tx *types.Transaction) (uint64, error) {
+	if isCoinbase(tx) {
+		return 0, nil
+	}
+
+	inputSum, err := sumInputs(tx)
+	if err != nil {
+		return 0, err
+	}
+
+	outputSum, err := sumOutputs(tx)
+	if err != nil {
+		return 0, err
+	}
+
+	if outputSum > inputSum {
+		return 0, fmt.Errorf("%w: inputs=%d, outputs=%d", ErrNegativeFee, inputSum, outputSum)
+	}
+
+	return inputSum - outputSum, nil
+}
+
+// isCoinbase returns true if the transaction's first input is TxInputGenesis.
+func isCoinbase(tx *types.Transaction) bool {
+	if len(tx.Vin) == 0 {
+		return false
+	}
+	_, ok := tx.Vin[0].(types.TxInputGenesis)
+	return ok
+}
+
+// sumInputs totals all TxInputToKey amounts, checking for overflow.
+func sumInputs(tx *types.Transaction) (uint64, error) {
+	var total uint64
+	for _, vin := range tx.Vin {
+		toKey, ok := vin.(types.TxInputToKey)
+		if !ok {
+			continue
+		}
+		if total > math.MaxUint64-toKey.Amount {
+			return 0, ErrInputOverflow
+		}
+		total += toKey.Amount
+	}
+	return total, nil
+}
+
+// sumOutputs totals all TxOutputBare amounts, checking for overflow.
+func sumOutputs(tx *types.Transaction) (uint64, error) {
+	var total uint64
+	for _, vout := range tx.Vout {
+		bare, ok := vout.(types.TxOutputBare)
+		if !ok {
+			continue
+		}
+		if total > math.MaxUint64-bare.Amount {
+			return 0, ErrOutputOverflow
+		}
+		total += bare.Amount
+	}
+	return total, nil
+}

consensus/fee_test.go

@@ -0,0 +1,73 @@
+// Copyright (c) 2017-2026 Lethean (https://lt.hn)
+//
+// Licensed under the European Union Public Licence (EUPL) version 1.2.
+// SPDX-License-Identifier: EUPL-1.2
+
+//go:build !integration
+
+package consensus
+
+import (
+	"testing"
+
+	"forge.lthn.ai/core/go-blockchain/types"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestTxFee_Good(t *testing.T) {
+	// Coinbase tx: fee is 0.
+	coinbase := &types.Transaction{
+		Version: types.VersionInitial,
+		Vin:     []types.TxInput{types.TxInputGenesis{Height: 1}},
+	}
+	fee, err := TxFee(coinbase)
+	require.NoError(t, err)
+	assert.Equal(t, uint64(0), fee)
+
+	// Normal v1 tx: fee = inputs - outputs.
+	tx := &types.Transaction{
+		Version: types.VersionPreHF4,
+		Vin: []types.TxInput{
+			types.TxInputToKey{Amount: 100},
+			types.TxInputToKey{Amount: 50},
+		},
+		Vout: []types.TxOutput{
+			types.TxOutputBare{Amount: 120},
+		},
+	}
+	fee, err = TxFee(tx)
+	require.NoError(t, err)
+	assert.Equal(t, uint64(30), fee)
+}
+
+func TestTxFee_Bad(t *testing.T) {
+	// Outputs exceed inputs.
+	tx := &types.Transaction{
+		Version: types.VersionPreHF4,
+		Vin: []types.TxInput{
+			types.TxInputToKey{Amount: 50},
+		},
+		Vout: []types.TxOutput{
+			types.TxOutputBare{Amount: 100},
+		},
+	}
+	_, err := TxFee(tx)
+	assert.ErrorIs(t, err, ErrNegativeFee)
+}
+
+func TestTxFee_Ugly(t *testing.T) {
+	// Input amounts that would overflow uint64.
+	tx := &types.Transaction{
+		Version: types.VersionPreHF4,
+		Vin: []types.TxInput{
+			types.TxInputToKey{Amount: ^uint64(0)},
+			types.TxInputToKey{Amount: 1},
+		},
+		Vout: []types.TxOutput{
+			types.TxOutputBare{Amount: 1},
+		},
+	}
+	_, err := TxFee(tx)
+	assert.ErrorIs(t, err, ErrInputOverflow)
+}