Step 4

- Transactions

block.go

@@ -2,8 +2,8 @@ package main
 
 import (
 	"bytes"
+	"crypto/sha256"
 	"encoding/gob"
-	"log"
 	"math"
 	"time"
 )
@@ -14,17 +14,17 @@ var (
 
 type Block struct {
 	Timestamp     int64
-	Data          []byte
+	Transactions  []*Transaction
 	PrevBlockHash []byte
 	Hash          []byte
 	Nonce         int
 }
 
 // NewBlock returns a new block, ready to be added to the chain
-func NewBlock(data string, prevBlockHash []byte) *Block {
+func NewBlock(transactions []*Transaction, prevBlockHash []byte) *Block {
 	block := &Block{
 		Timestamp:     time.Now().Unix(),
-		Data:          []byte(data),
+		Transactions:  transactions,
 		PrevBlockHash: prevBlockHash,
 		Hash:          []byte{},
 	}
@@ -41,7 +41,7 @@ func (b *Block) Serialize() []byte {
 	var result bytes.Buffer
 	encoder := gob.NewEncoder(&result)
 	if err := encoder.Encode(b); err != nil {
-		log.Panic(err)
+		exitWithError(err)
 	}
 	return result.Bytes()
 }
@@ -50,7 +50,19 @@ func DeserializeBlock(d []byte) *Block {
 	var block Block
 	decoder := gob.NewDecoder(bytes.NewReader(d))
 	if err := decoder.Decode(&block); err != nil {
-		log.Panic(err)
+		exitWithError(err)
 	}
 	return &block
 }
+
+func (b *Block) HashTransactions() []byte {
+	var txHashes [][]byte
+	var txHash [32]byte
+
+	for _, tx := range b.Transactions {
+		txHashes = append(txHashes, tx.ID)
+	}
+	txHash = sha256.Sum256(bytes.Join(txHashes, []byte{}))
+
+	return txHash[:]
+}

blockchain.go

@@ -1,108 +1,241 @@
 package main
 
 import (
+	"encoding/hex"
 	"fmt"
-	"log"
+	"os"
 
 	"github.com/boltdb/bolt"
 )
 
-var dbFile = "blockchain.db"
-
-const blocksBucket = "blocks"
+const (
+	dbFile              = "blockchain.db"
+	blocksBucket        = "blocks"
+	genesisCoinbaseData = "Arbitrary Genesis Data"
+)
 
 type Blockchain struct {
 	tip []byte
 	db  *bolt.DB
 }
 
-type BlockchainIterator struct {
-	currentHash []byte
-	db          *bolt.DB
+// Return whether the database file already exists
+func dbExists() bool {
+	if _, err := os.Stat(dbFile); os.IsNotExist(err) {
+		return false
+	}
+	return true
 }
 
+// NewBlockchain creates a new Blockchain object from the DB
 func NewBlockchain() *Blockchain {
+	if dbExists() == false {
+		fmt.Println("No existing blockchain found. Create one first.")
+		os.Exit(1)
+	}
+
 	var tip []byte
 	db, err := bolt.Open(dbFile, 0600, nil)
+	if err != nil {
+		exitWithError(err)
+	}
 
 	err = db.Update(func(tx *bolt.Tx) error {
 		b := tx.Bucket([]byte(blocksBucket))
+		tip = b.Get([]byte("1"))
 
-		if b == nil {
-			fmt.Println("No existing blockchain found. Creating a new one...")
-			genesis := NewGenesisBlock()
-
-			b, err := tx.CreateBucket([]byte(blocksBucket))
-			if err != nil {
-				log.Panic(err)
-			}
-
-			err = b.Put(genesis.Hash, genesis.Serialize())
-			if err != nil {
-				log.Panic(err)
-			}
-
-			err = b.Put([]byte("1"), genesis.Hash)
-			if err != nil {
-				log.Panic(err)
-			}
-			tip = genesis.Hash
-		} else {
-			tip = b.Get([]byte("1"))
-		}
 		return nil
 	})
 	if err != nil {
-		log.Panic(err)
+		exitWithError(err)
 	}
 
-	bc := Blockchain{tip, db}
+	bc := Blockchain{
+		tip: tip,
+		db:  db,
+	}
+	return &bc
+}
 
+// CreateBlockchain creates the blockchain DB and generate a new genesis block
+func CreateBlockchain(address string) *Blockchain {
+	if dbExists() {
+		fmt.Println("Blockchain already exists.")
+		os.Exit(1)
+	}
+
+	var tip []byte
+	db, err := bolt.Open(dbFile, 0600, nil)
+	if err != nil {
+		exitWithError(err)
+	}
+
+	err = db.Update(func(tx *bolt.Tx) error {
+		cbtx := NewCoinbaseTX(address, genesisCoinbaseData)
+		genesis := NewGenesisBlock(cbtx)
+
+		b, err := tx.CreateBucket([]byte(blocksBucket))
+		if err != nil {
+			exitWithError(err)
+		}
+
+		err = b.Put(genesis.Hash, genesis.Serialize())
+		if err != nil {
+			exitWithError(err)
+		}
+
+		err = b.Put([]byte("1"), genesis.Hash)
+		if err != nil {
+			exitWithError(err)
+		}
+		tip = genesis.Hash
+		return nil
+	})
+	if err != nil {
+		exitWithError(err)
+	}
+
+	bc := Blockchain{
+		tip: tip,
+		db:  db,
+	}
 	return &bc
 }
 
 // NewGenesisBlock creates a new Genesis Block to start the blockchain
-func NewGenesisBlock() *Block {
-	return NewBlock("Genesis Block", []byte{})
+func NewGenesisBlock(coinbase *Transaction) *Block {
+	return NewBlock([]*Transaction{coinbase}, []byte{})
 }
 
-// AddBlock adds a block to the blockchain
-func (bc *Blockchain) AddBlock(data string) {
+// FindUnspentTransactions returns a list of transactions containing unspent outputs
+func (bc *Blockchain) FindUnspentTransactions(address string) []Transaction {
+	var unspentTXs []Transaction
+	spentTXOs := make(map[string][]int)
+	bci := bc.Iterator()
+
+	for {
+		block := bci.Next()
+		for _, tx := range block.Transactions {
+			txID := hex.EncodeToString(tx.ID)
+
+		Outputs:
+			for outIdx, out := range tx.Vout {
+				// Was the output spent?
+				if spentTXOs[txID] != nil {
+					for _, spentOut := range spentTXOs[txID] {
+						if spentOut == outIdx {
+							continue Outputs
+						}
+					}
+				}
+
+				if out.CanBeUnlockedWith(address) {
+					unspentTXs = append(unspentTXs, *tx)
+				}
+			}
+
+			if tx.IsCoinbase() == false {
+				for _, in := range tx.Vin {
+					if in.CanUnlockOutputWith(address) {
+						inTxID := hex.EncodeToString(in.Txid)
+						spentTXOs[inTxID] = append(spentTXOs[inTxID], in.Vout)
+					}
+				}
+			}
+		}
+
+		if len(block.PrevBlockHash) == 0 {
+			break
+		}
+	}
+
+	return unspentTXs
+}
+
+// FindUTXO finds and returns all unspent transaction outputs
+func (bc *Blockchain) FindUTXO(address string) []TXOutput {
+	var UTXOs []TXOutput
+	unspentTransactions := bc.FindUnspentTransactions(address)
+	for _, tx := range unspentTransactions {
+		for _, out := range tx.Vout {
+			if out.CanBeUnlockedWith(address) {
+				UTXOs = append(UTXOs, out)
+			}
+		}
+	}
+	return UTXOs
+}
+
+// FindSpendableOutputs finds and returns unspent outputs to reference in inputs
+func (bc *Blockchain) FindSpendableOutputs(address string, amount int) (int, map[string][]int) {
+	unspentOutputs := make(map[string][]int)
+	unspentTXs := bc.FindUnspentTransactions(address)
+	accumulated := 0
+
+Work:
+	for _, tx := range unspentTXs {
+		txID := hex.EncodeToString(tx.ID)
+		for outIdx, out := range tx.Vout {
+			if out.CanBeUnlockedWith(address) && accumulated < amount {
+				accumulated += out.Value
+				unspentOutputs[txID] = append(unspentOutputs[txID], outIdx)
+				if accumulated >= amount {
+					break Work
+				}
+			}
+		}
+	}
+	return accumulated, unspentOutputs
+}
+
+// MineBlock mines a new block with the provided transactions
+func (bc *Blockchain) MineBlock(transactions []*Transaction) {
 	var lastHash []byte
 
 	err := bc.db.View(func(tx *bolt.Tx) error {
 		b := tx.Bucket([]byte(blocksBucket))
 		lastHash = b.Get([]byte("1"))
+
 		return nil
 	})
 	if err != nil {
-		log.Panic(err)
+		exitWithError(err)
 	}
 
-	newBlock := NewBlock(data, lastHash)
-
+	newBlock := NewBlock(transactions, lastHash)
 	err = bc.db.Update(func(tx *bolt.Tx) error {
-		var err error
 		b := tx.Bucket([]byte(blocksBucket))
-		if err = b.Put(newBlock.Hash, newBlock.Serialize()); err != nil {
-			log.Panic(err)
+		err := b.Put(newBlock.Hash, newBlock.Serialize())
+		if err != nil {
+			exitWithError(err)
 		}
-		if err = b.Put([]byte("1"), newBlock.Hash); err != nil {
-			log.Panic(err)
+
+		err = b.Put([]byte("1"), newBlock.Hash)
+		if err != nil {
+			exitWithError(err)
 		}
+
 		bc.tip = newBlock.Hash
 		return nil
 	})
 	if err != nil {
-		log.Panic(err)
+		exitWithError(err)
 	}
 }
 
+// Iterating the Blockchain
+type BlockchainIterator struct {
+	currentHash []byte
+	db          *bolt.DB
+}
+
+// Iterator returns a blockchain iterator
 func (bc *Blockchain) Iterator() *BlockchainIterator {
 	bci := &BlockchainIterator{bc.tip, bc.db}
 	return bci
 }
 
+// next returns the next block (starting from the tip)
 func (bci *BlockchainIterator) Next() *Block {
 	var block *Block
 
@@ -113,7 +246,7 @@ func (bci *BlockchainIterator) Next() *Block {
 		return nil
 	})
 	if err != nil {
-		log.Panic(err)
+		exitWithError(err)
 	}
 
 	bci.currentHash = block.PrevBlockHash

cli.go

@@ -7,66 +7,99 @@ import (
 	"strconv"
 )
 
-type CLI struct {
-	bc *Blockchain
-}
+const (
+	flagBalance = "balance"
+	flagCreate  = "create"
+	flagSend    = "send"
+	flagLedger  = "ledger"
+)
 
-func (cli *CLI) printUsage() {
-	fmt.Println("Usage:")
-	fmt.Println("\taddblock -data BLOCK_DATA\tadd a block to the blockchain")
-	fmt.Println("\tprintchain\t\t\tprint all the blocks of the blockchain")
-}
-
-func (cli *CLI) validateArgs() {
-	if len(os.Args) < 2 {
-		cli.printUsage()
-		os.Exit(1)
-	}
-}
+type CLI struct{}
 
+// Run parses command line arguments and processes commands
 func (cli *CLI) Run() {
 	cli.validateArgs()
 
 	// Valid CLI flags
-	addBlockCmd := flag.NewFlagSet("addblock", flag.ExitOnError)
-	printChainCmd := flag.NewFlagSet("printchain", flag.ExitOnError)
-	// CLI Flag Data
-	addBlockData := addBlockCmd.String("data", "", "Block data")
+	getBalanceCmd := flag.NewFlagSet(flagBalance, flag.ExitOnError)
+	createBlockchainCmd := flag.NewFlagSet(flagCreate, flag.ExitOnError)
+	sendCmd := flag.NewFlagSet(flagSend, flag.ExitOnError)
+	printChainCmd := flag.NewFlagSet(flagLedger, flag.ExitOnError)
+
+	getBalanceAddress := getBalanceCmd.String("address", "", "The address to get the balance of")
+	createBlockchainAddress := createBlockchainCmd.String("address", "", "The address to send the genesis block reward to")
+	sendFrom := sendCmd.String("from", "", "Source wallet address")
+	sendTo := sendCmd.String("to", "", "Destination wallet address")
+	sendAmount := sendCmd.Int("amount", 0, "Amount to send")
 
 	switch os.Args[1] {
-	case "addblock":
-		addBlockCmd.Parse(os.Args[2:])
-	case "printchain":
-		printChainCmd.Parse(os.Args[2:])
+	case flagBalance:
+		if err := getBalanceCmd.Parse(os.Args[2:]); err != nil {
+			exitWithError(err)
+		}
+	case flagCreate:
+		if err := createBlockchainCmd.Parse(os.Args[2:]); err != nil {
+			exitWithError(err)
+		}
+	case flagLedger:
+		if err := printChainCmd.Parse(os.Args[2:]); err != nil {
+			exitWithError(err)
+		}
+	case flagSend:
+		if err := sendCmd.Parse(os.Args[2:]); err != nil {
+			exitWithError(err)
+		}
 	default:
 		cli.printUsage()
 		os.Exit(1)
 	}
 
-	if addBlockCmd.Parsed() {
-		if *addBlockData == "" {
-			addBlockCmd.Usage()
+	if getBalanceCmd.Parsed() {
+		if *getBalanceAddress == "" {
+			getBalanceCmd.Usage()
 			os.Exit(1)
 		}
-		cli.addBlock(*addBlockData)
+		cli.getBalance(*getBalanceAddress)
+	}
+
+	if createBlockchainCmd.Parsed() {
+		if *createBlockchainAddress == "" {
+			createBlockchainCmd.Usage()
+			os.Exit(1)
+		}
+		cli.createBlockchain(*createBlockchainAddress)
 	}
 
 	if printChainCmd.Parsed() {
 		cli.printChain()
 	}
+
+	if sendCmd.Parsed() {
+		if *sendFrom == "" || *sendTo == "" || *sendAmount <= 0 {
+			sendCmd.Usage()
+			os.Exit(1)
+		}
+		cli.send(*sendFrom, *sendTo, *sendAmount)
+	}
 }
 
-func (cli *CLI) addBlock(data string) {
-	cli.bc.AddBlock(data)
-	fmt.Println("Success!")
+// createBlockchain creates a new blockchain with a genesis block
+func (cli *CLI) createBlockchain(address string) {
+	bc := CreateBlockchain(address)
+	bc.db.Close()
+	fmt.Println("Done!")
 }
 
+// printChain outputs the entire ledger
 func (cli *CLI) printChain() {
-	bci := cli.bc.Iterator()
+	bc := NewBlockchain()
+	defer bc.db.Close()
+
+	bci := bc.Iterator()
+
 	for {
 		block := bci.Next()
 		fmt.Printf("Prev. hash: %x\n", block.PrevBlockHash)
-		fmt.Printf("Data: %s\n", block.Data)
 		fmt.Printf("Hash: %x\n", block.Hash)
 		pow := NewProofOfWork(block)
 		fmt.Printf("PoW: %s\n", strconv.FormatBool(pow.Validate()))
@@ -77,3 +110,50 @@ func (cli *CLI) printChain() {
 		}
 	}
 }
+
+// getBalance retrieves the transaction balance for an address
+func (cli *CLI) getBalance(address string) {
+	bc := NewBlockchain()
+	defer bc.db.Close()
+
+	balance := 0
+	UTXOs := bc.FindUTXO(address)
+
+	for _, out := range UTXOs {
+		balance += out.Value
+	}
+
+	fmt.Printf("Balance of '%s': %d\n", address, balance)
+}
+
+// send Triggers a transaction exchange
+func (cli *CLI) send(from, to string, amount int) {
+	bc := NewBlockchain()
+	defer bc.db.Close()
+
+	tx := NewUTXOTransaction(from, to, amount, bc)
+	bc.MineBlock([]*Transaction{tx})
+	fmt.Println("Success!")
+}
+
+// printUsage prints the usage of the cli app
+func (cli *CLI) printUsage() {
+	fmt.Println("Usage:")
+	fmt.Println("\t" + flagBalance + " -address ADDRESS\tGet balance of ADDRESS")
+	fmt.Println("\t" + flagCreate + " -address ADDRESS\tCreate a blockchain and send genesis block reward to ADDRESS")
+	fmt.Println("\t" + flagLedger + "\t\t\tprint all the blocks of the blockchain")
+	fmt.Println("\t" + flagSend + " -from FROM -to TO -amount AMOUNT\tSend AMOUNT of coins from FROM address to TO address")
+}
+
+// validateArgs just makes sure that we've got a couple arguments
+func (cli *CLI) validateArgs() {
+	if len(os.Args) < 2 {
+		cli.printUsage()
+		os.Exit(1)
+	}
+}
+
+func exitWithError(err error) {
+	fmt.Println(err)
+	os.Exit(1)
+}

main.go

@@ -1,9 +1,6 @@
 package main
 
 func main() {
-	bc := NewBlockchain()
-	defer bc.db.Close()
-
-	cli := CLI{bc}
+	cli := CLI{}
 	cli.Run()
 }

proof.go

@@ -27,7 +27,7 @@ func (pow *ProofOfWork) prepareData(nonce int) []byte {
 	data := bytes.Join(
 		[][]byte{
 			pow.block.PrevBlockHash,
-			pow.block.Data,
+			pow.block.HashTransactions(),
 			IntToHex(pow.block.Timestamp),
 			IntToHex(int64(proofTargetBits)),
 			IntToHex(int64(nonce)),
@@ -42,7 +42,7 @@ func (pow *ProofOfWork) Run() (int, []byte) {
 	var hash [32]byte
 	nonce := 0
 
-	fmt.Printf("Mining the block containing \"%s\"\n", pow.block.Data)
+	fmt.Printf("Mining a new block")
 	for nonce < maxNonce {
 		data := pow.prepareData(nonce)
 		hash = sha256.Sum256(data)

transaction.go

@@ -0,0 +1,138 @@
+package main
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/gob"
+	"encoding/hex"
+	"fmt"
+	"os"
+)
+
+const subsidy = 10
+
+// Transaction represents a blockchain transaction
+type Transaction struct {
+	ID   []byte
+	Vin  []TXInput
+	Vout []TXOutput
+}
+
+// IsCoinbase checks whether the transaction is a coinbase
+func (tx Transaction) IsCoinbase() bool {
+	return len(tx.Vin) == 1 && len(tx.Vin[0].Txid) == 0 && tx.Vin[0].Vout == -1
+}
+
+// SetID sets the ID of a transaction
+func (tx Transaction) SetID() {
+	var encoded bytes.Buffer
+	var hash [32]byte
+
+	enc := gob.NewEncoder(&encoded)
+	err := enc.Encode(tx)
+	if err != nil {
+		exitWithError(err)
+	}
+	hash = sha256.Sum256(encoded.Bytes())
+	tx.ID = hash[:]
+}
+
+// NewCoinbaseTX creates a new coinbase transaction
+func NewCoinbaseTX(to, data string) *Transaction {
+	if data == "" {
+		data = fmt.Sprintf("Reward to '%s'", to)
+	}
+
+	txin := TXInput{
+		Txid:      []byte{},
+		Vout:      -1,
+		ScriptSig: data,
+	}
+	txout := TXOutput{
+		Value:        subsidy,
+		ScriptPubKey: to,
+	}
+
+	tx := Transaction{nil, []TXInput{txin}, []TXOutput{txout}}
+	tx.SetID()
+
+	return &tx
+}
+
+// NewUTXOTransaction creates a new transaction
+func NewUTXOTransaction(from, to string, amount int, bc *Blockchain) *Transaction {
+	var inputs []TXInput
+	var outputs []TXOutput
+
+	acc, validOutputs := bc.FindSpendableOutputs(from, amount)
+
+	if acc < amount {
+		fmt.Println("ERROR: Not enough funds")
+		os.Exit(1)
+	}
+
+	// Build a list of inputs
+	for txid, outs := range validOutputs {
+		txID, err := hex.DecodeString(txid)
+		if err != nil {
+			fmt.Println(err)
+			os.Exit(1)
+		}
+
+		for _, out := range outs {
+			input := TXInput{
+				Txid:      txID,
+				Vout:      out,
+				ScriptSig: from,
+			}
+			inputs = append(inputs, input)
+		}
+	}
+
+	// Build a list of outputs
+	output := TXOutput{
+		Value:        amount,
+		ScriptPubKey: to,
+	}
+	outputs = append(outputs, output)
+	if acc > amount {
+		// There is some change
+		chgOutput := TXOutput{
+			Value:        acc - amount,
+			ScriptPubKey: from,
+		}
+		outputs = append(outputs, chgOutput)
+	}
+
+	tx := Transaction{
+		ID:   nil,
+		Vin:  inputs,
+		Vout: outputs,
+	}
+	tx.SetID()
+
+	return &tx
+}
+
+// TXInput represents a transaction input
+type TXInput struct {
+	Txid      []byte
+	Vout      int
+	ScriptSig string
+}
+
+// CanUnlockOutputWith checks whether the unlockingData (address) initiated the transaction
+func (in *TXInput) CanUnlockOutputWith(unlockingData string) bool {
+	return in.ScriptSig == unlockingData
+}
+
+// TXOutput represents a transaction output
+type TXOutput struct {
+	Value        int
+	ScriptPubKey string
+}
+
+// CanBeUnlockedWith checks if the output can be unlocked with the provided data
+func (out *TXOutput) CanBeUnlockedWith(unlockingData string) bool {
+	return out.ScriptPubKey == unlockingData
+}

utils.go

@@ -3,7 +3,6 @@ package main
 import (
 	"bytes"
 	"encoding/binary"
-	"log"
 )
 
 // IntToHex converts an int64 to a byte array
@@ -11,7 +10,7 @@ func IntToHex(num int64) []byte {
 	buf := new(bytes.Buffer)
 	err := binary.Write(buf, binary.BigEndian, num)
 	if err != nil {
-		log.Panic(err)
+		exitWithError(err)
 	}
 	return buf.Bytes()
 }