Implement rpc server method: sendrawtransaction (#174)

* Added new config attributes: 'SecondsPerBlock','LowPriorityThreshold'

* Added new files:

* Added new method: CompareTo

* Fixed empty Slice case

* Added new methods: LessThan, GreaterThan, Equal, CompareTo

* Added new method: InputIntersection

* Added MaxTransactionSize, GroupOutputByAssetID

* Added ned method: ScriptHash

* Added new method: IsDoubleSpend

* Refactor blockchainer, Added Feer interface, Verify and GetMemPool method

* 1) Added MemPool
2) Added new methods to satisfy the blockchainer interface: IsLowPriority, Verify, GetMemPool

* Added new methods: RelayTxn, RelayDirectly

* Fixed tests

* Implemented RPC server method sendrawtransaction

* Refactor getrawtransaction, sendrawtransaction in separate methods

* Moved 'secondsPerBlock' to config file

* Implemented Kim suggestions:
1) Fixed data race issues
2) refactor Verify method
3) Get rid of unused InputIntersection  method due to refactoring Verify method
4) Fixed bug in https://github.com/CityOfZion/neo-go/pull/174#discussion_r264108135
5) minor simplications of the code

* Fixed minor issues related to

1) space
2) getter methods do not need pointer on the receiver
3) error message
4) refactoring  CompareTo method in uint256.go

* Fixed small issues

* Use sync.RWMutex instead of sync.Mutex

* Refined (R)Lock/(R)Unlock

* return error instead of bool in Verify methods

config/config.go

@@ -43,6 +43,8 @@ type (
 	ProtocolConfiguration struct {
 		Magic                   NetMode   `yaml:"Magic"`
 		AddressVersion          int64     `yaml:"AddressVersion"`
+		SecondsPerBlock         int       `yaml:"SecondsPerBlock"`
+		LowPriorityThreshold    float64   `yaml:"LowPriorityThreshold"`
 		MaxTransactionsPerBlock int64     `yaml:"MaxTransactionsPerBlock"`
 		StandbyValidators       []string  `yaml:"StandbyValidators"`
 		SeedList                []string  `yaml:"SeedList"`

config/protocol.mainnet.yml

@@ -1,6 +1,8 @@
 ProtocolConfiguration:
   Magic: 7630401
   AddressVersion: 23
+  SecondsPerBlock: 15
+  LowPriorityThreshold: 0.001
   StandbyValidators:
   - 03b209fd4f53a7170ea4444e0cb0a6bb6a53c2bd016926989cf85f9b0fba17a70c
   - 02df48f60e8f3e01c48ff40b9b7f1310d7a8b2a193188befe1c2e3df740e895093

config/protocol.privnet.yml

@@ -1,15 +1,17 @@
 ProtocolConfiguration:
   Magic: 56753
   AddressVersion: 23
+  SecondsPerBlock: 15
+  LowPriorityThreshold: 0.000
   StandbyValidators:
-  - 02b3622bf4017bdfe317c58aed5f4c753f206b7db896046fa7d774bbc4bf7f8dc2
-  - 02103a7f7dd016558597f7960d27c516a4394fd968b9e65155eb4b013e4040406e
-  - 03d90c07df63e690ce77912e10ab51acc944b66860237b608c4f8f8309e71ee699
-  - 02a7bc55fe8684e0119768d104ba30795bdcc86619e864add26156723ed185cd62
+    - 02b3622bf4017bdfe317c58aed5f4c753f206b7db896046fa7d774bbc4bf7f8dc2
+    - 02103a7f7dd016558597f7960d27c516a4394fd968b9e65155eb4b013e4040406e
+    - 03d90c07df63e690ce77912e10ab51acc944b66860237b608c4f8f8309e71ee699
+    - 02a7bc55fe8684e0119768d104ba30795bdcc86619e864add26156723ed185cd62
   SeedList:
-  - 127.0.0.1:20334
-  - 127.0.0.1:20335
-  - 127.0.0.1:20336
+    - 127.0.0.1:20334
+    - 127.0.0.1:20335
+    - 127.0.0.1:20336
   SystemFee:
     EnrollmentTransaction: 1000
     IssueTransaction: 500

config/protocol.testnet.yml

@@ -1,6 +1,8 @@
 ProtocolConfiguration:
   Magic: 1953787457
   AddressVersion: 23
+  SecondsPerBlock: 15
+  LowPriorityThreshold: 0.000
   StandbyValidators:
   - 0327da12b5c40200e9f65569476bbff2218da4f32548ff43b6387ec1416a231ee8
   - 026ce35b29147ad09e4afe4ec4a7319095f08198fa8babbe3c56e970b143528d22

config/protocol.unit_testnet.yml

@@ -1,15 +1,17 @@
 ProtocolConfiguration:
   Magic: 56753
   AddressVersion: 23
+  SecondsPerBlock: 15
+  LowPriorityThreshold: 0.000
   StandbyValidators:
-  - 02b3622bf4017bdfe317c58aed5f4c753f206b7db896046fa7d774bbc4bf7f8dc2
-  - 02103a7f7dd016558597f7960d27c516a4394fd968b9e65155eb4b013e4040406e
-  - 03d90c07df63e690ce77912e10ab51acc944b66860237b608c4f8f8309e71ee699
-  - 02a7bc55fe8684e0119768d104ba30795bdcc86619e864add26156723ed185cd62
+    - 02b3622bf4017bdfe317c58aed5f4c753f206b7db896046fa7d774bbc4bf7f8dc2
+    - 02103a7f7dd016558597f7960d27c516a4394fd968b9e65155eb4b013e4040406e
+    - 03d90c07df63e690ce77912e10ab51acc944b66860237b608c4f8f8309e71ee699
+    - 02a7bc55fe8684e0119768d104ba30795bdcc86619e864add26156723ed185cd62
   SeedList:
-  - 127.0.0.1:20334
-  - 127.0.0.1:20335
-  - 127.0.0.1:20336
+    - 127.0.0.1:20334
+    - 127.0.0.1:20335
+    - 127.0.0.1:20336
   SystemFee:
     EnrollmentTransaction: 1000
     IssueTransaction: 500

pkg/core/blockchain.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"encoding/binary"
 	"fmt"
+	"math"
 	"sync/atomic"
 	"time"
 
@@ -12,12 +13,12 @@ import (
 	"github.com/CityOfZion/neo-go/pkg/core/storage"
 	"github.com/CityOfZion/neo-go/pkg/core/transaction"
 	"github.com/CityOfZion/neo-go/pkg/util"
+	"github.com/pkg/errors"
 	log "github.com/sirupsen/logrus"
 )
 
 // tuning parameters
 const (
-	secondsPerBlock  = 15
 	headerBatchCount = 2000
 	version          = "0.0.1"
 )
@@ -55,6 +56,8 @@ type Blockchain struct {
 
 	// Whether we will verify received blocks.
 	verifyBlocks bool
+
+	memPool MemPool
 }
 
 type headersOpFunc func(headerList *HeaderHashList)
@@ -69,10 +72,11 @@ func NewBlockchain(ctx context.Context, s storage.Store, cfg config.ProtocolConf
 		headersOpDone: make(chan struct{}),
 		blockCache:    NewCache(),
 		verifyBlocks:  false,
+		memPool:       NewMemPool(50000),
 	}
 
-  go bc.run(ctx)
-  if err := bc.init(); err != nil {
+	go bc.run(ctx)
+	if err := bc.init(); err != nil {
 		return nil, err
 	}
 
@@ -457,6 +461,10 @@ func (bc *Blockchain) headerListLen() (n int) {
 
 // GetTransaction returns a TX and its height by the given hash.
 func (bc *Blockchain) GetTransaction(hash util.Uint256) (*transaction.Transaction, uint32, error) {
+	if tx, ok := bc.memPool.TryGetValue(hash); ok {
+		return tx, 0, nil // the height is not actually defined for memPool transaction. Not sure if zero is a good number in this case.
+	}
+
 	key := storage.AppendPrefix(storage.DataTransaction, hash.BytesReverse())
 	b, err := bc.Get(key)
 	if err != nil {
@@ -506,9 +514,17 @@ func (bc *Blockchain) GetHeader(hash util.Uint256) (*Header, error) {
 	return block.Header(), nil
 }
 
-// HasBlock return true if the blockchain contains he given
+// HasTransaction return true if the blockchain contains he given
 // transaction hash.
 func (bc *Blockchain) HasTransaction(hash util.Uint256) bool {
+	if bc.memPool.ContainsKey(hash) {
+		return true
+	}
+
+	key := storage.AppendPrefix(storage.DataTransaction, hash.BytesReverse())
+	if _, err := bc.Get(key); err == nil {
+		return true
+	}
 	return false
 }
 
@@ -596,7 +612,7 @@ func (bc *Blockchain) GetConfig() config.ProtocolConfiguration {
 // transaction package because of a import cycle problem. Perhaps we should think to re-design
 // the code base to avoid this situation.
 func (bc *Blockchain) References(t *transaction.Transaction) map[util.Uint256]*transaction.Output {
-	references := make(map[util.Uint256]*transaction.Output)
+	references := make(map[util.Uint256]*transaction.Output, 0)
 
 	for prevHash, inputs := range t.GroupInputsByPrevHash() {
 		if tx, _, err := bc.GetTransaction(prevHash); err != nil {
@@ -641,8 +657,279 @@ func (bc *Blockchain) SystemFee(t *transaction.Transaction) util.Fixed8 {
 	return bc.GetConfig().SystemFee.TryGetValue(t.Type)
 }
 
+// IsLowPriority flags a trnsaction as low priority if the network fee is less than
+// LowPriorityThreshold
+func (bc *Blockchain) IsLowPriority(t *transaction.Transaction) bool {
+	return bc.NetworkFee(t) < util.NewFixed8FromFloat(bc.GetConfig().LowPriorityThreshold)
+}
+
+// GetMemPool returns the memory pool of the blockchain.
+func (bc *Blockchain) GetMemPool() MemPool {
+	return bc.memPool
+}
+
+// Verify verifies whether a transaction is bonafide or not.
+// Golang implementation of Verify method in C# (https://github.com/neo-project/neo/blob/master/neo/Network/P2P/Payloads/Transaction.cs#L270).
+func (bc *Blockchain) Verify(t *transaction.Transaction) error {
+	if t.Size() > transaction.MaxTransactionSize {
+		return errors.Errorf("invalid transaction size = %d. It shoud be less then MaxTransactionSize = %d", t.Size(), transaction.MaxTransactionSize)
+	}
+	if ok := bc.verifyInputs(t); !ok {
+		return errors.New("invalid transaction's inputs")
+	}
+	if ok := bc.memPool.Verify(t); !ok {
+		return errors.New("invalid transaction due to conflicts with the memory pool")
+	}
+	if IsDoubleSpend(bc.Store, t) {
+		return errors.New("invalid transaction caused by double spending")
+	}
+	if ok := bc.verifyOutputs(t); !ok {
+		return errors.New("invalid transaction's outputs")
+	}
+	if ok := bc.verifyResults(t); !ok {
+		return errors.New("invalid transaction's results")
+	}
+
+	for _, a := range t.Attributes {
+		if a.Usage == transaction.ECDH02 || a.Usage == transaction.ECDH03 {
+			return errors.Errorf("invalid attribute's usage = %s ", a.Usage)
+		}
+	}
+
+	return bc.VerifyWitnesses(t)
+}
+
+func (bc *Blockchain) verifyInputs(t *transaction.Transaction) bool {
+	for i := 1; i < len(t.Inputs); i++ {
+		for j := 0; j < i; j++ {
+			if t.Inputs[i].PrevHash == t.Inputs[j].PrevHash && t.Inputs[i].PrevIndex == t.Inputs[j].PrevIndex {
+				return false
+			}
+		}
+	}
+
+	return true
+}
+
+func (bc *Blockchain) verifyOutputs(t *transaction.Transaction) bool {
+	for assetID, outputs := range t.GroupOutputByAssetID() {
+		assetState := bc.GetAssetState(assetID)
+		if assetState == nil {
+			return false
+		}
+
+		if assetState.Expiration < bc.blockHeight+1 && assetState.AssetType != transaction.GoverningToken && assetState.AssetType != transaction.UtilityToken {
+			return false
+		}
+
+		for _, out := range outputs {
+			if int64(out.Amount)%int64(math.Pow10(8-int(assetState.Precision))) != 0 {
+				return false
+			}
+		}
+	}
+
+	return true
+}
+
+func (bc *Blockchain) verifyResults(t *transaction.Transaction) bool {
+	results := bc.GetTransationResults(t)
+	if results == nil {
+		return false
+	}
+	var resultsDestroy []*transaction.Result
+	var resultsIssue []*transaction.Result
+	for _, re := range results {
+		if re.Amount.GreaterThan(util.Fixed8(0)) {
+			resultsDestroy = append(resultsDestroy, re)
+		}
+
+		if re.Amount.LessThan(util.Fixed8(0)) {
+			resultsIssue = append(resultsIssue, re)
+		}
+	}
+	if len(resultsDestroy) > 1 {
+		return false
+	}
+	if len(resultsDestroy) == 1 && resultsDestroy[0].AssetID != utilityTokenTX().Hash() {
+		return false
+	}
+	if bc.SystemFee(t).GreaterThan(util.Fixed8(0)) && (len(resultsDestroy) == 0 || resultsDestroy[0].Amount.LessThan(bc.SystemFee(t))) {
+		return false
+	}
+
+	switch t.Type {
+	case transaction.MinerType, transaction.ClaimType:
+		for _, r := range resultsIssue {
+			if r.AssetID != utilityTokenTX().Hash() {
+				return false
+			}
+		}
+		break
+	case transaction.IssueType:
+		for _, r := range resultsIssue {
+			if r.AssetID == utilityTokenTX().Hash() {
+				return false
+			}
+		}
+		break
+	default:
+		if len(resultsIssue) > 0 {
+			return false
+		}
+		break
+	}
+
+	return true
+}
+
+// GetTransationResults returns the transaction results aggregate by assetID.
+// Golang of GetTransationResults method in C# (https://github.com/neo-project/neo/blob/master/neo/Network/P2P/Payloads/Transaction.cs#L207)
+func (bc *Blockchain) GetTransationResults(t *transaction.Transaction) []*transaction.Result {
+	var tempResults []*transaction.Result
+	var results []*transaction.Result
+	tempGroupResult := make(map[util.Uint256]util.Fixed8)
+
+	if references := bc.References(t); references == nil {
+		return nil
+	} else {
+		for _, output := range references {
+			tempResults = append(tempResults, &transaction.Result{
+				AssetID: output.AssetID,
+				Amount:  output.Amount,
+			})
+		}
+		for _, output := range t.Outputs {
+			tempResults = append(tempResults, &transaction.Result{
+				AssetID: output.AssetID,
+				Amount:  -output.Amount,
+			})
+		}
+		for _, r := range tempResults {
+			if amount, ok := tempGroupResult[r.AssetID]; ok {
+				tempGroupResult[r.AssetID] = amount.Add(r.Amount)
+			} else {
+				tempGroupResult[r.AssetID] = r.Amount
+			}
+		}
+
+		results = []*transaction.Result{} // this assignment is necessary. (Most of the time amount == 0 and results is the empty slice.)
+		for assetID, amount := range tempGroupResult {
+			if amount != util.Fixed8(0) {
+				results = append(results, &transaction.Result{
+					AssetID: assetID,
+					Amount:  amount,
+				})
+			}
+		}
+
+		return results
+
+	}
+
+}
+
+// GetScriptHashesForVerifying returns all the ScriptHashes of a transaction which will be use
+// to verify whether the transaction is bonafide or not.
+// Golang implementation of GetScriptHashesForVerifying method in C# (https://github.com/neo-project/neo/blob/master/neo/Network/P2P/Payloads/Transaction.cs#L190)
+func (bc *Blockchain) GetScriptHashesForVerifying(t *transaction.Transaction) ([]util.Uint160, error) {
+	references := bc.References(t)
+	if references == nil {
+		return nil, errors.New("Invalid operation")
+	}
+	hashes := make(map[util.Uint160]bool)
+	for _, i := range t.Inputs {
+		h := references[i.PrevHash].ScriptHash
+		if _, ok := hashes[h]; !ok {
+			hashes[h] = true
+		}
+	}
+	for _, a := range t.Attributes {
+		if a.Usage == transaction.Script {
+			h, err := util.Uint160DecodeBytes(a.Data)
+			if err != nil {
+				return nil, err
+			}
+			if _, ok := hashes[h]; !ok {
+				hashes[h] = true
+			}
+		}
+	}
+
+	for a, outputs := range t.GroupOutputByAssetID() {
+		as := bc.GetAssetState(a)
+		if as == nil {
+			return nil, errors.New("Invalid operation")
+		}
+		if as.AssetType == transaction.DutyFlag {
+			for _, o := range outputs {
+				h := o.ScriptHash
+				if _, ok := hashes[h]; !ok {
+					hashes[h] = true
+				}
+			}
+		}
+	}
+	// convert hashes to []util.Uint160
+	hashesResult := make([]util.Uint160, 0, len(hashes))
+	for h := range hashes {
+		hashesResult = append(hashesResult, h)
+	}
+
+	return hashesResult, nil
+
+}
+
+// VerifyWitnesses verify the scripts (witnesses) that come with a transactions.
+// Golang implementation of VerifyWitnesses method in C# (https://github.com/neo-project/neo/blob/master/neo/SmartContract/Helper.cs#L87).
+// Unfortunately the IVerifiable interface could not be implemented because we can't move the References method in blockchain.go to the transaction.go file
+func (bc *Blockchain) VerifyWitnesses(t *transaction.Transaction) error {
+	hashes, err := bc.GetScriptHashesForVerifying(t)
+	if err != nil {
+		return err
+	}
+
+	witnesses := t.Scripts
+	if len(hashes) != len(witnesses) {
+		return errors.Errorf("expected len(hashes) == len(witnesses). got: %d != %d", len(hashes), len(witnesses))
+	}
+	for i := 0; i < len(hashes); i++ {
+		verification := witnesses[i].VerificationScript
+
+		if len(verification) == 0 {
+			/*TODO: replicate following C# code:
+			using (ScriptBuilder sb = new ScriptBuilder())
+			{
+				sb.EmitAppCall(hashes[i].ToArray());
+				verification = sb.ToArray();
+			}
+			*/
+
+		} else {
+			if h, err := witnesses[i].ScriptHash(); err != nil || hashes[i] != h {
+				return err
+			}
+		}
+
+		/*TODO: replicate following C# code:
+		using (ApplicationEngine engine = new ApplicationEngine(TriggerType.Verification, verifiable, snapshot, Fixed8.Zero))
+		{
+			engine.LoadScript(verification);
+			engine.LoadScript(verifiable.Witnesses[i].InvocationScript);
+			if (!engine.Execute()) return false;
+			if (engine.ResultStack.Count != 1 || !engine.ResultStack.Pop().GetBoolean()) return false;
+		}*/
+	}
+
+	return nil
+}
+
 func hashAndIndexToBytes(h util.Uint256, index uint32) []byte {
 	buf := make([]byte, 4)
 	binary.LittleEndian.PutUint32(buf, index)
 	return append(h.BytesReverse(), buf...)
 }
+
+func (bc *Blockchain) secondsPerBlock() int {
+	return bc.config.SecondsPerBlock
+}

pkg/core/blockchainer.go

@@ -25,7 +25,7 @@ type Blockchainer interface {
 	GetAccountState(util.Uint160) *AccountState
 	GetTransaction(util.Uint256) (*transaction.Transaction, uint32, error)
 	References(t *transaction.Transaction) map[util.Uint256]*transaction.Output
-	FeePerByte(t *transaction.Transaction) util.Fixed8
-	SystemFee(t *transaction.Transaction) util.Fixed8
-	NetworkFee(t *transaction.Transaction) util.Fixed8
+	Feer // fee interface
+	Verify(t *transaction.Transaction) error
+	GetMemPool() MemPool
 }

pkg/core/feer.go

@@ -0,0 +1,14 @@
+package core
+
+import (
+	"github.com/CityOfZion/neo-go/pkg/core/transaction"
+	"github.com/CityOfZion/neo-go/pkg/util"
+)
+
+// Feer is an interface that abstract the implementation of the fee calculation.
+type Feer interface {
+	NetworkFee(t *transaction.Transaction) util.Fixed8
+	IsLowPriority(t *transaction.Transaction) bool
+	FeePerByte(t *transaction.Transaction) util.Fixed8
+	SystemFee(t *transaction.Transaction) util.Fixed8
+}

pkg/core/mem_pool.go

@@ -0,0 +1,267 @@
+package core
+
+import (
+	"sort"
+	"sync"
+	"time"
+
+	"github.com/CityOfZion/neo-go/pkg/core/transaction"
+	"github.com/CityOfZion/neo-go/pkg/util"
+)
+
+// PoolItem represents a transaction in the the Memory pool.
+type PoolItem struct {
+	txn       *transaction.Transaction
+	timeStamp time.Time
+	fee       Feer
+}
+
+// PoolItems slice of PoolItem
+type PoolItems []*PoolItem
+
+// MemPool stores the unconfirms transactions.
+type MemPool struct {
+	lock                        *sync.RWMutex
+	unsortedTxn                 map[util.Uint256]*PoolItem
+	unverifiedTxn               map[util.Uint256]*PoolItem
+	sortedHighPrioTxn           PoolItems
+	sortedLowPrioTxn            PoolItems
+	unverifiedSortedHighPrioTxn PoolItems
+	unverifiedSortedLowPrioTxn  PoolItems
+
+	capacity int
+}
+
+func (p PoolItems) Len() int           { return len(p) }
+func (p PoolItems) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }
+func (p PoolItems) Less(i, j int) bool { return p[i].CompareTo(p[j]) < 0 }
+
+// CompareTo returns the difference between two PoolItems.
+// difference < 0 implies p < otherP.
+// difference = 0 implies p = otherP.
+// difference > 0 implies p > otherP.
+func (p PoolItem) CompareTo(otherP *PoolItem) int {
+	if otherP == nil {
+		return 1
+	}
+
+	if p.fee.IsLowPriority(p.txn) && p.fee.IsLowPriority(otherP.txn) {
+		thisIsClaimTx := p.txn.Type == transaction.ClaimType
+		otherIsClaimTx := otherP.txn.Type == transaction.ClaimType
+
+		if thisIsClaimTx != otherIsClaimTx {
+			// This is a claim Tx and other isn't.
+			if thisIsClaimTx {
+				return 1
+			}
+			// The other is claim Tx and this isn't.
+			return -1
+		}
+	}
+
+	// Fees sorted ascending
+	pFPB := p.fee.FeePerByte(p.txn)
+	otherFPB := p.fee.FeePerByte(otherP.txn)
+	if ret := pFPB.CompareTo(otherFPB); ret != 0 {
+		return ret
+	}
+
+	pNF := p.fee.NetworkFee(p.txn)
+	otherNF := p.fee.NetworkFee(otherP.txn)
+	if ret := pNF.CompareTo(otherNF); ret != 0 {
+		return ret
+	}
+
+	// Transaction hash sorted descending
+	return otherP.txn.Hash().CompareTo(p.txn.Hash())
+}
+
+// Count returns the total number of uncofirm transactions.
+func (mp MemPool) Count() int {
+	mp.lock.RLock()
+	defer mp.lock.RUnlock()
+
+	return len(mp.unsortedTxn) + len(mp.unverifiedTxn)
+}
+
+// ContainsKey checks if a transactions hash is in the MemPool.
+func (mp MemPool) ContainsKey(hash util.Uint256) bool {
+	mp.lock.RLock()
+	defer mp.lock.RUnlock()
+
+	if _, ok := mp.unsortedTxn[hash]; ok {
+		return true
+	}
+
+	if _, ok := mp.unverifiedTxn[hash]; ok {
+		return true
+	}
+
+	return false
+}
+
+// TryAdd try to add the PoolItem to the MemPool.
+func (mp MemPool) TryAdd(hash util.Uint256, pItem *PoolItem) bool {
+	var pool PoolItems
+
+	mp.lock.RLock()
+	if _, ok := mp.unsortedTxn[hash]; ok {
+		return false
+	}
+	mp.unsortedTxn[hash] = pItem
+	mp.lock.RUnlock()
+
+	if pItem.fee.IsLowPriority(pItem.txn) {
+		pool = mp.sortedLowPrioTxn
+	} else {
+		pool = mp.sortedHighPrioTxn
+	}
+
+	mp.lock.Lock()
+	pool = append(pool, pItem)
+	sort.Sort(pool)
+	mp.lock.Unlock()
+
+	if mp.Count() > mp.capacity {
+		(&mp).RemoveOverCapacity()
+	}
+	mp.lock.RLock()
+	_, ok := mp.unsortedTxn[hash]
+	mp.lock.RUnlock()
+	return ok
+}
+
+// RemoveOverCapacity removes transactions with lowest fees until the total number of transactions
+// in the MemPool is within the capacity of the MemPool.
+func (mp *MemPool) RemoveOverCapacity() {
+	for mp.Count()-mp.capacity > 0 {
+		mp.lock.Lock()
+		if minItem, argPosition := getLowestFeeTransaction(mp.sortedLowPrioTxn, mp.unverifiedSortedLowPrioTxn); minItem != nil {
+			if argPosition == 1 {
+				// minItem belongs to the mp.sortedLowPrioTxn slice.
+				// The corresponding unsorted pool is is mp.unsortedTxn.
+				delete(mp.unsortedTxn, minItem.txn.Hash())
+				mp.sortedLowPrioTxn = append(mp.sortedLowPrioTxn[:0], mp.sortedLowPrioTxn[1:]...)
+			} else {
+				// minItem belongs to the mp.unverifiedSortedLowPrioTxn slice.
+				// The corresponding unsorted pool is is mp.unverifiedTxn.
+				delete(mp.unverifiedTxn, minItem.txn.Hash())
+				mp.unverifiedSortedLowPrioTxn = append(mp.unverifiedSortedLowPrioTxn[:0], mp.unverifiedSortedLowPrioTxn[1:]...)
+
+			}
+		} else if minItem, argPosition := getLowestFeeTransaction(mp.sortedHighPrioTxn, mp.unverifiedSortedHighPrioTxn); minItem != nil {
+			if argPosition == 1 {
+				// minItem belongs to the mp.sortedHighPrioTxn slice.
+				// The corresponding unsorted pool is is mp.unsortedTxn.
+				delete(mp.unsortedTxn, minItem.txn.Hash())
+				mp.sortedHighPrioTxn = append(mp.sortedHighPrioTxn[:0], mp.sortedHighPrioTxn[1:]...)
+			} else {
+				// minItem belongs to the mp.unverifiedSortedHighPrioTxn slice.
+				// The corresponding unsorted pool is is mp.unverifiedTxn.
+				delete(mp.unverifiedTxn, minItem.txn.Hash())
+				mp.unverifiedSortedHighPrioTxn = append(mp.unverifiedSortedHighPrioTxn[:0], mp.unverifiedSortedHighPrioTxn[1:]...)
+
+			}
+		}
+		mp.lock.Unlock()
+	}
+
+}
+
+// NewPoolItem returns a new PoolItem.
+func NewPoolItem(t *transaction.Transaction, fee Feer) *PoolItem {
+	return &PoolItem{
+		txn:       t,
+		timeStamp: time.Now().UTC(),
+		fee:       fee,
+	}
+}
+
+// NewMemPool returns a new MemPool struct.
+func NewMemPool(capacity int) MemPool {
+	return MemPool{
+		lock:          new(sync.RWMutex),
+		unsortedTxn:   make(map[util.Uint256]*PoolItem),
+		unverifiedTxn: make(map[util.Uint256]*PoolItem),
+		capacity:      capacity,
+	}
+}
+
+// TryGetValue returns a transactions if it esists in the memory pool.
+func (mp MemPool) TryGetValue(hash util.Uint256) (*transaction.Transaction, bool) {
+	mp.lock.Lock()
+	defer mp.lock.Unlock()
+	if pItem, ok := mp.unsortedTxn[hash]; ok {
+		return pItem.txn, ok
+	}
+
+	if pItem, ok := mp.unverifiedTxn[hash]; ok {
+		return pItem.txn, ok
+	}
+
+	return nil, false
+}
+
+// getLowestFeeTransaction returns the PoolItem with the lowest fee amongst the "verifiedTxnSorted"
+// and "unverifiedTxnSorted" PoolItems along with a integer. The integer can assume two values, 1 and 2 which indicate
+// that the PoolItem with the lowest fee was found in "verifiedTxnSorted" respectively in "unverifiedTxnSorted".
+// "verifiedTxnSorted" and "unverifiedTxnSorted" are sorted slice order by transaction fee ascending. This means that
+// the transaction with lowest fee start at index 0.
+// Reference: GetLowestFeeTransaction method in C# (https://github.com/neo-project/neo/blob/master/neo/Ledger/MemoryPool.cs)
+func getLowestFeeTransaction(verifiedTxnSorted PoolItems, unverifiedTxnSorted PoolItems) (*PoolItem, int) {
+	minItem := min(unverifiedTxnSorted)
+	verifiedMin := min(verifiedTxnSorted)
+	if verifiedMin == nil || (minItem != nil && verifiedMin.CompareTo(minItem) >= 0) {
+		return minItem, 2
+	}
+
+	minItem = verifiedMin
+	return minItem, 1
+
+}
+
+// min return the minimum item in a ascending sorted slice of pool items.
+// The  function can't be applied to unsorted slice!
+func min(sortedPool PoolItems) *PoolItem {
+	if len(sortedPool) == 0 {
+		return nil
+	}
+	return sortedPool[0]
+}
+
+// GetVerifiedTransactions returns a slice of Input from all the transactions in the memory pool
+// whose hash is not included in excludedHashes.
+func (mp *MemPool) GetVerifiedTransactions() []*transaction.Transaction {
+	var t []*transaction.Transaction
+
+	mp.lock.Lock()
+	defer mp.lock.Unlock()
+	for _, p := range mp.unsortedTxn {
+		t = append(t, p.txn)
+	}
+
+	return t
+}
+
+// Verify verifies if the inputs of a transaction tx are already used in any other transaction in the memory pool.
+// If yes, the transaction tx is not a valid transaction and the function return false.
+// If no, the transaction tx is a valid transaction and the function return true.
+func (mp MemPool) Verify(tx *transaction.Transaction) bool {
+	count := 0
+	inputs := make([]*transaction.Input, 0)
+	for _, item := range mp.GetVerifiedTransactions() {
+		if tx.Hash().Equals(item.Hash()) {
+			inputs = append(inputs, item.Inputs...)
+		}
+	}
+
+	for i := 0; i < len(inputs); i++ {
+		for j := 0; j < len(tx.Inputs); j++ {
+			if inputs[i].PrevHash.Equals(tx.Inputs[j].PrevHash) {
+				count++
+			}
+		}
+	}
+
+	return count == 0
+}

pkg/core/transaction/input.go

@@ -36,6 +36,6 @@ func (in *Input) EncodeBinary(w io.Writer) error {
 }
 
 // Size returns the size in bytes of the Input
-func (in *Input) Size() int {
+func (in Input) Size() int {
 	return in.PrevHash.Size() + 2 // 2 = sizeOf uint16
 }

pkg/core/transaction/result.go

@@ -0,0 +1,9 @@
+package transaction
+
+import "github.com/CityOfZion/neo-go/pkg/util"
+
+// Result represents the Result of a transaction.
+type Result struct {
+	AssetID util.Uint256
+	Amount  util.Fixed8
+}

pkg/core/transaction/transaction.go

@@ -10,6 +10,12 @@ import (
 	log "github.com/sirupsen/logrus"
 )
 
+const (
+	// MaxTransactionSize is the upper limit size in bytes that a transaction can reach. It is
+	// set to be 102400.
+	MaxTransactionSize = 102400
+)
+
 // Transaction is a process recorded in the NEO blockchain.
 type Transaction struct {
 	// The type of the transaction.
@@ -251,6 +257,15 @@ func (t *Transaction) GroupInputsByPrevHash() map[util.Uint256][]*Input {
 	return m
 }
 
+// GroupOutputByAssetID groups all TX outputs by their assetID.
+func (t Transaction) GroupOutputByAssetID() map[util.Uint256][]*Output {
+	m := make(map[util.Uint256][]*Output)
+	for _, out := range t.Outputs {
+		m[out.AssetID] = append(m[out.AssetID], out)
+	}
+	return m
+}
+
 // Size returns the size of the transaction in term of bytes
 func (t *Transaction) Size() int {
 	attrSize := util.GetVarSize(t.Attributes)

pkg/core/transaction/witness.go

@@ -55,3 +55,8 @@ func (w *Witness) MarshalJSON() ([]byte, error) {
 func (w *Witness) Size() int {
 	return util.GetVarSize(w.InvocationScript) + util.GetVarSize(w.VerificationScript)
 }
+
+// ScriptHash returns the hash of the VerificationScript.
+func (w Witness) ScriptHash() (util.Uint160, error) {
+	return util.Uint160FromScript(w.VerificationScript)
+}

pkg/core/unspent_coin_state.go

@@ -7,6 +7,7 @@ import (
 	"io"
 
 	"github.com/CityOfZion/neo-go/pkg/core/storage"
+	"github.com/CityOfZion/neo-go/pkg/core/transaction"
 	"github.com/CityOfZion/neo-go/pkg/util"
 )
 
@@ -91,3 +92,32 @@ func (s *UnspentCoinState) DecodeBinary(r io.Reader) error {
 	}
 	return nil
 }
+
+// IsDoubleSpend verifies that the input transactions are not double spent.
+func IsDoubleSpend(s storage.Store, tx *transaction.Transaction) bool {
+	if len(tx.Inputs) == 0 {
+		return false
+	}
+
+	for prevHash, inputs := range tx.GroupInputsByPrevHash() {
+		unspent := &UnspentCoinState{}
+		key := storage.AppendPrefix(storage.STCoin, prevHash.BytesReverse())
+		if b, err := s.Get(key); err == nil {
+			if err := unspent.DecodeBinary(bytes.NewReader(b)); err != nil {
+				return false
+			}
+			if unspent == nil {
+				return true
+			}
+
+			for _, input := range inputs {
+				if int(input.PrevIndex) >= len(unspent.states) || unspent.states[input.PrevIndex] == CoinStateSpent {
+					return true
+				}
+			}
+		}
+
+	}
+
+	return false
+}

pkg/network/helper_test.go

@@ -77,6 +77,18 @@ func (chain testChain) GetTransaction(util.Uint256) (*transaction.Transaction, u
 	panic("TODO")
 }
 
+func (chain testChain) GetMemPool() core.MemPool {
+	panic("TODO")
+}
+
+func (chain testChain) IsLowPriority(*transaction.Transaction) bool {
+	panic("TODO")
+}
+
+func (chain testChain) Verify(*transaction.Transaction) error {
+	panic("TODO")
+}
+
 type testDiscovery struct{}
 
 func (d testDiscovery) BackFill(addrs ...string)   {}

pkg/network/relay_reason.go

@@ -0,0 +1,15 @@
+package network
+
+// RelayReason is the type which describes the different relay outcome
+type RelayReason uint8
+
+// List of valid RelayReason.
+const (
+	RelaySucceed RelayReason = iota
+	RelayAlreadyExists
+	RelayOutOfMemory
+	RelayUnableToVerify
+	RelayInvalid
+	RelayPolicyFail
+	RelayUnknown
+)

pkg/network/server.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/CityOfZion/neo-go/pkg/core"
+	"github.com/CityOfZion/neo-go/pkg/core/transaction"
 	"github.com/CityOfZion/neo-go/pkg/network/payload"
 	"github.com/CityOfZion/neo-go/pkg/util"
 	log "github.com/sirupsen/logrus"
@@ -310,3 +311,41 @@ func (s *Server) handleMessage(peer Peer, msg *Message) error {
 	}
 	return nil
 }
+
+// RelayTxn a new transaction to the local node and the connected peers.
+// Reference: the method OnRelay in C#: https://github.com/neo-project/neo/blob/master/neo/Network/P2P/LocalNode.cs#L159
+func (s *Server) RelayTxn(t *transaction.Transaction) RelayReason {
+	if t.Type == transaction.MinerType {
+		return RelayInvalid
+	}
+	if s.chain.HasTransaction(t.Hash()) {
+		return RelayAlreadyExists
+	}
+	if err := s.chain.Verify(t); err != nil {
+		return RelayInvalid
+	}
+	// TODO: Implement Plugin.CheckPolicy?
+	//if (!Plugin.CheckPolicy(transaction))
+	// return RelayResultReason.PolicyFail;
+	if ok := s.chain.GetMemPool().TryAdd(t.Hash(), core.NewPoolItem(t, s.chain)); !ok {
+		return RelayOutOfMemory
+	}
+
+	for p := range s.Peers() {
+		payload := payload.NewInventory(payload.TXType, []util.Uint256{t.Hash()})
+		s.RelayDirectly(p, payload)
+	}
+
+	return RelaySucceed
+}
+
+// RelayDirectly relay directly the inventory to the remote peers.
+// Reference: the method OnRelayDirectly in C#: https://github.com/neo-project/neo/blob/master/neo/Network/P2P/LocalNode.cs#L166
+func (s *Server) RelayDirectly(p Peer, inv *payload.Inventory) {
+	if !p.Version().Relay {
+		return
+	}
+
+	p.WriteMsg(NewMessage(s.Net, CMDInv, inv))
+
+}

pkg/rpc/server.go

@@ -1,6 +1,7 @@
 package rpc
 
 import (
+	"bytes"
 	"context"
 	"encoding/hex"
 	"fmt"
@@ -8,6 +9,7 @@ import (
 	"strconv"
 
 	"github.com/CityOfZion/neo-go/pkg/core"
+	"github.com/CityOfZion/neo-go/pkg/core/transaction"
 	"github.com/CityOfZion/neo-go/pkg/crypto"
 	"github.com/CityOfZion/neo-go/pkg/network"
 	"github.com/CityOfZion/neo-go/pkg/rpc/result"
@@ -183,7 +185,7 @@ Methods:
 
 		results = peers
 
-	case "getblocksysfee", "getcontractstate", "getrawmempool", "getstorage", "submitblock", "gettxout", "invoke", "invokefunction", "invokescript", "sendrawtransaction":
+	case "getblocksysfee", "getcontractstate", "getrawmempool", "getstorage", "submitblock", "gettxout", "invoke", "invokefunction", "invokescript":
 
 		results = "TODO"
 
@@ -227,37 +229,10 @@ Methods:
 			results = "Invalid public account address"
 		}
 	case "getrawtransaction":
-		param0, err := reqParams.ValueWithType(0, "string")
-		if err != nil {
-			resultsErr = err
-		} else if txHash, err := util.Uint256DecodeString(param0.StringVal); err != nil {
-			err = errors.Wrapf(err, "param at index 0, (%s), could not be decode to Uint256", param0.StringVal)
-			resultsErr = NewInvalidParamsError(err.Error(), err)
-		} else if tx, height, err := s.chain.GetTransaction(txHash); err != nil {
-			err = errors.Wrapf(err, "Invalid transaction hash: %s", txHash)
-			resultsErr = NewInvalidParamsError(err.Error(), err)
-		} else if len(reqParams) >= 2 {
-			_header := s.chain.GetHeaderHash(int(height))
-			header, err := s.chain.GetHeader(_header)
-			if err != nil {
-				resultsErr = NewInvalidParamsError(err.Error(), err)
-			}
+		results, resultsErr = s.getrawtransaction(reqParams)
 
-			param1, _ := reqParams.ValueAt(1)
-			switch v := param1.RawValue.(type) {
-
-			case int, float64, bool, string:
-				if v == 0 || v == "0" || v == 0.0 || v == false || v == "false" {
-					results = hex.EncodeToString(tx.Bytes())
-				} else {
-					results = wrappers.NewTransactionOutputRaw(tx, header, s.chain)
-				}
-			default:
-				results = wrappers.NewTransactionOutputRaw(tx, header, s.chain)
-			}
-		} else {
-			results = hex.EncodeToString(tx.Bytes())
-		}
+	case "sendrawtransaction":
+		results, resultsErr = s.sendrawtransaction(reqParams)
 
 	default:
 		resultsErr = NewMethodNotFoundError(fmt.Sprintf("Method '%s' not supported", req.Method), nil)
@@ -271,6 +246,86 @@ Methods:
 	req.WriteResponse(w, results)
 }
 
+func (s *Server) getrawtransaction(reqParams Params) (interface{}, error) {
+	var resultsErr error
+	var results interface{}
+
+	param0, err := reqParams.ValueWithType(0, "string")
+	if err != nil {
+		resultsErr = err
+	} else if txHash, err := util.Uint256DecodeString(param0.StringVal); err != nil {
+		resultsErr = errInvalidParams
+	} else if tx, height, err := s.chain.GetTransaction(txHash); err != nil {
+		err = errors.Wrapf(err, "Invalid transaction hash: %s", txHash)
+		resultsErr = NewInvalidParamsError(err.Error(), err)
+	} else if len(reqParams) >= 2 {
+		_header := s.chain.GetHeaderHash(int(height))
+		header, err := s.chain.GetHeader(_header)
+		if err != nil {
+			resultsErr = NewInvalidParamsError(err.Error(), err)
+		}
+
+		param1, _ := reqParams.ValueAt(1)
+		switch v := param1.RawValue.(type) {
+
+		case int, float64, bool, string:
+			if v == 0 || v == "0" || v == 0.0 || v == false || v == "false" {
+				results = hex.EncodeToString(tx.Bytes())
+			} else {
+				results = wrappers.NewTransactionOutputRaw(tx, header, s.chain)
+			}
+		default:
+			results = wrappers.NewTransactionOutputRaw(tx, header, s.chain)
+		}
+	} else {
+		results = hex.EncodeToString(tx.Bytes())
+	}
+
+	return results, resultsErr
+}
+
+func (s *Server) sendrawtransaction(reqParams Params) (interface{}, error) {
+	var resultsErr error
+	var results interface{}
+
+	param, err := reqParams.ValueWithType(0, "string")
+	if err != nil {
+		resultsErr = err
+	} else if byteTx, err := hex.DecodeString(param.StringVal); err != nil {
+		resultsErr = errInvalidParams
+	} else {
+		r := bytes.NewReader(byteTx)
+		tx := &transaction.Transaction{}
+		err = tx.DecodeBinary(r)
+		if err != nil {
+			err = errors.Wrap(err, "transaction DecodeBinary failed")
+		}
+		relayReason := s.coreServer.RelayTxn(tx)
+		switch relayReason {
+		case network.RelaySucceed:
+			results = true
+		case network.RelayAlreadyExists:
+			err = errors.New("block or transaction already exists and cannot be sent repeatedly")
+		case network.RelayOutOfMemory:
+			err = errors.New("the memory pool is full and no more transactions can be sent")
+		case network.RelayUnableToVerify:
+			err = errors.New("the block cannot be validated")
+		case network.RelayInvalid:
+			err = errors.New("block or transaction validation failed")
+		case network.RelayPolicyFail:
+			err = errors.New("one of the Policy filters failed")
+		default:
+			err = errors.New("unknown error")
+
+		}
+		if err != nil {
+			resultsErr = NewInternalServerError(err.Error(), err)
+		}
+	}
+
+	return results, resultsErr
+}
+
 func (s Server) validBlockHeight(param *Param) bool {
 	return param.IntVal >= 0 && param.IntVal <= int(s.chain.BlockHeight())
 }

pkg/rpc/server_test.go

@@ -116,7 +116,7 @@ var testRpcCases = []tc{
 	{
 		rpcCall:        `{ "jsonrpc": "2.0", "id": 1, "method": "getrawtransaction", "params": ["45a41306c846ea80290416143e8e856559818065be3f4e143c60e43a", 1] }`,
 		method:         "getrawtransaction_4",
-		expectedResult: `{"jsonrpc":"2.0","error":{"code":-32602,"message":"Invalid Params","data":"param at index 0, (45a41306c846ea80290416143e8e856559818065be3f4e143c60e43a), could not be decode to Uint256: expected string size of 64 got 56"},"id":1}`,
+		expectedResult: `{"jsonrpc":"2.0","error":{"code":-32602,"message":"Invalid Params"},"id":1}`,
 	},
 
 	// Good case, valid transaction
@@ -216,6 +216,27 @@ var testRpcCases = []tc{
 		method:         "validateaddress_4",
 		expectedResult: `{"jsonrpc":"2.0","error":{"code":-32602,"message":"Invalid Params"},"id":1}`,
 	},
+
+	// Good case
+	{
+		rpcCall:        `{ "jsonrpc": "2.0", "id": 1, "method": "sendrawtransaction", "params": ["80000190274d792072617720636f6e7472616374207472616e73616374696f6e206465736372697074696f6e01949354ea0a8b57dfee1e257a1aedd1e0eea2e5837de145e8da9c0f101bfccc8e0100029b7cffdaa674beae0f930ebe6085af9093e5fe56b34a5c220ccdcf6efc336fc500a3e11100000000ea610aa6db39bd8c8556c9569d94b5e5a5d0ad199b7cffdaa674beae0f930ebe6085af9093e5fe56b34a5c220ccdcf6efc336fc5004f2418010000001cc9c05cefffe6cdd7b182816a9152ec218d2ec0014140dbd3cddac5cb2bd9bf6d93701f1a6f1c9dbe2d1b480c54628bbb2a4d536158c747a6af82698edf9f8af1cac3850bcb772bd9c8e4ac38f80704751cc4e0bd0e67232103cbb45da6072c14761c9da545749d9cfd863f860c351066d16df480602a2024c6ac"] }`,
+		method:         "sendrawtransaction_1",
+		expectedResult: `{"jsonrpc":"2.0","result":true,"id":1}`,
+	},
+
+	/* Good case: TODO: uncomment this test case once https://github.com/CityOfZion/neo-go/issues/173 is fixed!
+	{
+		rpcCall:        `{ "jsonrpc": "2.0", "id": 1, "method": "sendrawtransaction", "params": ["d1001b00046e616d6567d3d8602814a429a91afdbaa3914884a1c90c733101201cc9c05cefffe6cdd7b182816a9152ec218d2ec000000141403387ef7940a5764259621e655b3c621a6aafd869a611ad64adcc364d8dd1edf84e00a7f8b11b630a377eaef02791d1c289d711c08b7ad04ff0d6c9caca22cfe6232103cbb45da6072c14761c9da545749d9cfd863f860c351066d16df480602a2024c6ac"] }`,
+		method:         "sendrawtransaction_2",
+		expectedResult: `{"jsonrpc":"2.0","result":true,"id":1}`,
+	},*/
+
+	// Bad case, incorrect raw transaction
+	{
+		rpcCall:        `{ "jsonrpc": "2.0", "id": 1, "method": "sendrawtransaction", "params": ["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"] }`,
+		method:         "sendrawtransaction_1",
+		expectedResult: `{"jsonrpc":"2.0","error":{"code":-32602,"message":"Invalid Params"},"id":1}`,
+	},
 }
 
 func TestHandler(t *testing.T) {

pkg/util/fixed8.go

@@ -128,3 +128,26 @@ func (f Fixed8) Add(g Fixed8) Fixed8 {
 func (f Fixed8) Sub(g Fixed8) Fixed8 {
 	return NewFixed8(f.Value() - g.Value())
 }
+
+// LessThan implements Fixd8 < operator.
+func (f Fixed8) LessThan(g Fixed8) bool {
+	return f.Value() < g.Value()
+}
+
+// GreaterThan implements Fixd8 < operator.
+func (f Fixed8) GreaterThan(g Fixed8) bool {
+	return f.Value() > g.Value()
+}
+
+// Equal implements Fixd8 == operator.
+func (f Fixed8) Equal(g Fixed8) bool {
+	return f.Value() == g.Value()
+}
+
+// CompareTo returns the difference between the f and g.
+// difference < 0 implies f < g.
+// difference = 0 implies f = g.
+// difference > 0 implies f > g.
+func (f Fixed8) CompareTo(g Fixed8) int {
+	return int(f.Value() - g.Value())
+}

pkg/util/size.go

@@ -65,20 +65,22 @@ func GetVarSize(value interface{}) int {
 		valueLength := v.Len()
 		valueSize := 0
 
-		switch reflect.ValueOf(value).Index(0).Interface().(type) {
-		case io.Serializable:
-			for i := 0; i < valueLength; i++ {
-				elem := v.Index(i).Interface().(io.Serializable)
-				valueSize += elem.Size()
+		if valueLength != 0 {
+			switch reflect.ValueOf(value).Index(0).Interface().(type) {
+			case io.Serializable:
+				for i := 0; i < valueLength; i++ {
+					elem := v.Index(i).Interface().(io.Serializable)
+					valueSize += elem.Size()
+				}
+			case uint8, int8:
+				valueSize = valueLength
+			case uint16, int16:
+				valueSize = valueLength * 2
+			case uint32, int32:
+				valueSize = valueLength * 4
+			case uint64, int64:
+				valueSize = valueLength * 8
 			}
-		case uint8, int8:
-			valueSize = valueLength
-		case uint16, int16:
-			valueSize = valueLength * 2
-		case uint32, int32:
-			valueSize = valueLength * 4
-		case uint64, int64:
-			valueSize = valueLength * 8
 		}
 
 		return GetVarIntSize(valueLength) + valueSize

pkg/util/uint256.go

@@ -1,6 +1,7 @@
 package util
 
 import (
+	"bytes"
 	"encoding/hex"
 	"encoding/json"
 	"fmt"
@@ -74,3 +75,9 @@ func (u Uint256) Size() int {
 func (u Uint256) MarshalJSON() ([]byte, error) {
 	return []byte(`"0x` + u.String() + `"`), nil
 }
+
+// CompareTo compares two Uint256 with each other. Possible output: 1, -1, 0
+//  1 implies u > other.
+// -1 implies u < other.
+//  0 implies  u = other.
+func (u Uint256) CompareTo(other Uint256) int { return bytes.Compare(u[:], other[:]) }