package core

import (
	"errors"
	"fmt"
	"math/big"
	"sync"
	"sync/atomic"
	"time"

	"github.com/nspcc-dev/neo-go/pkg/config"
	"github.com/nspcc-dev/neo-go/pkg/core/block"
	"github.com/nspcc-dev/neo-go/pkg/core/dao"
	"github.com/nspcc-dev/neo-go/pkg/core/interop"
	"github.com/nspcc-dev/neo-go/pkg/core/mempool"
	"github.com/nspcc-dev/neo-go/pkg/core/native"
	"github.com/nspcc-dev/neo-go/pkg/core/state"
	"github.com/nspcc-dev/neo-go/pkg/core/storage"
	"github.com/nspcc-dev/neo-go/pkg/core/transaction"
	"github.com/nspcc-dev/neo-go/pkg/crypto"
	"github.com/nspcc-dev/neo-go/pkg/crypto/hash"
	"github.com/nspcc-dev/neo-go/pkg/crypto/keys"
	"github.com/nspcc-dev/neo-go/pkg/encoding/bigint"
	"github.com/nspcc-dev/neo-go/pkg/io"
	"github.com/nspcc-dev/neo-go/pkg/smartcontract"
	"github.com/nspcc-dev/neo-go/pkg/smartcontract/manifest"
	"github.com/nspcc-dev/neo-go/pkg/smartcontract/trigger"
	"github.com/nspcc-dev/neo-go/pkg/util"
	"github.com/nspcc-dev/neo-go/pkg/vm"
	"github.com/nspcc-dev/neo-go/pkg/vm/stackitem"
	"go.uber.org/zap"
)

// Tuning parameters.
const (
	headerBatchCount = 2000
	version          = "0.1.0"

	defaultMemPoolSize   = 50000
	verificationGasLimit = 100000000 // 1 GAS
)

var (
	// ErrAlreadyExists is returned when trying to add some already existing
	// transaction into the pool (not specifying whether it exists in the
	// chain or mempool).
	ErrAlreadyExists = errors.New("already exists")
	// ErrOOM is returned when adding transaction to the memory pool because
	// it reached its full capacity.
	ErrOOM = errors.New("no space left in the memory pool")
	// ErrPolicy is returned on attempt to add transaction that doesn't
	// comply with node's configured policy into the mempool.
	ErrPolicy = errors.New("not allowed by policy")
	// ErrInvalidBlockIndex is returned when trying to add block with index
	// other than expected height of the blockchain.
	ErrInvalidBlockIndex error = errors.New("invalid block index")
)
var (
	genAmount         = []int{6, 5, 4, 3, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1}
	decrementInterval = 2000000
	persistInterval   = 1 * time.Second
)

// Blockchain represents the blockchain. It maintans internal state representing
// the state of the ledger that can be accessed in various ways and changed by
// adding new blocks or headers.
type Blockchain struct {
	config config.ProtocolConfiguration

	// The only way chain state changes is by adding blocks, so we can't
	// allow concurrent block additions. It differs from the next lock in
	// that it's only for AddBlock method itself, the chain state is
	// protected by the lock below, but holding it during all of AddBlock
	// is too expensive (because the state only changes when persisting
	// change cache).
	addLock sync.Mutex

	// This lock ensures blockchain immutability for operations that need
	// that while performing their tasks. It's mostly used as a read lock
	// with the only writer being the block addition logic.
	lock sync.RWMutex

	// Data access object for CRUD operations around storage.
	dao *dao.Simple

	// Current index/height of the highest block.
	// Read access should always be called by BlockHeight().
	// Write access should only happen in storeBlock().
	blockHeight uint32

	// Current top Block wrapped in an atomic.Value for safe access.
	topBlock atomic.Value

	// Current persisted block count.
	persistedHeight uint32

	// Number of headers stored in the chain file.
	storedHeaderCount uint32

	generationAmount  []int
	decrementInterval int

	// All operations on headerList must be called from an
	// headersOp to be routine safe.
	headerList *HeaderHashList

	// Only for operating on the headerList.
	headersOp     chan headersOpFunc
	headersOpDone chan struct{}

	// Stop synchronization mechanisms.
	stopCh      chan struct{}
	runToExitCh chan struct{}

	memPool *mempool.Pool

	sbCommittee keys.PublicKeys

	log *zap.Logger

	lastBatch *storage.MemBatch

	contracts native.Contracts

	// Notification subsystem.
	events  chan bcEvent
	subCh   chan interface{}
	unsubCh chan interface{}
}

// bcEvent is an internal event generated by the Blockchain and then
// broadcasted to other parties. It joins the new block and associated
// invocation logs, all the other events visible from outside can be produced
// from this combination.
type bcEvent struct {
	block          *block.Block
	appExecResults []*state.AppExecResult
}

type headersOpFunc func(headerList *HeaderHashList)

// NewBlockchain returns a new blockchain object the will use the
// given Store as its underlying storage. For it to work correctly you need
// to spawn a goroutine for its Run method after this initialization.
func NewBlockchain(s storage.Store, cfg config.ProtocolConfiguration, log *zap.Logger) (*Blockchain, error) {
	if log == nil {
		return nil, errors.New("empty logger")
	}

	if cfg.MemPoolSize <= 0 {
		cfg.MemPoolSize = defaultMemPoolSize
		log.Info("mempool size is not set or wrong, setting default value", zap.Int("MemPoolSize", cfg.MemPoolSize))
	}
	committee, err := committeeFromConfig(cfg)
	if err != nil {
		return nil, err
	}
	bc := &Blockchain{
		config:        cfg,
		dao:           dao.NewSimple(s, cfg.Magic),
		headersOp:     make(chan headersOpFunc),
		headersOpDone: make(chan struct{}),
		stopCh:        make(chan struct{}),
		runToExitCh:   make(chan struct{}),
		memPool:       mempool.New(cfg.MemPoolSize),
		sbCommittee:   committee,
		log:           log,
		events:        make(chan bcEvent),
		subCh:         make(chan interface{}),
		unsubCh:       make(chan interface{}),

		generationAmount:  genAmount,
		decrementInterval: decrementInterval,

		contracts: *native.NewContracts(),
	}

	if err := bc.init(); err != nil {
		return nil, err
	}

	return bc, nil
}

func (bc *Blockchain) init() error {
	// If we could not find the version in the Store, we know that there is nothing stored.
	ver, err := bc.dao.GetVersion()
	if err != nil {
		bc.log.Info("no storage version found! creating genesis block")
		if err = bc.dao.PutVersion(version); err != nil {
			return err
		}
		genesisBlock, err := createGenesisBlock(bc.config)
		if err != nil {
			return err
		}
		bc.headerList = NewHeaderHashList(genesisBlock.Hash())
		err = bc.dao.PutCurrentHeader(hashAndIndexToBytes(genesisBlock.Hash(), genesisBlock.Index))
		if err != nil {
			return err
		}
		return bc.storeBlock(genesisBlock, nil)
	}
	if ver != version {
		return fmt.Errorf("storage version mismatch betweeen %s and %s", version, ver)
	}

	// At this point there was no version found in the storage which
	// implies a creating fresh storage with the version specified
	// and the genesis block as first block.
	bc.log.Info("restoring blockchain", zap.String("version", version))

	bHeight, err := bc.dao.GetCurrentBlockHeight()
	if err != nil {
		return err
	}
	bc.blockHeight = bHeight
	bc.persistedHeight = bHeight
	if err = bc.dao.InitMPT(bHeight); err != nil {
		return fmt.Errorf("can't init MPT at height %d: %w", bHeight, err)
	}

	hashes, err := bc.dao.GetHeaderHashes()
	if err != nil {
		return err
	}

	bc.headerList = NewHeaderHashList(hashes...)
	bc.storedHeaderCount = uint32(len(hashes))

	currHeaderHeight, currHeaderHash, err := bc.dao.GetCurrentHeaderHeight()
	if err != nil {
		return err
	}
	if bc.storedHeaderCount == 0 && currHeaderHeight == 0 {
		bc.headerList.Add(currHeaderHash)
	}

	// There is a high chance that the Node is stopped before the next
	// batch of 2000 headers was stored. Via the currentHeaders stored we can sync
	// that with stored blocks.
	if currHeaderHeight >= bc.storedHeaderCount {
		hash := currHeaderHash
		var targetHash util.Uint256
		if bc.headerList.Len() > 0 {
			targetHash = bc.headerList.Get(bc.headerList.Len() - 1)
		} else {
			genesisBlock, err := createGenesisBlock(bc.config)
			if err != nil {
				return err
			}
			targetHash = genesisBlock.Hash()
			bc.headerList.Add(targetHash)
		}
		headers := make([]*block.Header, 0)

		for hash != targetHash {
			header, err := bc.GetHeader(hash)
			if err != nil {
				return fmt.Errorf("could not get header %s: %w", hash, err)
			}
			headers = append(headers, header)
			hash = header.PrevHash
		}
		headerSliceReverse(headers)
		for _, h := range headers {
			if !h.Verify() {
				return fmt.Errorf("bad header %d/%s in the storage", h.Index, h.Hash())
			}
			bc.headerList.Add(h.Hash())
		}
	}

	return nil
}

// Run runs chain loop, it needs to be run as goroutine and executing it is
// critical for correct Blockchain operation.
func (bc *Blockchain) Run() {
	persistTimer := time.NewTimer(persistInterval)
	defer func() {
		persistTimer.Stop()
		if err := bc.persist(); err != nil {
			bc.log.Warn("failed to persist", zap.Error(err))
		}
		if err := bc.dao.Store.Close(); err != nil {
			bc.log.Warn("failed to close db", zap.Error(err))
		}
		close(bc.runToExitCh)
	}()
	go bc.notificationDispatcher()
	for {
		select {
		case <-bc.stopCh:
			return
		case op := <-bc.headersOp:
			op(bc.headerList)
			bc.headersOpDone <- struct{}{}
		case <-persistTimer.C:
			go func() {
				err := bc.persist()
				if err != nil {
					bc.log.Warn("failed to persist blockchain", zap.Error(err))
				}
				persistTimer.Reset(persistInterval)
			}()
		}
	}
}

// notificationDispatcher manages subscription to events and broadcasts new events.
func (bc *Blockchain) notificationDispatcher() {
	var (
		// These are just sets of subscribers, though modelled as maps
		// for ease of management (not a lot of subscriptions is really
		// expected, but maps are convenient for adding/deleting elements).
		blockFeed        = make(map[chan<- *block.Block]bool)
		txFeed           = make(map[chan<- *transaction.Transaction]bool)
		notificationFeed = make(map[chan<- *state.NotificationEvent]bool)
		executionFeed    = make(map[chan<- *state.AppExecResult]bool)
	)
	for {
		select {
		case <-bc.stopCh:
			return
		case sub := <-bc.subCh:
			switch ch := sub.(type) {
			case chan<- *block.Block:
				blockFeed[ch] = true
			case chan<- *transaction.Transaction:
				txFeed[ch] = true
			case chan<- *state.NotificationEvent:
				notificationFeed[ch] = true
			case chan<- *state.AppExecResult:
				executionFeed[ch] = true
			default:
				panic(fmt.Sprintf("bad subscription: %T", sub))
			}
		case unsub := <-bc.unsubCh:
			switch ch := unsub.(type) {
			case chan<- *block.Block:
				delete(blockFeed, ch)
			case chan<- *transaction.Transaction:
				delete(txFeed, ch)
			case chan<- *state.NotificationEvent:
				delete(notificationFeed, ch)
			case chan<- *state.AppExecResult:
				delete(executionFeed, ch)
			default:
				panic(fmt.Sprintf("bad unsubscription: %T", unsub))
			}
		case event := <-bc.events:
			// We don't want to waste time looping through transactions when there are no
			// subscribers.
			if len(txFeed) != 0 || len(notificationFeed) != 0 || len(executionFeed) != 0 {
				aer := event.appExecResults[0]
				if !aer.TxHash.Equals(event.block.Hash()) {
					panic("inconsistent application execution results")
				}
				for ch := range executionFeed {
					ch <- aer
				}
				for i := range aer.Events {
					for ch := range notificationFeed {
						ch <- &aer.Events[i]
					}
				}

				aerIdx := 1
				for _, tx := range event.block.Transactions {
					aer := event.appExecResults[aerIdx]
					if !aer.TxHash.Equals(tx.Hash()) {
						panic("inconsistent application execution results")
					}
					aerIdx++
					for ch := range executionFeed {
						ch <- aer
					}
					if aer.VMState == vm.HaltState {
						for i := range aer.Events {
							for ch := range notificationFeed {
								ch <- &aer.Events[i]
							}
						}
					}
					for ch := range txFeed {
						ch <- tx
					}
				}
			}
			for ch := range blockFeed {
				ch <- event.block
			}
		}
	}
}

// Close stops Blockchain's internal loop, syncs changes to persistent storage
// and closes it. The Blockchain is no longer functional after the call to Close.
func (bc *Blockchain) Close() {
	// If there is a block addition in progress, wait for it to finish and
	// don't allow new ones.
	bc.addLock.Lock()
	close(bc.stopCh)
	<-bc.runToExitCh
	bc.addLock.Unlock()
}

// AddBlock accepts successive block for the Blockchain, verifies it and
// stores internally. Eventually it will be persisted to the backing storage.
func (bc *Blockchain) AddBlock(block *block.Block) error {
	bc.addLock.Lock()
	defer bc.addLock.Unlock()

	var mp *mempool.Pool
	expectedHeight := bc.BlockHeight() + 1
	if expectedHeight != block.Index {
		return fmt.Errorf("expected %d, got %d: %w", expectedHeight, block.Index, ErrInvalidBlockIndex)
	}

	headerLen := bc.headerListLen()
	if int(block.Index) == headerLen {
		err := bc.addHeaders(bc.config.VerifyBlocks, block.Header())
		if err != nil {
			return err
		}
	}
	if bc.config.VerifyBlocks {
		err := block.Verify()
		if err != nil {
			return fmt.Errorf("block %s is invalid: %w", block.Hash().StringLE(), err)
		}
		mp = mempool.New(len(block.Transactions))
		for _, tx := range block.Transactions {
			var err error
			// Transactions are verified before adding them
			// into the pool, so there is no point in doing
			// it again even if we're verifying in-block transactions.
			if bc.memPool.ContainsKey(tx.Hash()) {
				err = mp.Add(tx, bc)
				if err == nil {
					continue
				}
			} else {
				err = bc.verifyAndPoolTx(tx, mp)
			}
			if err != nil && bc.config.VerifyTransactions {
				return fmt.Errorf("transaction %s failed to verify: %w", tx.Hash().StringLE(), err)
			}
		}
	}
	return bc.storeBlock(block, mp)
}

// AddHeaders processes the given headers and add them to the
// HeaderHashList. It expects headers to be sorted by index.
func (bc *Blockchain) AddHeaders(headers ...*block.Header) error {
	return bc.addHeaders(bc.config.VerifyBlocks, headers...)
}

// addHeaders is an internal implementation of AddHeaders (`verify` parameter
// tells it to verify or not verify given headers).
func (bc *Blockchain) addHeaders(verify bool, headers ...*block.Header) (err error) {
	var (
		start = time.Now()
		batch = bc.dao.Store.Batch()
	)

	if len(headers) > 0 {
		var i int
		curHeight := bc.HeaderHeight()
		for i = range headers {
			if headers[i].Index > curHeight {
				break
			}
		}
		headers = headers[i:]
	}

	if len(headers) == 0 {
		return nil
	} else if verify {
		// Verify that the chain of the headers is consistent.
		var lastHeader *block.Header
		if lastHeader, err = bc.GetHeader(headers[0].PrevHash); err != nil {
			return fmt.Errorf("previous header was not found: %w", err)
		}
		for _, h := range headers {
			if err = bc.verifyHeader(h, lastHeader); err != nil {
				return
			}
			lastHeader = h
		}
	}

	bc.headersOp <- func(headerList *HeaderHashList) {
		oldlen := headerList.Len()
		for _, h := range headers {
			if int(h.Index-1) >= headerList.Len() {
				err = fmt.Errorf(
					"height of received header %d is higher then the current header %d",
					h.Index, headerList.Len(),
				)
				return
			}
			if int(h.Index) < headerList.Len() {
				continue
			}
			if !h.Verify() {
				err = fmt.Errorf("header %v is invalid", h)
				return
			}
			if err = bc.processHeader(h, batch, headerList); err != nil {
				return
			}
		}

		if oldlen != headerList.Len() {
			updateHeaderHeightMetric(headerList.Len() - 1)
			if err = bc.dao.Store.PutBatch(batch); err != nil {
				return
			}
			bc.log.Debug("done processing headers",
				zap.Int("headerIndex", headerList.Len()-1),
				zap.Uint32("blockHeight", bc.BlockHeight()),
				zap.Duration("took", time.Since(start)))
		}
	}
	<-bc.headersOpDone
	return err
}

// processHeader processes the given header. Note that this is only thread safe
// if executed in headers operation.
func (bc *Blockchain) processHeader(h *block.Header, batch storage.Batch, headerList *HeaderHashList) error {
	headerList.Add(h.Hash())

	buf := io.NewBufBinWriter()
	for int(h.Index)-headerBatchCount >= int(bc.storedHeaderCount) {
		if err := headerList.Write(buf.BinWriter, int(bc.storedHeaderCount), headerBatchCount); err != nil {
			return err
		}
		key := storage.AppendPrefixInt(storage.IXHeaderHashList, int(bc.storedHeaderCount))
		batch.Put(key, buf.Bytes())
		bc.storedHeaderCount += headerBatchCount
		buf.Reset()
	}

	buf.Reset()
	h.EncodeBinary(buf.BinWriter)
	if buf.Err != nil {
		return buf.Err
	}

	key := storage.AppendPrefix(storage.DataBlock, h.Hash().BytesLE())
	batch.Put(key, buf.Bytes())
	batch.Put(storage.SYSCurrentHeader.Bytes(), hashAndIndexToBytes(h.Hash(), h.Index))

	return nil
}

// GetStateRoot returns state root for a given height.
func (bc *Blockchain) GetStateRoot(height uint32) (*state.MPTRootState, error) {
	return bc.dao.GetStateRoot(height)
}

// storeBlock performs chain update using the block given, it executes all
// transactions with all appropriate side-effects and updates Blockchain state.
// This is the only way to change Blockchain state.
func (bc *Blockchain) storeBlock(block *block.Block, txpool *mempool.Pool) error {
	cache := dao.NewCached(bc.dao)
	writeBuf := io.NewBufBinWriter()
	appExecResults := make([]*state.AppExecResult, 0, 1+len(block.Transactions))
	if err := cache.StoreAsBlock(block, writeBuf); err != nil {
		return err
	}
	writeBuf.Reset()

	if err := cache.StoreAsCurrentBlock(block, writeBuf); err != nil {
		return err
	}
	writeBuf.Reset()

	if block.Index > 0 {
		systemInterop := bc.newInteropContext(trigger.System, cache, block, nil)
		v := systemInterop.SpawnVM()
		v.LoadScriptWithFlags(bc.contracts.GetPersistScript(), smartcontract.AllowModifyStates|smartcontract.AllowCall)
		v.SetPriceGetter(getPrice)
		if err := v.Run(); err != nil {
			return fmt.Errorf("onPersist run failed: %w", err)
		} else if _, err := systemInterop.DAO.Persist(); err != nil {
			return fmt.Errorf("can't save onPersist changes: %w", err)
		}
		for i := range systemInterop.Notifications {
			bc.handleNotification(&systemInterop.Notifications[i], cache, block, block.Hash())
		}
		aer := &state.AppExecResult{
			TxHash:      block.Hash(), // application logs can be retrieved by block hash
			Trigger:     trigger.System,
			VMState:     v.State(),
			GasConsumed: v.GasConsumed(),
			Stack:       v.Estack().ToArray(),
			Events:      systemInterop.Notifications,
		}
		appExecResults = append(appExecResults, aer)
		err := cache.PutAppExecResult(aer, writeBuf)
		if err != nil {
			return fmt.Errorf("failed to store onPersist exec result: %w", err)
		}
		writeBuf.Reset()
	}

	for _, tx := range block.Transactions {
		if err := cache.StoreAsTransaction(tx, block.Index, writeBuf); err != nil {
			return err
		}
		writeBuf.Reset()

		systemInterop := bc.newInteropContext(trigger.Application, cache, block, tx)
		v := systemInterop.SpawnVM()
		v.LoadScriptWithFlags(tx.Script, smartcontract.All)
		v.SetPriceGetter(getPrice)
		v.GasLimit = tx.SystemFee

		err := v.Run()
		if !v.HasFailed() {
			_, err := systemInterop.DAO.Persist()
			if err != nil {
				return fmt.Errorf("failed to persist invocation results: %w", err)
			}
			for j := range systemInterop.Notifications {
				bc.handleNotification(&systemInterop.Notifications[j], cache, block, tx.Hash())
			}
		} else {
			bc.log.Warn("contract invocation failed",
				zap.String("tx", tx.Hash().StringLE()),
				zap.Uint32("block", block.Index),
				zap.Error(err))
		}
		aer := &state.AppExecResult{
			TxHash:      tx.Hash(),
			Trigger:     trigger.Application,
			VMState:     v.State(),
			GasConsumed: v.GasConsumed(),
			Stack:       v.Estack().ToArray(),
			Events:      systemInterop.Notifications,
		}
		appExecResults = append(appExecResults, aer)
		err = cache.PutAppExecResult(aer, writeBuf)
		if err != nil {
			return fmt.Errorf("failed to store tx exec result: %w", err)
		}
		writeBuf.Reset()
	}

	root := bc.dao.MPT.StateRoot()
	var prevHash util.Uint256
	if block.Index > 0 {
		prev, err := bc.dao.GetStateRoot(block.Index - 1)
		if err != nil {
			return fmt.Errorf("can't get previous state root: %w", err)
		}
		prevHash = hash.DoubleSha256(prev.GetSignedPart())
	}
	err := bc.AddStateRoot(&state.MPTRoot{
		MPTRootBase: state.MPTRootBase{
			Index:    block.Index,
			PrevHash: prevHash,
			Root:     root,
		},
	})
	if err != nil {
		return err
	}

	if bc.config.SaveStorageBatch {
		bc.lastBatch = cache.DAO.GetBatch()
	}

	bc.lock.Lock()
	_, err = cache.Persist()
	if err != nil {
		bc.lock.Unlock()
		return err
	}
	bc.contracts.Policy.OnPersistEnd(bc.dao)
	bc.dao.MPT.Flush()
	// Every persist cycle we also compact our in-memory MPT.
	persistedHeight := atomic.LoadUint32(&bc.persistedHeight)
	if persistedHeight == block.Index-1 {
		// 10 is good and roughly estimated to fit remaining trie into 1M of memory.
		bc.dao.MPT.Collapse(10)
	}
	bc.topBlock.Store(block)
	atomic.StoreUint32(&bc.blockHeight, block.Index)
	bc.memPool.RemoveStale(func(tx *transaction.Transaction) bool { return bc.isTxStillRelevant(tx, txpool) }, bc)
	bc.lock.Unlock()

	updateBlockHeightMetric(block.Index)
	// Genesis block is stored when Blockchain is not yet running, so there
	// is no one to read this event. And it doesn't make much sense as event
	// anyway.
	if block.Index != 0 {
		bc.events <- bcEvent{block, appExecResults}
	}
	return nil
}

func (bc *Blockchain) handleNotification(note *state.NotificationEvent, d *dao.Cached, b *block.Block, h util.Uint256) {
	if note.Name != "transfer" && note.Name != "Transfer" {
		return
	}
	arr, ok := note.Item.Value().([]stackitem.Item)
	if !ok || len(arr) != 3 {
		return
	}
	var from []byte
	fromValue := arr[0].Value()
	// we don't have `from` set when we are minting tokens
	if fromValue != nil {
		from, ok = fromValue.([]byte)
		if !ok {
			return
		}
	}
	var to []byte
	toValue := arr[1].Value()
	// we don't have `to` set when we are burning tokens
	if toValue != nil {
		to, ok = toValue.([]byte)
		if !ok {
			return
		}
	}
	amount, ok := arr[2].Value().(*big.Int)
	if !ok {
		bs, ok := arr[2].Value().([]byte)
		if !ok {
			return
		}
		amount = bigint.FromBytes(bs)
	}
	bc.processNEP5Transfer(d, h, b, note.ScriptHash, from, to, amount)
}

func parseUint160(addr []byte) util.Uint160 {
	if u, err := util.Uint160DecodeBytesBE(addr); err == nil {
		return u
	}
	return util.Uint160{}
}

func (bc *Blockchain) processNEP5Transfer(cache *dao.Cached, h util.Uint256, b *block.Block, sc util.Uint160, from, to []byte, amount *big.Int) {
	toAddr := parseUint160(to)
	fromAddr := parseUint160(from)
	var id int32
	nativeContract := bc.contracts.ByHash(sc)
	if nativeContract != nil {
		id = nativeContract.Metadata().ContractID
	} else {
		assetContract, err := cache.GetContractState(sc)
		if err != nil {
			return
		}
		id = assetContract.ID
	}
	transfer := &state.NEP5Transfer{
		Asset:     id,
		From:      fromAddr,
		To:        toAddr,
		Block:     b.Index,
		Timestamp: b.Timestamp,
		Tx:        h,
	}
	if !fromAddr.Equals(util.Uint160{}) {
		balances, err := cache.GetNEP5Balances(fromAddr)
		if err != nil {
			return
		}
		bs := balances.Trackers[id]
		bs.Balance = *new(big.Int).Sub(&bs.Balance, amount)
		bs.LastUpdatedBlock = b.Index
		balances.Trackers[id] = bs
		transfer.Amount = *new(big.Int).Sub(&transfer.Amount, amount)
		isBig, err := cache.AppendNEP5Transfer(fromAddr, balances.NextTransferBatch, transfer)
		if err != nil {
			return
		}
		if isBig {
			balances.NextTransferBatch++
		}
		if err := cache.PutNEP5Balances(fromAddr, balances); err != nil {
			return
		}
	}
	if !toAddr.Equals(util.Uint160{}) {
		balances, err := cache.GetNEP5Balances(toAddr)
		if err != nil {
			return
		}
		bs := balances.Trackers[id]
		bs.Balance = *new(big.Int).Add(&bs.Balance, amount)
		bs.LastUpdatedBlock = b.Index
		balances.Trackers[id] = bs

		transfer.Amount = *amount
		isBig, err := cache.AppendNEP5Transfer(toAddr, balances.NextTransferBatch, transfer)
		if err != nil {
			return
		}
		if isBig {
			balances.NextTransferBatch++
		}
		if err := cache.PutNEP5Balances(toAddr, balances); err != nil {
			return
		}
	}
}

// ForEachNEP5Transfer executes f for each nep5 transfer in log.
func (bc *Blockchain) ForEachNEP5Transfer(acc util.Uint160, f func(*state.NEP5Transfer) error) error {
	balances, err := bc.dao.GetNEP5Balances(acc)
	if err != nil {
		return nil
	}
	for i := uint32(0); i <= balances.NextTransferBatch; i++ {
		lg, err := bc.dao.GetNEP5TransferLog(acc, i)
		if err != nil {
			return nil
		}
		err = lg.ForEach(f)
		if err != nil {
			return err
		}
	}
	return nil
}

// GetNEP5Balances returns NEP5 balances for the acc.
func (bc *Blockchain) GetNEP5Balances(acc util.Uint160) *state.NEP5Balances {
	bs, err := bc.dao.GetNEP5Balances(acc)
	if err != nil {
		return nil
	}
	return bs
}

// GetUtilityTokenBalance returns utility token (GAS) balance for the acc.
func (bc *Blockchain) GetUtilityTokenBalance(acc util.Uint160) *big.Int {
	bs, err := bc.dao.GetNEP5Balances(acc)
	if err != nil {
		return big.NewInt(0)
	}
	balance := bs.Trackers[bc.contracts.GAS.ContractID].Balance
	return &balance
}

// GetGoverningTokenBalance returns governing token (NEO) balance and the height
// of the last balance change for the account.
func (bc *Blockchain) GetGoverningTokenBalance(acc util.Uint160) (*big.Int, uint32) {
	bs, err := bc.dao.GetNEP5Balances(acc)
	if err != nil {
		return big.NewInt(0), 0
	}
	neo := bs.Trackers[bc.contracts.NEO.ContractID]
	return &neo.Balance, neo.LastUpdatedBlock
}

// LastBatch returns last persisted storage batch.
func (bc *Blockchain) LastBatch() *storage.MemBatch {
	return bc.lastBatch
}

// persist flushes current in-memory Store contents to the persistent storage.
func (bc *Blockchain) persist() error {
	var (
		start     = time.Now()
		persisted int
		err       error
	)

	persisted, err = bc.dao.Persist()
	if err != nil {
		return err
	}
	if persisted > 0 {
		bHeight, err := bc.dao.GetCurrentBlockHeight()
		if err != nil {
			return err
		}
		oldHeight := atomic.SwapUint32(&bc.persistedHeight, bHeight)
		diff := bHeight - oldHeight

		storedHeaderHeight, _, err := bc.dao.GetCurrentHeaderHeight()
		if err != nil {
			return err
		}
		bc.log.Info("blockchain persist completed",
			zap.Uint32("persistedBlocks", diff),
			zap.Int("persistedKeys", persisted),
			zap.Uint32("headerHeight", storedHeaderHeight),
			zap.Uint32("blockHeight", bHeight),
			zap.Duration("took", time.Since(start)))

		// update monitoring metrics.
		updatePersistedHeightMetric(bHeight)
	}

	return nil
}

func (bc *Blockchain) headerListLen() (n int) {
	bc.headersOp <- func(headerList *HeaderHashList) {
		n = headerList.Len()
	}
	<-bc.headersOpDone
	return
}

// 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.
	}
	return bc.dao.GetTransaction(hash)
}

// GetAppExecResult returns application execution result by the given
// tx hash.
func (bc *Blockchain) GetAppExecResult(hash util.Uint256) (*state.AppExecResult, error) {
	return bc.dao.GetAppExecResult(hash)
}

// GetStorageItem returns an item from storage.
func (bc *Blockchain) GetStorageItem(id int32, key []byte) *state.StorageItem {
	return bc.dao.GetStorageItem(id, key)
}

// GetStorageItems returns all storage items for a given contract id.
func (bc *Blockchain) GetStorageItems(id int32) (map[string]*state.StorageItem, error) {
	return bc.dao.GetStorageItems(id)
}

// GetBlock returns a Block by the given hash.
func (bc *Blockchain) GetBlock(hash util.Uint256) (*block.Block, error) {
	topBlock := bc.topBlock.Load()
	if topBlock != nil {
		if tb, ok := topBlock.(*block.Block); ok && tb.Hash().Equals(hash) {
			return tb, nil
		}
	}

	block, err := bc.dao.GetBlock(hash)
	if err != nil {
		return nil, err
	}
	for _, tx := range block.Transactions {
		stx, _, err := bc.dao.GetTransaction(tx.Hash())
		if err != nil {
			return nil, err
		}
		*tx = *stx
	}
	return block, nil
}

// GetHeader returns data block header identified with the given hash value.
func (bc *Blockchain) GetHeader(hash util.Uint256) (*block.Header, error) {
	topBlock := bc.topBlock.Load()
	if topBlock != nil {
		if tb, ok := topBlock.(*block.Block); ok && tb.Hash().Equals(hash) {
			return tb.Header(), nil
		}
	}
	block, err := bc.dao.GetBlock(hash)
	if err != nil {
		return nil, err
	}
	return block.Header(), nil
}

// HasTransaction returns true if the blockchain contains he given
// transaction hash.
func (bc *Blockchain) HasTransaction(hash util.Uint256) bool {
	return bc.memPool.ContainsKey(hash) || bc.dao.HasTransaction(hash)
}

// HasBlock returns true if the blockchain contains the given
// block hash.
func (bc *Blockchain) HasBlock(hash util.Uint256) bool {
	if header, err := bc.GetHeader(hash); err == nil {
		return header.Index <= bc.BlockHeight()
	}
	return false
}

// CurrentBlockHash returns the highest processed block hash.
func (bc *Blockchain) CurrentBlockHash() (hash util.Uint256) {
	bc.headersOp <- func(headerList *HeaderHashList) {
		hash = headerList.Get(int(bc.BlockHeight()))
	}
	<-bc.headersOpDone
	return
}

// CurrentHeaderHash returns the hash of the latest known header.
func (bc *Blockchain) CurrentHeaderHash() (hash util.Uint256) {
	bc.headersOp <- func(headerList *HeaderHashList) {
		hash = headerList.Last()
	}
	<-bc.headersOpDone
	return
}

// GetHeaderHash returns the hash from the headerList by its
// height/index.
func (bc *Blockchain) GetHeaderHash(i int) (hash util.Uint256) {
	bc.headersOp <- func(headerList *HeaderHashList) {
		hash = headerList.Get(i)
	}
	<-bc.headersOpDone
	return
}

// BlockHeight returns the height/index of the highest block.
func (bc *Blockchain) BlockHeight() uint32 {
	return atomic.LoadUint32(&bc.blockHeight)
}

// HeaderHeight returns the index/height of the highest header.
func (bc *Blockchain) HeaderHeight() uint32 {
	return uint32(bc.headerListLen() - 1)
}

// GetContractState returns contract by its script hash.
func (bc *Blockchain) GetContractState(hash util.Uint160) *state.Contract {
	contract, err := bc.dao.GetContractState(hash)
	if contract == nil && err != storage.ErrKeyNotFound {
		bc.log.Warn("failed to get contract state", zap.Error(err))
	}
	return contract
}

// GetContractScriptHash returns contract script hash by its ID.
func (bc *Blockchain) GetContractScriptHash(id int32) (util.Uint160, error) {
	return bc.dao.GetContractScriptHash(id)
}

// GetConfig returns the config stored in the blockchain.
func (bc *Blockchain) GetConfig() config.ProtocolConfiguration {
	return bc.config
}

// SubscribeForBlocks adds given channel to new block event broadcasting, so when
// there is a new block added to the chain you'll receive it via this channel.
// Make sure it's read from regularly as not reading these events might affect
// other Blockchain functions.
func (bc *Blockchain) SubscribeForBlocks(ch chan<- *block.Block) {
	bc.subCh <- ch
}

// SubscribeForTransactions adds given channel to new transaction event
// broadcasting, so when there is a new transaction added to the chain (in a
// block) you'll receive it via this channel. Make sure it's read from regularly
// as not reading these events might affect other Blockchain functions.
func (bc *Blockchain) SubscribeForTransactions(ch chan<- *transaction.Transaction) {
	bc.subCh <- ch
}

// SubscribeForNotifications adds given channel to new notifications event
// broadcasting, so when an in-block transaction execution generates a
// notification you'll receive it via this channel. Only notifications from
// successful transactions are broadcasted, if you're interested in failed
// transactions use SubscribeForExecutions instead. Make sure this channel is
// read from regularly as not reading these events might affect other Blockchain
// functions.
func (bc *Blockchain) SubscribeForNotifications(ch chan<- *state.NotificationEvent) {
	bc.subCh <- ch
}

// SubscribeForExecutions adds given channel to new transaction execution event
// broadcasting, so when an in-block transaction execution happens you'll receive
// the result of it via this channel. Make sure it's read from regularly as not
// reading these events might affect other Blockchain functions.
func (bc *Blockchain) SubscribeForExecutions(ch chan<- *state.AppExecResult) {
	bc.subCh <- ch
}

// UnsubscribeFromBlocks unsubscribes given channel from new block notifications,
// you can close it afterwards. Passing non-subscribed channel is a no-op.
func (bc *Blockchain) UnsubscribeFromBlocks(ch chan<- *block.Block) {
	bc.unsubCh <- ch
}

// UnsubscribeFromTransactions unsubscribes given channel from new transaction
// notifications, you can close it afterwards. Passing non-subscribed channel is
// a no-op.
func (bc *Blockchain) UnsubscribeFromTransactions(ch chan<- *transaction.Transaction) {
	bc.unsubCh <- ch
}

// UnsubscribeFromNotifications unsubscribes given channel from new
// execution-generated notifications, you can close it afterwards. Passing
// non-subscribed channel is a no-op.
func (bc *Blockchain) UnsubscribeFromNotifications(ch chan<- *state.NotificationEvent) {
	bc.unsubCh <- ch
}

// UnsubscribeFromExecutions unsubscribes given channel from new execution
// notifications, you can close it afterwards. Passing non-subscribed channel is
// a no-op.
func (bc *Blockchain) UnsubscribeFromExecutions(ch chan<- *state.AppExecResult) {
	bc.unsubCh <- ch
}

// CalculateClaimable calculates the amount of GAS generated by owning specified
// amount of NEO between specified blocks. The amount of NEO being passed is in
// its natural non-divisible form (1 NEO as 1, 2 NEO as 2, no multiplication by
// 10⁸ is needed as for Fixed8).
func (bc *Blockchain) CalculateClaimable(value *big.Int, startHeight, endHeight uint32) *big.Int {
	var amount int64
	di := uint32(bc.decrementInterval)

	ustart := startHeight / di
	if genSize := uint32(len(bc.generationAmount)); ustart < genSize {
		uend := endHeight / di
		iend := endHeight % di
		if uend >= genSize {
			uend = genSize - 1
			iend = di
		} else if iend == 0 {
			uend--
			iend = di
		}

		istart := startHeight % di
		for ustart < uend {
			amount += int64(di-istart) * int64(bc.generationAmount[ustart])
			ustart++
			istart = 0
		}

		amount += int64(iend-istart) * int64(bc.generationAmount[ustart])
	}

	return new(big.Int).Mul(big.NewInt(amount), value)
}

// FeePerByte returns transaction network fee per byte.
func (bc *Blockchain) FeePerByte() int64 {
	return bc.contracts.Policy.GetFeePerByteInternal(bc.dao)
}

// GetMaxBlockSize returns maximum allowed block size from native Policy contract.
func (bc *Blockchain) GetMaxBlockSize() uint32 {
	return bc.contracts.Policy.GetMaxBlockSizeInternal(bc.dao)
}

// GetMaxBlockSystemFee returns maximum block system fee from native Policy contract.
func (bc *Blockchain) GetMaxBlockSystemFee() int64 {
	return bc.contracts.Policy.GetMaxBlockSystemFeeInternal(bc.dao)
}

// GetMemPool returns the memory pool of the blockchain.
func (bc *Blockchain) GetMemPool() *mempool.Pool {
	return bc.memPool
}

// ApplyPolicyToTxSet applies configured policies to given transaction set. It
// expects slice to be ordered by fee and returns a subslice of it.
func (bc *Blockchain) ApplyPolicyToTxSet(txes []*transaction.Transaction) []*transaction.Transaction {
	maxTx := bc.contracts.Policy.GetMaxTransactionsPerBlockInternal(bc.dao)
	if maxTx != 0 && len(txes) > int(maxTx) {
		txes = txes[:maxTx]
	}
	maxBlockSize := bc.contracts.Policy.GetMaxBlockSizeInternal(bc.dao)
	maxBlockSysFee := bc.contracts.Policy.GetMaxBlockSystemFeeInternal(bc.dao)
	var (
		blockSize uint32
		sysFee    int64
	)
	blockSize = uint32(io.GetVarSize(new(block.Block)) + io.GetVarSize(len(txes)+1))
	for i, tx := range txes {
		blockSize += uint32(tx.Size())
		sysFee += tx.SystemFee
		if blockSize > maxBlockSize || sysFee > maxBlockSysFee {
			txes = txes[:i]
			break
		}
	}
	return txes
}

// Various errors that could be returns upon header verification.
var (
	ErrHdrHashMismatch     = errors.New("previous header hash doesn't match")
	ErrHdrIndexMismatch    = errors.New("previous header index doesn't match")
	ErrHdrInvalidTimestamp = errors.New("block is not newer than the previous one")
)

func (bc *Blockchain) verifyHeader(currHeader, prevHeader *block.Header) error {
	if prevHeader.Hash() != currHeader.PrevHash {
		return ErrHdrHashMismatch
	}
	if prevHeader.Index+1 != currHeader.Index {
		return ErrHdrIndexMismatch
	}
	if prevHeader.Timestamp >= currHeader.Timestamp {
		return ErrHdrInvalidTimestamp
	}
	return bc.verifyHeaderWitnesses(currHeader, prevHeader)
}

// Various errors that could be returned upon verification.
var (
	ErrTxExpired           = errors.New("transaction has expired")
	ErrInsufficientFunds   = errors.New("insufficient funds")
	ErrTxSmallNetworkFee   = errors.New("too small network fee")
	ErrTxTooBig            = errors.New("too big transaction")
	ErrMemPoolConflict     = errors.New("invalid transaction due to conflicts with the memory pool")
	ErrTxInvalidWitnessNum = errors.New("number of signers doesn't match witnesses")
	ErrInvalidAttribute    = errors.New("invalid attribute")
)

// verifyAndPoolTx verifies whether a transaction is bonafide or not and tries
// to add it to the mempool given.
func (bc *Blockchain) verifyAndPoolTx(t *transaction.Transaction, pool *mempool.Pool) error {
	height := bc.BlockHeight()
	if t.ValidUntilBlock <= height || t.ValidUntilBlock > height+transaction.MaxValidUntilBlockIncrement {
		return fmt.Errorf("%w: ValidUntilBlock = %d, current height = %d", ErrTxExpired, t.ValidUntilBlock, height)
	}
	// Policying.
	if err := bc.contracts.Policy.CheckPolicy(bc.dao, t); err != nil {
		// Only one %w can be used.
		return fmt.Errorf("%w: %v", ErrPolicy, err)
	}
	size := t.Size()
	if size > transaction.MaxTransactionSize {
		return fmt.Errorf("%w: (%d > MaxTransactionSize %d)", ErrTxTooBig, size, transaction.MaxTransactionSize)
	}
	needNetworkFee := int64(size) * bc.FeePerByte()
	netFee := t.NetworkFee - needNetworkFee
	if netFee < 0 {
		return fmt.Errorf("%w: net fee is %v, need %v", ErrTxSmallNetworkFee, t.NetworkFee, needNetworkFee)
	}
	if bc.dao.HasTransaction(t.Hash()) {
		return fmt.Errorf("blockchain: %w", ErrAlreadyExists)
	}
	err := bc.verifyTxWitnesses(t, nil)
	if err != nil {
		return err
	}
	err = pool.Add(t, bc)
	if err != nil {
		switch {
		case errors.Is(err, mempool.ErrConflict):
			return ErrMemPoolConflict
		case errors.Is(err, mempool.ErrDup):
			return fmt.Errorf("mempool: %w", ErrAlreadyExists)
		case errors.Is(err, mempool.ErrInsufficientFunds):
			return ErrInsufficientFunds
		case errors.Is(err, mempool.ErrOOM):
			return ErrOOM
		default:
			return err
		}
	}
	if err := bc.verifyTxAttributes(t); err != nil {
		return err
	}

	return nil
}

func (bc *Blockchain) verifyTxAttributes(tx *transaction.Transaction) error {
	for i := range tx.Attributes {
		switch tx.Attributes[i].Type {
		case transaction.HighPriority:
			pubs, err := bc.contracts.NEO.GetCommitteeMembers(bc, bc.dao)
			if err != nil {
				return err
			}
			s, err := smartcontract.CreateMajorityMultiSigRedeemScript(pubs)
			if err != nil {
				return err
			}
			h := hash.Hash160(s)
			for i := range tx.Signers {
				if tx.Signers[i].Account.Equals(h) {
					return nil
				}
			}
			return fmt.Errorf("%w: high priority tx is not signed by committee", ErrInvalidAttribute)
		}
	}
	return nil
}

// isTxStillRelevant is a callback for mempool transaction filtering after the
// new block addition. It returns false for transactions added by the new block
// (passed via txpool) and does witness reverification for non-standard
// contracts. It operates under the assumption that full transaction verification
// was already done so we don't need to check basic things like size, input/output
// correctness, presence in blocks before the new one, etc.
func (bc *Blockchain) isTxStillRelevant(t *transaction.Transaction, txpool *mempool.Pool) bool {
	var recheckWitness bool

	if txpool == nil {
		if bc.dao.HasTransaction(t.Hash()) {
			return false
		}
	} else if txpool.ContainsKey(t.Hash()) {
		return false
	}
	if err := bc.verifyTxAttributes(t); err != nil {
		return false
	}
	for i := range t.Scripts {
		if !vm.IsStandardContract(t.Scripts[i].VerificationScript) {
			recheckWitness = true
			break
		}
	}
	if recheckWitness {
		return bc.verifyTxWitnesses(t, nil) == nil
	}
	return true

}

// AddStateRoot add new (possibly unverified) state root to the blockchain.
func (bc *Blockchain) AddStateRoot(r *state.MPTRoot) error {
	our, err := bc.GetStateRoot(r.Index)
	if err == nil {
		if our.Flag == state.Verified {
			return bc.updateStateHeight(r.Index)
		} else if r.Witness == nil && our.Witness != nil {
			r.Witness = our.Witness
		}
	}
	if err := bc.verifyStateRoot(r); err != nil {
		return fmt.Errorf("invalid state root: %w", err)
	}
	if r.Index > bc.BlockHeight() { // just put it into the store for future checks
		return bc.dao.PutStateRoot(&state.MPTRootState{
			MPTRoot: *r,
			Flag:    state.Unverified,
		})
	}

	flag := state.Unverified
	if r.Witness != nil {
		if err := bc.verifyStateRootWitness(r); err != nil {
			return fmt.Errorf("can't verify signature: %w", err)
		}
		flag = state.Verified
	}
	err = bc.dao.PutStateRoot(&state.MPTRootState{
		MPTRoot: *r,
		Flag:    flag,
	})
	if err != nil {
		return err
	}
	return bc.updateStateHeight(r.Index)
}

func (bc *Blockchain) updateStateHeight(newHeight uint32) error {
	h, err := bc.dao.GetCurrentStateRootHeight()
	if err != nil {
		return fmt.Errorf("can't get current state root height: %w", err)
	} else if newHeight == h+1 {
		updateStateHeightMetric(newHeight)
		return bc.dao.PutCurrentStateRootHeight(h + 1)
	}
	return nil
}

// verifyStateRoot checks if state root is valid.
func (bc *Blockchain) verifyStateRoot(r *state.MPTRoot) error {
	if r.Index == 0 {
		return nil
	}
	prev, err := bc.GetStateRoot(r.Index - 1)
	if err != nil {
		return errors.New("can't get previous state root")
	} else if !r.PrevHash.Equals(hash.DoubleSha256(prev.GetSignedPart())) {
		return errors.New("previous hash mismatch")
	} else if prev.Version != r.Version {
		return errors.New("version mismatch")
	}
	return nil
}

// verifyStateRootWitness verifies that state root signature is correct.
func (bc *Blockchain) verifyStateRootWitness(r *state.MPTRoot) error {
	b, err := bc.GetBlock(bc.GetHeaderHash(int(r.Index)))
	if err != nil {
		return err
	}
	return bc.VerifyWitness(b.NextConsensus, r, r.Witness, bc.contracts.Policy.GetMaxVerificationGas(bc.dao))
}

// VerifyTx verifies whether transaction is bonafide or not relative to the
// current blockchain state. Note that this verification is completely isolated
// from the main node's mempool.
func (bc *Blockchain) VerifyTx(t *transaction.Transaction) error {
	var mp = mempool.New(1)
	bc.lock.RLock()
	defer bc.lock.RUnlock()
	return bc.verifyAndPoolTx(t, mp)
}

// PoolTx verifies and tries to add given transaction into the mempool. If not
// given, the default mempool is used. Passing multiple pools is not supported.
func (bc *Blockchain) PoolTx(t *transaction.Transaction, pools ...*mempool.Pool) error {
	var pool = bc.memPool

	bc.lock.RLock()
	defer bc.lock.RUnlock()
	// Programmer error.
	if len(pools) > 1 {
		panic("too many pools given")
	}
	if len(pools) == 1 {
		pool = pools[0]
	}
	return bc.verifyAndPoolTx(t, pool)
}

//GetStandByValidators returns validators from the configuration.
func (bc *Blockchain) GetStandByValidators() keys.PublicKeys {
	return bc.sbCommittee[:bc.config.ValidatorsCount].Copy()
}

// GetStandByCommittee returns standby committee from the configuration.
func (bc *Blockchain) GetStandByCommittee() keys.PublicKeys {
	return bc.sbCommittee.Copy()
}

// GetValidators returns current validators.
func (bc *Blockchain) GetValidators() ([]*keys.PublicKey, error) {
	return bc.contracts.NEO.GetValidatorsInternal(bc, bc.dao)
}

// GetNextBlockValidators returns next block validators.
func (bc *Blockchain) GetNextBlockValidators() ([]*keys.PublicKey, error) {
	return bc.contracts.NEO.GetNextBlockValidatorsInternal(bc, bc.dao)
}

// GetEnrollments returns all registered validators.
func (bc *Blockchain) GetEnrollments() ([]state.Validator, error) {
	return bc.contracts.NEO.GetCandidates(bc.dao)
}

// GetTestVM returns a VM and a Store setup for a test run of some sort of code.
func (bc *Blockchain) GetTestVM(tx *transaction.Transaction) *vm.VM {
	systemInterop := bc.newInteropContext(trigger.Application, bc.dao, nil, tx)
	vm := systemInterop.SpawnVM()
	vm.SetPriceGetter(getPrice)
	return vm
}

// Various witness verification errors.
var (
	ErrWitnessHashMismatch         = errors.New("witness hash mismatch")
	ErrVerificationFailed          = errors.New("signature check failed")
	ErrUnknownVerificationContract = errors.New("unknown verification contract")
	ErrInvalidVerificationContract = errors.New("verification contract is missing `verify` method")
)

// initVerificationVM initializes VM for witness check.
func initVerificationVM(ic *interop.Context, hash util.Uint160, witness *transaction.Witness) error {
	var offset int
	var initMD *manifest.Method
	verification := witness.VerificationScript
	if len(verification) != 0 {
		if witness.ScriptHash() != hash {
			return ErrWitnessHashMismatch
		}
	} else {
		cs, err := ic.DAO.GetContractState(hash)
		if err != nil {
			return ErrUnknownVerificationContract
		}
		md := cs.Manifest.ABI.GetMethod(manifest.MethodVerify)
		if md == nil {
			return ErrInvalidVerificationContract
		}
		verification = cs.Script
		offset = md.Offset
		initMD = cs.Manifest.ABI.GetMethod(manifest.MethodInit)
	}

	v := ic.VM
	v.LoadScriptWithFlags(verification, smartcontract.NoneFlag)
	v.Jump(v.Context(), offset)
	if initMD != nil {
		v.Call(v.Context(), initMD.Offset)
	}
	v.LoadScript(witness.InvocationScript)
	return nil
}

// VerifyWitness checks that w is a correct witness for c signed by h.
func (bc *Blockchain) VerifyWitness(h util.Uint160, c crypto.Verifiable, w *transaction.Witness, gas int64) error {
	ic := bc.newInteropContext(trigger.Verification, bc.dao, nil, nil)
	ic.Container = c
	return bc.verifyHashAgainstScript(h, w, ic, gas)
}

// verifyHashAgainstScript verifies given hash against the given witness.
func (bc *Blockchain) verifyHashAgainstScript(hash util.Uint160, witness *transaction.Witness, interopCtx *interop.Context, gas int64) error {
	gasPolicy := bc.contracts.Policy.GetMaxVerificationGas(interopCtx.DAO)
	if gas > gasPolicy {
		gas = gasPolicy
	}

	vm := interopCtx.SpawnVM()
	vm.SetPriceGetter(getPrice)
	vm.GasLimit = gas
	if err := initVerificationVM(interopCtx, hash, witness); err != nil {
		return err
	}
	err := vm.Run()
	if vm.HasFailed() {
		return fmt.Errorf("%w: vm execution has failed: %v", ErrVerificationFailed, err)
	}
	resEl := vm.Estack().Pop()
	if resEl != nil {
		res, err := resEl.Item().TryBool()
		if err != nil {
			return fmt.Errorf("%w: invalid return value", ErrVerificationFailed)
		}
		if !res {
			return fmt.Errorf("%w: invalid signature", ErrVerificationFailed)
		}
		if vm.Estack().Len() != 0 {
			return fmt.Errorf("%w: expected exactly one returned value", ErrVerificationFailed)
		}
	} else {
		return fmt.Errorf("%w: no result returned from the script", ErrVerificationFailed)
	}
	return nil
}

// verifyTxWitnesses verifies the scripts (witnesses) that come with a given
// transaction. It can reorder them by ScriptHash, because that's required to
// match a slice of script hashes from the Blockchain. Block parameter
// is used for easy interop access and can be omitted for transactions that are
// not yet added into any block.
// Golang implementation of VerifyWitnesses method in C# (https://github.com/neo-project/neo/blob/master/neo/SmartContract/Helper.cs#L87).
func (bc *Blockchain) verifyTxWitnesses(t *transaction.Transaction, block *block.Block) error {
	if len(t.Signers) != len(t.Scripts) {
		return fmt.Errorf("%w: %d vs %d", ErrTxInvalidWitnessNum, len(t.Signers), len(t.Scripts))
	}
	interopCtx := bc.newInteropContext(trigger.Verification, bc.dao, block, t)
	for i := range t.Signers {
		err := bc.verifyHashAgainstScript(t.Signers[i].Account, &t.Scripts[i], interopCtx, t.NetworkFee)
		if err != nil {
			return fmt.Errorf("witness #%d: %w", i, err)
		}
	}

	return nil
}

// verifyHeaderWitnesses is a block-specific implementation of VerifyWitnesses logic.
func (bc *Blockchain) verifyHeaderWitnesses(currHeader, prevHeader *block.Header) error {
	var hash util.Uint160
	if prevHeader == nil && currHeader.PrevHash.Equals(util.Uint256{}) {
		hash = currHeader.Script.ScriptHash()
	} else {
		hash = prevHeader.NextConsensus
	}
	return bc.VerifyWitness(hash, currHeader, &currHeader.Script, verificationGasLimit)
}

// GoverningTokenHash returns the governing token (NEO) native contract hash.
func (bc *Blockchain) GoverningTokenHash() util.Uint160 {
	return bc.contracts.NEO.Hash
}

// UtilityTokenHash returns the utility token (GAS) native contract hash.
func (bc *Blockchain) UtilityTokenHash() util.Uint160 {
	return bc.contracts.GAS.Hash
}

func hashAndIndexToBytes(h util.Uint256, index uint32) []byte {
	buf := io.NewBufBinWriter()
	buf.WriteBytes(h.BytesLE())
	buf.WriteU32LE(index)
	return buf.Bytes()
}

func (bc *Blockchain) newInteropContext(trigger trigger.Type, d dao.DAO, block *block.Block, tx *transaction.Transaction) *interop.Context {
	ic := interop.NewContext(trigger, bc, d, bc.contracts.Contracts, block, tx, bc.log)
	ic.Functions = [][]interop.Function{systemInterops, neoInterops}
	switch {
	case tx != nil:
		ic.Container = tx
	case block != nil:
		ic.Container = block
	}
	return ic
}