dcrd/mining.go

// Copyright (c) 2014-2016 The btcsuite developers
// Copyright (c) 2015-2018 The Decred developers
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.

package main

import (
	"container/heap"
	"encoding/binary"
	"fmt"
	"math"
	"sort"
	"time"

	"github.com/decred/dcrd/blockchain"
	"github.com/decred/dcrd/blockchain/stake"
	"github.com/decred/dcrd/chaincfg"
	"github.com/decred/dcrd/chaincfg/chainhash"
	"github.com/decred/dcrd/dcrutil"
	"github.com/decred/dcrd/mining"
	"github.com/decred/dcrd/txscript"
	"github.com/decred/dcrd/wire"
)

const (
	// generatedBlockVersion is the version of the block being generated for
	// the main network.  It is defined as a constant here rather than using
	// the wire.BlockVersion constant since a change in the block version
	// will require changes to the generated block.  Using the wire constant
	// for generated block version could allow creation of invalid blocks
	// for the updated version.
	generatedBlockVersion = 5

	// generatedBlockVersionTest is the version of the block being generated
	// for networks other than the main network.
	generatedBlockVersionTest = 6

	// blockHeaderOverhead is the max number of bytes it takes to serialize
	// a block header and max possible transaction count.
	blockHeaderOverhead = wire.MaxBlockHeaderPayload + wire.MaxVarIntPayload

	// coinbaseFlags is some extra data appended to the coinbase script
	// sig.
	coinbaseFlags = "/dcrd/"

	// kilobyte is the size of a kilobyte.
	kilobyte = 1000
)

// txPrioItem houses a transaction along with extra information that allows the
// transaction to be prioritized and track dependencies on other transactions
// which have not been mined into a block yet.
type txPrioItem struct {
	tx       *dcrutil.Tx
	txType   stake.TxType
	fee      int64
	priority float64
	feePerKB float64

	// dependsOn holds a map of transaction hashes which this one depends
	// on.  It will only be set when the transaction references other
	// transactions in the source pool and hence must come after them in
	// a block.
	dependsOn map[chainhash.Hash]struct{}
}

// txPriorityQueueLessFunc describes a function that can be used as a compare
// function for a transaction priority queue (txPriorityQueue).
type txPriorityQueueLessFunc func(*txPriorityQueue, int, int) bool

// txPriorityQueue implements a priority queue of txPrioItem elements that
// supports an arbitrary compare function as defined by txPriorityQueueLessFunc.
type txPriorityQueue struct {
	lessFunc txPriorityQueueLessFunc
	items    []*txPrioItem
}

// Len returns the number of items in the priority queue.  It is part of the
// heap.Interface implementation.
func (pq *txPriorityQueue) Len() int {
	return len(pq.items)
}

// Less returns whether the item in the priority queue with index i should sort
// before the item with index j by deferring to the assigned less function.  It
// is part of the heap.Interface implementation.
func (pq *txPriorityQueue) Less(i, j int) bool {
	return pq.lessFunc(pq, i, j)
}

// Swap swaps the items at the passed indices in the priority queue.  It is
// part of the heap.Interface implementation.
func (pq *txPriorityQueue) Swap(i, j int) {
	pq.items[i], pq.items[j] = pq.items[j], pq.items[i]
}

// Push pushes the passed item onto the priority queue.  It is part of the
// heap.Interface implementation.
func (pq *txPriorityQueue) Push(x interface{}) {
	pq.items = append(pq.items, x.(*txPrioItem))
}

// Pop removes the highest priority item (according to Less) from the priority
// queue and returns it.  It is part of the heap.Interface implementation.
func (pq *txPriorityQueue) Pop() interface{} {
	n := len(pq.items)
	item := pq.items[n-1]
	pq.items[n-1] = nil
	pq.items = pq.items[0 : n-1]
	return item
}

// SetLessFunc sets the compare function for the priority queue to the provided
// function.  It also invokes heap.Init on the priority queue using the new
// function so it can immediately be used with heap.Push/Pop.
func (pq *txPriorityQueue) SetLessFunc(lessFunc txPriorityQueueLessFunc) {
	pq.lessFunc = lessFunc
	heap.Init(pq)
}

// stakePriority is an integer that is used to sort stake transactions
// by importance when they enter the min heap for block construction.
// 2 is for votes (highest), followed by 1 for tickets (2nd highest),
// followed by 0 for regular transactions and revocations (lowest).
type stakePriority int

const (
	regOrRevocPriority stakePriority = iota
	ticketPriority
	votePriority
)

// stakePriority assigns a stake priority based on a transaction type.
func txStakePriority(txType stake.TxType) stakePriority {
	prio := regOrRevocPriority
	switch txType {
	case stake.TxTypeSSGen:
		prio = votePriority
	case stake.TxTypeSStx:
		prio = ticketPriority
	}

	return prio
}

// compareStakePriority compares the stake priority of two transactions.
// It uses votes > tickets > regular transactions or revocations. It
// returns 1 if i > j, 0 if i == j, and -1 if i < j in terms of stake
// priority.
func compareStakePriority(i, j *txPrioItem) int {
	iStakePriority := txStakePriority(i.txType)
	jStakePriority := txStakePriority(j.txType)

	if iStakePriority > jStakePriority {
		return 1
	}
	if iStakePriority < jStakePriority {
		return -1
	}
	return 0
}

// txPQByStakeAndFee sorts a txPriorityQueue by stake priority, followed by
// fees per kilobyte, and then transaction priority.
func txPQByStakeAndFee(pq *txPriorityQueue, i, j int) bool {
	// Sort by stake priority, continue if they're the same stake priority.
	cmp := compareStakePriority(pq.items[i], pq.items[j])
	if cmp == 1 {
		return true
	}
	if cmp == -1 {
		return false
	}

	// Using > here so that pop gives the highest fee item as opposed
	// to the lowest.  Sort by fee first, then priority.
	if pq.items[i].feePerKB == pq.items[j].feePerKB {
		return pq.items[i].priority > pq.items[j].priority
	}

	// The stake priorities are equal, so return based on fees
	// per KB.
	return pq.items[i].feePerKB > pq.items[j].feePerKB
}

// txPQByStakeAndFeeAndThenPriority sorts a txPriorityQueue by stake priority,
// followed by fees per kilobyte, and then if the transaction type is regular
// or a revocation it sorts it by priority.
func txPQByStakeAndFeeAndThenPriority(pq *txPriorityQueue, i, j int) bool {
	// Sort by stake priority, continue if they're the same stake priority.
	cmp := compareStakePriority(pq.items[i], pq.items[j])
	if cmp == 1 {
		return true
	}
	if cmp == -1 {
		return false
	}

	bothAreLowStakePriority :=
		txStakePriority(pq.items[i].txType) == regOrRevocPriority &&
			txStakePriority(pq.items[j].txType) == regOrRevocPriority

	// Use fees per KB on high stake priority transactions.
	if !bothAreLowStakePriority {
		return pq.items[i].feePerKB > pq.items[j].feePerKB
	}

	// Both transactions are of low stake importance. Use > here so that
	// pop gives the highest priority item as opposed to the lowest.
	// Sort by priority first, then fee.
	if pq.items[i].priority == pq.items[j].priority {
		return pq.items[i].feePerKB > pq.items[j].feePerKB
	}

	return pq.items[i].priority > pq.items[j].priority
}

// newTxPriorityQueue returns a new transaction priority queue that reserves the
// passed amount of space for the elements.  The new priority queue uses the
// less than function lessFunc to sort the items in the min heap. The priority
// queue can grow larger than the reserved space, but extra copies of the
// underlying array can be avoided by reserving a sane value.
func newTxPriorityQueue(reserve int, lessFunc func(*txPriorityQueue, int, int) bool) *txPriorityQueue {
	pq := &txPriorityQueue{
		items: make([]*txPrioItem, 0, reserve),
	}
	pq.SetLessFunc(lessFunc)
	return pq
}

// containsTx is a helper function that checks to see if a list of transactions
// contains any of the TxIns of some transaction.
func containsTxIns(txs []*dcrutil.Tx, tx *dcrutil.Tx) bool {
	for _, txToCheck := range txs {
		for _, txIn := range tx.MsgTx().TxIn {
			if txIn.PreviousOutPoint.Hash.IsEqual(txToCheck.Hash()) {
				return true
			}
		}
	}

	return false
}

// blockWithNumVotes is a block with the number of votes currently present
// for that block. Just used for sorting.
type blockWithNumVotes struct {
	Hash     chainhash.Hash
	NumVotes uint16
}

// byNumberOfVotes implements sort.Interface to sort a slice of blocks by their
// number of votes.
type byNumberOfVotes []*blockWithNumVotes

// Len returns the number of elements in the slice.  It is part of the
// sort.Interface implementation.
func (b byNumberOfVotes) Len() int {
	return len(b)
}

// Swap swaps the elements at the passed indices.  It is part of the
// sort.Interface implementation.
func (b byNumberOfVotes) Swap(i, j int) {
	b[i], b[j] = b[j], b[i]
}

// Less returns whether the block with index i should sort before the block with
// index j.  It is part of the sort.Interface implementation.
func (b byNumberOfVotes) Less(i, j int) bool {
	return b[i].NumVotes < b[j].NumVotes
}

// SortParentsByVotes takes a list of block header hashes and sorts them
// by the number of votes currently available for them in the votes map of
// mempool.  It then returns all blocks that are eligible to be used (have
// at least a majority number of votes) sorted by number of votes, descending.
//
// This function is safe for concurrent access.
func SortParentsByVotes(txSource mining.TxSource, currentTopBlock chainhash.Hash, blocks []chainhash.Hash, params *chaincfg.Params) []chainhash.Hash {
	// Return now when no blocks were provided.
	lenBlocks := len(blocks)
	if lenBlocks == 0 {
		return nil
	}

	// Fetch the vote metadata for the provided block hashes from the
	// mempool and filter out any blocks that do not have the minimum
	// required number of votes.
	minVotesRequired := (params.TicketsPerBlock / 2) + 1
	voteMetadata := txSource.VotesForBlocks(blocks)
	filtered := make([]*blockWithNumVotes, 0, lenBlocks)
	for i := range blocks {
		numVotes := uint16(len(voteMetadata[i]))
		if numVotes >= minVotesRequired {
			filtered = append(filtered, &blockWithNumVotes{
				Hash:     blocks[i],
				NumVotes: numVotes,
			})
		}
	}

	// Return now if there are no blocks with enough votes to be eligible to
	// build on top of.
	if len(filtered) == 0 {
		return nil
	}

	// Blocks with the most votes appear at the top of the list.
	sort.Sort(sort.Reverse(byNumberOfVotes(filtered)))
	sortedUsefulBlocks := make([]chainhash.Hash, 0, len(filtered))
	for _, bwnv := range filtered {
		sortedUsefulBlocks = append(sortedUsefulBlocks, bwnv.Hash)
	}

	// Make sure we don't reorganize the chain needlessly if the top block has
	// the same amount of votes as the current leader after the sort. After this
	// point, all blocks listed in sortedUsefulBlocks definitely also have the
	// minimum number of votes required.
	curVoteMetadata := txSource.VotesForBlocks([]chainhash.Hash{currentTopBlock})
	numTopBlockVotes := uint16(len(curVoteMetadata))
	if filtered[0].NumVotes == numTopBlockVotes && filtered[0].Hash !=
		currentTopBlock {

		// Attempt to find the position of the current block being built
		// from in the list.
		pos := 0
		for i, bwnv := range filtered {
			if bwnv.Hash == currentTopBlock {
				pos = i
				break
			}
		}

		// Swap the top block into the first position. We directly access
		// sortedUsefulBlocks useful blocks here with the assumption that
		// since the values were accumulated from filtered, they should be
		// in the same positions and we shouldn't be able to access anything
		// out of bounds.
		if pos != 0 {
			sortedUsefulBlocks[0], sortedUsefulBlocks[pos] =
				sortedUsefulBlocks[pos], sortedUsefulBlocks[0]
		}
	}

	return sortedUsefulBlocks
}

// BlockTemplate houses a block that has yet to be solved along with additional
// details about the fees and the number of signature operations for each
// transaction in the block.
type BlockTemplate struct {
	// Block is a block that is ready to be solved by miners.  Thus, it is
	// completely valid with the exception of satisfying the proof-of-work
	// requirement.
	Block *wire.MsgBlock

	// Fees contains the amount of fees each transaction in the generated
	// template pays in base units.  Since the first transaction is the
	// coinbase, the first entry (offset 0) will contain the negative of the
	// sum of the fees of all other transactions.
	Fees []int64

	// SigOpCounts contains the number of signature operations each
	// transaction in the generated template performs.
	SigOpCounts []int64

	// Height is the height at which the block template connects to the main
	// chain.
	Height int64

	// ValidPayAddress indicates whether or not the template coinbase pays
	// to an address or is redeemable by anyone.  See the documentation on
	// NewBlockTemplate for details on which this can be useful to generate
	// templates without a coinbase payment address.
	ValidPayAddress bool
}

// mergeUtxoView adds all of the entries in view to viewA.  The result is that
// viewA will contain all of its original entries plus all of the entries
// in viewB.  It will replace any entries in viewB which also exist in viewA
// if the entry in viewA is fully spent.
func mergeUtxoView(viewA *blockchain.UtxoViewpoint, viewB *blockchain.UtxoViewpoint) {
	viewAEntries := viewA.Entries()
	for hash, entryB := range viewB.Entries() {
		if entryA, exists := viewAEntries[hash]; !exists ||
			entryA == nil || entryA.IsFullySpent() {
			viewAEntries[hash] = entryB
		}
	}
}

// hashExistsInList checks if a hash exists in a list of hash pointers.
func hashInSlice(h chainhash.Hash, list []chainhash.Hash) bool {
	for i := range list {
		if h == list[i] {
			return true
		}
	}

	return false
}

// txIndexFromTxList returns a transaction's index in a list, or -1 if it
// can not be found.
func txIndexFromTxList(hash chainhash.Hash, list []*dcrutil.Tx) int {
	for i, tx := range list {
		h := tx.Hash()
		if hash == *h {
			return i
		}
	}

	return -1
}

// standardCoinbaseOpReturn creates a standard OP_RETURN output to insert into
// coinbase to use as extranonces. The OP_RETURN pushes 32 bytes.
func standardCoinbaseOpReturn(height uint32, extraNonce uint64) ([]byte, error) {
	enData := make([]byte, 12)
	binary.LittleEndian.PutUint32(enData[0:4], height)
	binary.LittleEndian.PutUint64(enData[4:12], extraNonce)
	extraNonceScript, err := txscript.GenerateProvablyPruneableOut(enData)
	if err != nil {
		return nil, err
	}

	return extraNonceScript, nil
}

// extractCoinbaseTxExtraNonce extracts the extra nonce from a standard coinbase
// OP_RETURN output.  It will return 0 if either the provided transaction does
// not have the relevant output or the script is not large enough to perform the
// extraction.
func extractCoinbaseTxExtraNonce(coinbaseTx *wire.MsgTx) uint64 {
	if len(coinbaseTx.TxOut) < 2 {
		return 0
	}
	script := coinbaseTx.TxOut[1].PkScript
	if len(script) < 14 {
		return 0
	}
	return binary.LittleEndian.Uint64(script[6:14])
}

// extractCoinbaseExtraNonce extracts the extra nonce from a block template's
// coinbase transaction.
func (bt *BlockTemplate) extractCoinbaseExtraNonce() uint64 {
	return extractCoinbaseTxExtraNonce(bt.Block.Transactions[0])
}

// extractCoinbaseExtraNonce extracts the extra nonce from a block template's
// coinbase transaction.
func extractCoinbaseExtraNonce(msgBlock *wire.MsgBlock) uint64 {
	return extractCoinbaseTxExtraNonce(msgBlock.Transactions[0])
}

// UpdateExtraNonce updates the extra nonce in the coinbase script of the passed
// block by regenerating the coinbase script with the passed value and block
// height.  It also recalculates and updates the new merkle root that results
// from changing the coinbase script.
func UpdateExtraNonce(msgBlock *wire.MsgBlock, blockHeight int64, extraNonce uint64) error {
	// First block has no extranonce.
	if blockHeight == 1 {
		return nil
	}

	coinbaseOpReturn, err := standardCoinbaseOpReturn(uint32(blockHeight),
		extraNonce)
	if err != nil {
		return err
	}
	msgBlock.Transactions[0].TxOut[1].PkScript = coinbaseOpReturn

	// TODO(davec): A dcrutil.Block should use saved in the state to avoid
	// recalculating all of the other transaction hashes.
	// block.Transactions[0].InvalidateCache()

	// Recalculate the merkle root with the updated extra nonce.
	block := dcrutil.NewBlockDeepCopyCoinbase(msgBlock)
	merkles := blockchain.BuildMerkleTreeStore(block.Transactions())
	msgBlock.Header.MerkleRoot = *merkles[len(merkles)-1]
	return nil
}

// createCoinbaseTx returns a coinbase transaction paying an appropriate subsidy
// based on the passed block height to the provided address.  When the address
// is nil, the coinbase transaction will instead be redeemable by anyone.
//
// See the comment for NewBlockTemplate for more information about why the nil
// address handling is useful.
func createCoinbaseTx(subsidyCache *blockchain.SubsidyCache, coinbaseScript []byte, opReturnPkScript []byte, nextBlockHeight int64, addr dcrutil.Address, voters uint16, params *chaincfg.Params) (*dcrutil.Tx, error) {
	tx := wire.NewMsgTx()
	tx.AddTxIn(&wire.TxIn{
		// Coinbase transactions have no inputs, so previous outpoint is
		// zero hash and max index.
		PreviousOutPoint: *wire.NewOutPoint(&chainhash.Hash{},
			wire.MaxPrevOutIndex, wire.TxTreeRegular),
		Sequence:        wire.MaxTxInSequenceNum,
		BlockHeight:     wire.NullBlockHeight,
		BlockIndex:      wire.NullBlockIndex,
		SignatureScript: coinbaseScript,
	})

	// Block one is a special block that might pay out tokens to a ledger.
	if nextBlockHeight == 1 && len(params.BlockOneLedger) != 0 {
		// Convert the addresses in the ledger into useable format.
		addrs := make([]dcrutil.Address, len(params.BlockOneLedger))
		for i, payout := range params.BlockOneLedger {
			addr, err := dcrutil.DecodeAddress(payout.Address)
			if err != nil {
				return nil, err
			}
			addrs[i] = addr
		}

		for i, payout := range params.BlockOneLedger {
			// Make payout to this address.
			pks, err := txscript.PayToAddrScript(addrs[i])
			if err != nil {
				return nil, err
			}
			tx.AddTxOut(&wire.TxOut{
				Value:    payout.Amount,
				PkScript: pks,
			})
		}

		tx.TxIn[0].ValueIn = params.BlockOneSubsidy()

		return dcrutil.NewTx(tx), nil
	}

	// Create a coinbase with correct block subsidy and extranonce.
	subsidy := blockchain.CalcBlockWorkSubsidy(subsidyCache,
		nextBlockHeight,
		voters,
		activeNetParams.Params)
	tax := blockchain.CalcBlockTaxSubsidy(subsidyCache,
		nextBlockHeight,
		voters,
		activeNetParams.Params)

	// Tax output.
	if params.BlockTaxProportion > 0 {
		tx.AddTxOut(&wire.TxOut{
			Value:    tax,
			PkScript: params.OrganizationPkScript,
		})
	} else {
		// Tax disabled.
		scriptBuilder := txscript.NewScriptBuilder()
		trueScript, err := scriptBuilder.AddOp(txscript.OP_TRUE).Script()
		if err != nil {
			return nil, err
		}
		tx.AddTxOut(&wire.TxOut{
			Value:    tax,
			PkScript: trueScript,
		})
	}
	// Extranonce.
	tx.AddTxOut(&wire.TxOut{
		Value:    0,
		PkScript: opReturnPkScript,
	})
	// ValueIn.
	tx.TxIn[0].ValueIn = subsidy + tax

	// Create the script to pay to the provided payment address if one was
	// specified.  Otherwise create a script that allows the coinbase to be
	// redeemable by anyone.
	var pksSubsidy []byte
	if addr != nil {
		var err error
		pksSubsidy, err = txscript.PayToAddrScript(addr)
		if err != nil {
			return nil, err
		}
	} else {
		var err error
		scriptBuilder := txscript.NewScriptBuilder()
		pksSubsidy, err = scriptBuilder.AddOp(txscript.OP_TRUE).Script()
		if err != nil {
			return nil, err
		}
	}
	// Subsidy paid to miner.
	tx.AddTxOut(&wire.TxOut{
		Value:    subsidy,
		PkScript: pksSubsidy,
	})

	return dcrutil.NewTx(tx), nil
}

// spendTransaction updates the passed view by marking the inputs to the passed
// transaction as spent.  It also adds all outputs in the passed transaction
// which are not provably unspendable as available unspent transaction outputs.
func spendTransaction(utxoView *blockchain.UtxoViewpoint, tx *dcrutil.Tx, height int64) error {
	for _, txIn := range tx.MsgTx().TxIn {
		originHash := &txIn.PreviousOutPoint.Hash
		originIndex := txIn.PreviousOutPoint.Index
		entry := utxoView.LookupEntry(originHash)
		if entry != nil {
			entry.SpendOutput(originIndex)
		}

	}

	utxoView.AddTxOuts(tx, height, wire.NullBlockIndex)
	return nil
}

// logSkippedDeps logs any dependencies which are also skipped as a result of
// skipping a transaction while generating a block template at the trace level.
func logSkippedDeps(tx *dcrutil.Tx, deps map[chainhash.Hash]*txPrioItem) {
	if deps == nil {
		return
	}

	for _, item := range deps {
		minrLog.Tracef("Skipping tx %s since it depends on %s\n",
			item.tx.Hash(), tx.Hash())
	}
}

// minimumMedianTime returns the minimum allowed timestamp for a block building
// on the end of the current best chain.  In particular, it is one second after
// the median timestamp of the last several blocks per the chain consensus
// rules.
func minimumMedianTime(best *blockchain.BestState) time.Time {
	return best.MedianTime.Add(time.Second)
}

// medianAdjustedTime returns the current time adjusted to ensure it is at least
// one second after the median timestamp of the last several blocks per the
// chain consensus rules.
func medianAdjustedTime(best *blockchain.BestState, timeSource blockchain.MedianTimeSource) time.Time {
	// The timestamp for the block must not be before the median timestamp
	// of the last several blocks.  Thus, choose the maximum between the
	// current time and one second after the past median time.  The current
	// timestamp is truncated to a second boundary before comparison since a
	// block timestamp does not support a precision greater than one second.
	newTimestamp := timeSource.AdjustedTime()
	minTimestamp := minimumMedianTime(best)
	if newTimestamp.Before(minTimestamp) {
		newTimestamp = minTimestamp
	}

	// Adjust by the amount requested from the command line argument.
	newTimestamp = newTimestamp.Add(
		time.Duration(-cfg.MiningTimeOffset) * time.Second)

	return newTimestamp
}

// maybeInsertStakeTx checks to make sure that a stake tx is
// valid from the perspective of the mainchain (not necessarily
// the mempool or block) before inserting into a tx tree.
// If it fails the check, it returns false; otherwise true.
func maybeInsertStakeTx(bm *blockManager, stx *dcrutil.Tx, treeValid bool) bool {
	missingInput := false

	view, err := bm.chain.FetchUtxoView(stx, treeValid)
	if err != nil {
		minrLog.Warnf("Unable to fetch transaction store for "+
			"stx %s: %v", stx.Hash(), err)
		return false
	}
	mstx := stx.MsgTx()
	isSSGen := stake.IsSSGen(mstx)
	for i, txIn := range mstx.TxIn {
		// Evaluate if this is a stakebase input or not. If it
		// is, continue without evaluation of the input.
		// if isStakeBase
		if isSSGen && (i == 0) {
			txIn.BlockHeight = wire.NullBlockHeight
			txIn.BlockIndex = wire.NullBlockIndex

			continue
		}

		originHash := &txIn.PreviousOutPoint.Hash
		utxIn := view.LookupEntry(originHash)
		if utxIn == nil {
			missingInput = true
			break
		} else {
			originIdx := txIn.PreviousOutPoint.Index
			txIn.ValueIn = utxIn.AmountByIndex(originIdx)
			txIn.BlockHeight = uint32(utxIn.BlockHeight())
			txIn.BlockIndex = utxIn.BlockIndex()
		}
	}
	return !missingInput
}

// deepCopyBlockTemplate returns a deeply copied block template that copies all
// data except a block's references to transactions, which are kept as pointers
// in the block. This is considered safe because transaction data is generally
// immutable, with the exception of coinbases which we alternatively also
// deep copy.
func deepCopyBlockTemplate(blockTemplate *BlockTemplate) *BlockTemplate {
	if blockTemplate == nil {
		return nil
	}

	// Deep copy the header, which we hash on.
	headerCopy := blockTemplate.Block.Header

	// Copy transactions pointers. Duplicate the coinbase
	// transaction, because it might update it by modifying
	// the extra nonce.
	transactionsCopy := make([]*wire.MsgTx, len(blockTemplate.Block.Transactions))
	coinbaseCopy :=
		dcrutil.NewTxDeep(blockTemplate.Block.Transactions[0])
	for i, mtx := range blockTemplate.Block.Transactions {
		if i == 0 {
			transactionsCopy[i] = coinbaseCopy.MsgTx()
		} else {
			transactionsCopy[i] = mtx
		}
	}

	sTransactionsCopy := make([]*wire.MsgTx, len(blockTemplate.Block.STransactions))
	copy(sTransactionsCopy, blockTemplate.Block.STransactions)

	msgBlockCopy := &wire.MsgBlock{
		Header:        headerCopy,
		Transactions:  transactionsCopy,
		STransactions: sTransactionsCopy,
	}

	fees := make([]int64, len(blockTemplate.Fees))
	copy(fees, blockTemplate.Fees)

	sigOps := make([]int64, len(blockTemplate.SigOpCounts))
	copy(sigOps, blockTemplate.SigOpCounts)

	return &BlockTemplate{
		Block:           msgBlockCopy,
		Fees:            fees,
		SigOpCounts:     sigOps,
		Height:          blockTemplate.Height,
		ValidPayAddress: blockTemplate.ValidPayAddress,
	}
}

// handleTooFewVoters handles the situation in which there are too few voters on
// of the blockchain. If there are too few voters and a cached parent template to
// work off of is present, it will return a copy of that template to pass to the
// miner.
// Safe for concurrent access.
func handleTooFewVoters(subsidyCache *blockchain.SubsidyCache, nextHeight int64, miningAddress dcrutil.Address, bm *blockManager) (*BlockTemplate, error) {
	timeSource := bm.server.timeSource
	stakeValidationHeight := bm.server.chainParams.StakeValidationHeight
	curTemplate := bm.GetCurrentTemplate()

	// Check to see if we've fallen off the chain, for example if a
	// reorganization had recently occurred. If this is the case,
	// nuke the templates.
	best := bm.chain.BestSnapshot()
	if curTemplate != nil {
		if !best.PrevHash.IsEqual(
			&curTemplate.Block.Header.PrevBlock) {
			minrLog.Debugf("Cached mining templates are no longer current, " +
				"resetting")
			bm.SetCurrentTemplate(nil)
			bm.SetParentTemplate(nil)
		}
	}

	// Handle not enough voters being present if we're set to mine aggressively
	// (default behaviour).
	if nextHeight >= stakeValidationHeight {
		if bm.AggressiveMining {
			if curTemplate != nil {
				cptCopy := deepCopyBlockTemplate(curTemplate)

				// Update the timestamp of the old template.
				ts := medianAdjustedTime(best, timeSource)
				cptCopy.Block.Header.Timestamp = ts

				// If we're on testnet, the time since this last block
				// listed as the parent must be taken into consideration.
				if bm.server.chainParams.ReduceMinDifficulty {
					parentHash := cptCopy.Block.Header.PrevBlock

					requiredDifficulty, err :=
						bm.CalcNextRequiredDiffNode(&parentHash, ts)
					if err != nil {
						return nil, miningRuleError(ErrGettingDifficulty,
							err.Error())
					}

					cptCopy.Block.Header.Bits = requiredDifficulty
				}

				// Choose a new extra nonce value that is one greater
				// than the previous extra nonce, so we don't remine the
				// same block and choose the same winners as before.
				en := cptCopy.extractCoinbaseExtraNonce() + 1
				err := UpdateExtraNonce(cptCopy.Block, cptCopy.Height, en)
				if err != nil {
					return nil, err
				}

				// Update extranonce of the original template too, so
				// we keep getting unique numbers.
				err = UpdateExtraNonce(curTemplate.Block, curTemplate.Height, en)
				if err != nil {
					return nil, err
				}

				// Make sure the block validates.
				block := dcrutil.NewBlockDeepCopyCoinbase(cptCopy.Block)
				err = bm.chain.CheckConnectBlockTemplate(block)
				if err != nil {
					minrLog.Errorf("failed to check template while "+
						"duplicating a parent: %v", err.Error())
					return nil, miningRuleError(ErrCheckConnectBlock,
						err.Error())
				}

				return cptCopy, nil
			}

			// We may have just started mining and stored the current block
			// template, so we don't have a parent.
			if curTemplate == nil {
				// Fetch the latest block and head and begin working
				// off of it with an empty transaction tree regular
				// and the contents of that stake tree. In the future
				// we should have the option of readding some
				// transactions from this block, too.
				topBlock, err := bm.chain.BlockByHash(&best.Hash)
				if err != nil {
					str := fmt.Sprintf("unable to get tip block %s",
						best.PrevHash)
					return nil, miningRuleError(ErrGetTopBlock, str)
				}
				btMsgBlock := new(wire.MsgBlock)
				rand, err := wire.RandomUint64()
				if err != nil {
					return nil, err
				}
				coinbaseScript := make([]byte, len(coinbaseFlags)+2)
				copy(coinbaseScript[2:], coinbaseFlags)
				opReturnPkScript, err :=
					standardCoinbaseOpReturn(topBlock.MsgBlock().Header.Height,
						rand)
				if err != nil {
					return nil, err
				}
				coinbaseTx, err := createCoinbaseTx(subsidyCache,
					coinbaseScript,
					opReturnPkScript,
					topBlock.Height(),
					miningAddress,
					topBlock.MsgBlock().Header.Voters,
					bm.server.chainParams)
				if err != nil {
					return nil, err
				}
				btMsgBlock.AddTransaction(coinbaseTx.MsgTx())

				for _, stx := range topBlock.STransactions() {
					btMsgBlock.AddSTransaction(stx.MsgTx())
				}

				// Copy the rest of the header.
				btMsgBlock.Header = topBlock.MsgBlock().Header

				// Set a fresh timestamp.
				ts := medianAdjustedTime(best, timeSource)
				btMsgBlock.Header.Timestamp = ts

				// If we're on testnet, the time since this last block
				// listed as the parent must be taken into consideration.
				if bm.server.chainParams.ReduceMinDifficulty {
					parentHash := topBlock.MsgBlock().Header.PrevBlock

					requiredDifficulty, err :=
						bm.CalcNextRequiredDiffNode(&parentHash, ts)
					if err != nil {
						return nil, miningRuleError(ErrGettingDifficulty,
							err.Error())
					}

					btMsgBlock.Header.Bits = requiredDifficulty
				}

				// Recalculate the size.
				btMsgBlock.Header.Size = uint32(btMsgBlock.SerializeSize())

				bt := &BlockTemplate{
					Block:           btMsgBlock,
					Fees:            []int64{0},
					SigOpCounts:     []int64{0},
					Height:          int64(topBlock.MsgBlock().Header.Height),
					ValidPayAddress: miningAddress != nil,
				}

				// Recalculate the merkle roots. Use a temporary 'immutable'
				// block object as we're changing the header contents.
				btBlockTemp := dcrutil.NewBlockDeepCopyCoinbase(btMsgBlock)
				merkles :=
					blockchain.BuildMerkleTreeStore(btBlockTemp.Transactions())
				merklesStake :=
					blockchain.BuildMerkleTreeStore(btBlockTemp.STransactions())
				btMsgBlock.Header.MerkleRoot = *merkles[len(merkles)-1]
				btMsgBlock.Header.StakeRoot = *merklesStake[len(merklesStake)-1]

				// Make sure the block validates.
				btBlock := dcrutil.NewBlockDeepCopyCoinbase(btMsgBlock)
				err = bm.chain.CheckConnectBlockTemplate(btBlock)
				if err != nil {
					str := fmt.Sprintf("failed to check template: %v while "+
						"constructing a new parent", err.Error())
					return nil, miningRuleError(ErrCheckConnectBlock,
						str)
				}

				// Make a copy to return.
				cptCopy := deepCopyBlockTemplate(bt)

				return cptCopy, nil
			}
		}
	}

	bmgrLog.Debugf("Not enough voters on top block to generate " +
		"new block template")

	return nil, nil
}

// handleCreatedBlockTemplate stores a successfully created block template to
// the appropriate cache if needed, then returns the template to the miner to
// work on. The stored template is a copy of the template, to prevent races
// from occurring in case the template is mined on by the CPUminer.
func handleCreatedBlockTemplate(blockTemplate *BlockTemplate, bm *blockManager) (*BlockTemplate, error) {
	curTemplate := bm.GetCurrentTemplate()
	nextBlockHeight := blockTemplate.Height
	stakeValidationHeight := bm.server.chainParams.StakeValidationHeight
	// This is where we begin storing block templates, when either the
	// program is freshly started or the chain is matured to stake
	// validation height.
	if curTemplate == nil && nextBlockHeight >= stakeValidationHeight-2 {
		bm.SetCurrentTemplate(blockTemplate)
	}

	// We're at the height where the next block needs to include SSGens,
	// so we check to if CachedCurrentTemplate is out of date. If it is,
	// we store it as the cached parent template, and store the new block
	// template as the currenct template.
	if curTemplate != nil && nextBlockHeight >= stakeValidationHeight-1 {
		if curTemplate.Height < nextBlockHeight {
			bm.SetParentTemplate(curTemplate)
			bm.SetCurrentTemplate(blockTemplate)
		}
	}

	// Overwrite the old cached block if it's out of date.
	if curTemplate != nil {
		if curTemplate.Height == nextBlockHeight {
			bm.SetCurrentTemplate(blockTemplate)
		}
	}

	return blockTemplate, nil
}

// BlkTmplGenerator generates block templates based on a given mining policy
// and a transactions source. It also houses additional state required in
// order to ensure the templates adhere to the consensus rules and are built
// on top of the best chain tip or its parent if the best chain tip is
// unable to get enough votes.
//
// See the NewBlockTemplate method for a detailed description of how the block
// template is generated.
type BlkTmplGenerator struct {
	policy       *mining.Policy
	txSource     mining.TxSource
	sigCache     *txscript.SigCache
	chainParams  *chaincfg.Params
	chain        *blockchain.BlockChain
	blockManager *blockManager
	timeSource   blockchain.MedianTimeSource
}

// newBlkTmplGenerator returns a new block template generator for the given
// policy using transactions from the provided transaction source.
func newBlkTmplGenerator(policy *mining.Policy, txSource mining.TxSource,
	timeSource blockchain.MedianTimeSource, sigCache *txscript.SigCache,
	chainParams *chaincfg.Params, chain *blockchain.BlockChain,
	blockManager *blockManager) *BlkTmplGenerator {
	return &BlkTmplGenerator{
		policy:       policy,
		txSource:     txSource,
		sigCache:     sigCache,
		chainParams:  chainParams,
		chain:        chain,
		blockManager: blockManager,
		timeSource:   timeSource,
	}
}

// NewBlockTemplate returns a new block template that is ready to be solved
// using the transactions from the passed transaction source pool and a coinbase
// that either pays to the passed address if it is not nil, or a coinbase that
// is redeemable by anyone if the passed address is nil.  The nil address
// functionality is useful since there are cases such as the getblocktemplate
// RPC where external mining software is responsible for creating their own
// coinbase which will replace the one generated for the block template.  Thus
// the need to have configured address can be avoided.
//
// The transactions selected and included are prioritized according to several
// factors.  First, each transaction has a priority calculated based on its
// value, age of inputs, and size.  Transactions which consist of larger
// amounts, older inputs, and small sizes have the highest priority.  Second, a
// fee per kilobyte is calculated for each transaction.  Transactions with a
// higher fee per kilobyte are preferred.  Finally, the block generation related
// policy settings are all taken into account.
//
// Transactions which only spend outputs from other transactions already in the
// block chain are immediately added to a priority queue which either
// prioritizes based on the priority (then fee per kilobyte) or the fee per
// kilobyte (then priority) depending on whether or not the BlockPrioritySize
// policy setting allots space for high-priority transactions.  Transactions
// which spend outputs from other transactions in the source pool are added to a
// dependency map so they can be added to the priority queue once the
// transactions they depend on have been included.
//
// Once the high-priority area (if configured) has been filled with
// transactions, or the priority falls below what is considered high-priority,
// the priority queue is updated to prioritize by fees per kilobyte (then
// priority).
//
// When the fees per kilobyte drop below the TxMinFreeFee policy setting, the
// transaction will be skipped unless the BlockMinSize policy setting is
// nonzero, in which case the block will be filled with the low-fee/free
// transactions until the block size reaches that minimum size.
//
// Any transactions which would cause the block to exceed the BlockMaxSize
// policy setting, exceed the maximum allowed signature operations per block, or
// otherwise cause the block to be invalid are skipped.
//
// Given the above, a block generated by this function is of the following form:
//
//   -----------------------------------  --  --
//  |      Coinbase Transaction         |   |   |
//  |-----------------------------------|   |   |
//  |                                   |   |   | ----- policy.BlockPrioritySize
//  |   High-priority Transactions      |   |   |
//  |                                   |   |   |
//  |-----------------------------------|   | --
//  |                                   |   |
//  |                                   |   |
//  |                                   |   |--- (policy.BlockMaxSize) / 2
//  |  Transactions prioritized by fee  |   |
//  |  until <= policy.TxMinFreeFee     |   |
//  |                                   |   |
//  |                                   |   |
//  |                                   |   |
//  |-----------------------------------|   |
//  |  Low-fee/Non high-priority (free) |   |
//  |  transactions (while block size   |   |
//  |  <= policy.BlockMinSize)          |   |
//   -----------------------------------  --
//
// Which also includes a stake tree that looks like the following:
//
//   -----------------------------------  --  --
//  |                                   |   |   |
//  |             Votes                 |   |   | --- >= (chaincfg.TicketsPerBlock/2) + 1
//  |                                   |   |   |
//  |-----------------------------------|   | --
//  |                                   |   |   |
//  |            Tickets                |   |   | --- <= chaincfg.MaxFreshStakePerBlock
//  |                                   |   |   |
//  |-----------------------------------|   | --
//  |                                   |   |
//  |          Revocations              |   |
//  |                                   |   |
//   -----------------------------------  --
//
//  This function returns nil, nil if there are not enough voters on any of
//  the current top blocks to create a new block template.
func (g *BlkTmplGenerator) NewBlockTemplate(payToAddress dcrutil.Address) (*BlockTemplate, error) {
	subsidyCache := g.chain.FetchSubsidyCache()

	// All transaction scripts are verified using the more strict standarad
	// flags.
	scriptFlags, err := standardScriptVerifyFlags(g.chain)
	if err != nil {
		return nil, err
	}

	// Extend the most recently known best block.
	// The most recently known best block is the top block that has the most
	// ssgen votes for it. We only need this after the height in which stake voting
	// has kicked in.
	// To figure out which block has the most ssgen votes, we need to run the
	// following algorithm:
	// 1. Acquire the HEAD block and all of its orphans. Record their block header
	// hashes.
	// 2. Create a map of [blockHeaderHash] --> [mempoolTxnList].
	// 3. for blockHeaderHash in candidateBlocks:
	//		if mempoolTx.StakeDesc == SSGen &&
	//			mempoolTx.SSGenParseBlockHeader() == blockHeaderHash:
	//			map[blockHeaderHash].append(mempoolTx)
	// 4. Check len of each map entry and store.
	// 5. Query the ticketdb and check how many eligible ticket holders there are
	//    for the given block you are voting on.
	// 6. Divide #ofvotes (len(map entry)) / totalPossibleVotes --> penalty ratio
	// 7. Store penalty ratios for all block candidates.
	// 8. Select the one with the largest penalty ratio (highest block reward).
	//    This block is then selected to build upon instead of the others, because
	//    it yields the greater amount of rewards.

	best := g.chain.BestSnapshot()
	prevHash := best.Hash
	nextBlockHeight := best.Height + 1
	stakeValidationHeight := g.chainParams.StakeValidationHeight

	if nextBlockHeight >= stakeValidationHeight {
		// Obtain the entire generation of blocks stemming from this parent.
		children, err := g.blockManager.TipGeneration()
		if err != nil {
			return nil, miningRuleError(ErrFailedToGetGeneration, err.Error())
		}

		// Get the list of blocks that we can actually build on top of. If we're
		// not currently on the block that has the most votes, switch to that
		// block.
		eligibleParents := SortParentsByVotes(g.txSource, prevHash, children,
			g.chainParams)
		if len(eligibleParents) == 0 {
			minrLog.Debugf("Too few voters found on any HEAD block, " +
				"recycling a parent block to mine on")
			return handleTooFewVoters(subsidyCache, nextBlockHeight,
				payToAddress, g.blockManager)
		}

		minrLog.Debugf("Found eligible parent %v with enough votes to build "+
			"block on, proceeding to create a new block template",
			eligibleParents[0])

		// Force a reorganization to the parent with the most votes if we need
		// to.
		if eligibleParents[0] != prevHash {
			for i := range eligibleParents {
				newHead := &eligibleParents[i]
				err := g.blockManager.ForceReorganization(prevHash, *newHead)
				if err != nil {
					minrLog.Errorf("failed to reorganize to new parent: %v", err)
					continue
				}

				// Check to make sure we actually have the transactions
				// (votes) we need in the mempool.
				voteHashes := g.txSource.VoteHashesForBlock(newHead)
				if len(voteHashes) == 0 {
					return nil, fmt.Errorf("no vote metadata for block %v",
						newHead)
				}

				if exist := g.txSource.HaveAllTransactions(voteHashes); !exist {
					continue
				} else {
					prevHash = *newHead
					break
				}
			}
		}
	}

	// Get the current source transactions and create a priority queue to
	// hold the transactions which are ready for inclusion into a block
	// along with some priority related and fee metadata.  Reserve the same
	// number of items that are available for the priority queue.  Also,
	// choose the initial sort order for the priority queue based on whether
	// or not there is an area allocated for high-priority transactions.
	sourceTxns := g.txSource.MiningDescs()
	sortedByFee := g.policy.BlockPrioritySize == 0
	lessFunc := txPQByStakeAndFeeAndThenPriority
	if sortedByFee {
		lessFunc = txPQByStakeAndFee
	}
	priorityQueue := newTxPriorityQueue(len(sourceTxns), lessFunc)

	// Create a slice to hold the transactions to be included in the
	// generated block with reserved space.  Also create a utxo view to
	// house all of the input transactions so multiple lookups can be
	// avoided.
	blockTxns := make([]*dcrutil.Tx, 0, len(sourceTxns))
	blockUtxos := blockchain.NewUtxoViewpoint()

	// dependers is used to track transactions which depend on another
	// transaction in the source pool.  This, in conjunction with the
	// dependsOn map kept with each dependent transaction helps quickly
	// determine which dependent transactions are now eligible for inclusion
	// in the block once each transaction has been included.
	dependers := make(map[chainhash.Hash]map[chainhash.Hash]*txPrioItem)

	// Create slices to hold the fees and number of signature operations
	// for each of the selected transactions and add an entry for the
	// coinbase.  This allows the code below to simply append details about
	// a transaction as it is selected for inclusion in the final block.
	// However, since the total fees aren't known yet, use a dummy value for
	// the coinbase fee which will be updated later.
	txFees := make([]int64, 0, len(sourceTxns))
	txFeesMap := make(map[chainhash.Hash]int64)
	txSigOpCounts := make([]int64, 0, len(sourceTxns))
	txSigOpCountsMap := make(map[chainhash.Hash]int64)
	txFees = append(txFees, -1) // Updated once known

	minrLog.Debugf("Considering %d transactions for inclusion to new block",
		len(sourceTxns))
	treeKnownInvalid := g.txSource.IsTxTreeKnownInvalid(&prevHash)

mempoolLoop:
	for _, txDesc := range sourceTxns {
		// A block can't have more than one coinbase or contain
		// non-finalized transactions.
		tx := txDesc.Tx
		msgTx := tx.MsgTx()
		if blockchain.IsCoinBaseTx(msgTx) {
			minrLog.Tracef("Skipping coinbase tx %s", tx.Hash())
			continue
		}
		if !blockchain.IsFinalizedTransaction(tx, nextBlockHeight,
			best.MedianTime) {
			minrLog.Tracef("Skipping non-finalized tx %s", tx.Hash())
			continue
		}

		// Need this for a check below for stake base input, and to check
		// the ticket number.
		isSSGen := txDesc.Type == stake.TxTypeSSGen
		if isSSGen {
			blockHash, blockHeight := stake.SSGenBlockVotedOn(msgTx)
			if !((blockHash == prevHash) &&
				(int64(blockHeight) == nextBlockHeight-1)) {
				minrLog.Tracef("Skipping ssgen tx %s because it does "+
					"not vote on the correct block", tx.Hash())
				continue
			}
		}

		// Fetch all of the utxos referenced by the this transaction.
		// NOTE: This intentionally does not fetch inputs from the
		// mempool since a transaction which depends on other
		// transactions in the mempool must come after those
		utxos, err := g.chain.FetchUtxoView(tx, !treeKnownInvalid)
		if err != nil {
			minrLog.Warnf("Unable to fetch utxo view for tx %s: "+
				"%v", tx.Hash(), err)
			continue
		}

		// Setup dependencies for any transactions which reference
		// other transactions in the mempool so they can be properly
		// ordered below.
		prioItem := &txPrioItem{tx: txDesc.Tx, txType: txDesc.Type}
		for i, txIn := range tx.MsgTx().TxIn {
			// Evaluate if this is a stakebase input or not. If it is, continue
			// without evaluation of the input.
			// if isStakeBase
			if isSSGen && (i == 0) {
				continue
			}

			originHash := &txIn.PreviousOutPoint.Hash
			originIndex := txIn.PreviousOutPoint.Index
			utxoEntry := utxos.LookupEntry(originHash)
			if utxoEntry == nil || utxoEntry.IsOutputSpent(originIndex) {
				if !g.txSource.HaveTransaction(originHash) {
					minrLog.Tracef("Skipping tx %s because "+
						"it references unspent output "+
						"%s which is not available",
						tx.Hash(), txIn.PreviousOutPoint)
					continue mempoolLoop
				}

				// The transaction is referencing another
				// transaction in the source pool, so setup an
				// ordering dependency.
				deps, exists := dependers[*originHash]
				if !exists {
					deps = make(map[chainhash.Hash]*txPrioItem)
					dependers[*originHash] = deps
				}
				deps[*prioItem.tx.Hash()] = prioItem
				if prioItem.dependsOn == nil {
					prioItem.dependsOn = make(
						map[chainhash.Hash]struct{})
				}
				prioItem.dependsOn[*originHash] = struct{}{}

				// Skip the check below. We already know the
				// referenced transaction is available.
				continue
			}
		}

		// Calculate the final transaction priority using the input
		// value age sum as well as the adjusted transaction size.  The
		// formula is: sum(inputValue * inputAge) / adjustedTxSize
		prioItem.priority = mining.CalcPriority(tx.MsgTx(), utxos,
			nextBlockHeight)

		// Calculate the fee in Atoms/KB.
		// NOTE: This is a more precise value than the one calculated
		// during calcMinRelayFee which rounds up to the nearest full
		// kilobyte boundary.  This is beneficial since it provides an
		// incentive to create smaller transactions.
		txSize := tx.MsgTx().SerializeSize()
		prioItem.feePerKB = (float64(txDesc.Fee) * float64(kilobyte)) /
			float64(txSize)
		prioItem.fee = txDesc.Fee

		// Add the transaction to the priority queue to mark it ready
		// for inclusion in the block unless it has dependencies.
		if prioItem.dependsOn == nil {
			heap.Push(priorityQueue, prioItem)
		}

		// Merge the referenced outputs from the input transactions to
		// this transaction into the block utxo view.  This allows the
		// code below to avoid a second lookup.
		mergeUtxoView(blockUtxos, utxos)
	}

	minrLog.Tracef("Priority queue len %d, dependers len %d",
		priorityQueue.Len(), len(dependers))

	// The starting block size is the size of the block header plus the max
	// possible transaction count size, plus the size of the coinbase
	// transaction.
	blockSize := uint32(blockHeaderOverhead)

	// Guesstimate for sigops based on valid txs in loop below. This number
	// tends to overestimate sigops because of the way the loop below is
	// coded and the fact that tx can sometimes be removed from the tx
	// trees if they fail one of the stake checks below the priorityQueue
	// pop loop. This is buggy, but not catastrophic behaviour. A future
	// release should fix it. TODO
	blockSigOps := int64(0)
	totalFees := int64(0)

	numSStx := 0

	foundWinningTickets := make(map[chainhash.Hash]bool, len(best.NextWinningTickets))
	for _, ticketHash := range best.NextWinningTickets {
		foundWinningTickets[ticketHash] = false
	}

	// Choose which transactions make it into the block.
	for priorityQueue.Len() > 0 {
		// Grab the highest priority (or highest fee per kilobyte
		// depending on the sort order) transaction.
		prioItem := heap.Pop(priorityQueue).(*txPrioItem)
		tx := prioItem.tx

		// Store if this is an SStx or not.
		isSStx := prioItem.txType == stake.TxTypeSStx

		// Store if this is an SSGen or not.
		isSSGen := prioItem.txType == stake.TxTypeSSGen

		// Store if this is an SSRtx or not.
		isSSRtx := prioItem.txType == stake.TxTypeSSRtx

		// Grab the list of transactions which depend on this one (if any).
		deps := dependers[*tx.Hash()]

		// Skip if we already have too many SStx.
		if isSStx && (numSStx >=
			int(g.chainParams.MaxFreshStakePerBlock)) {
			minrLog.Tracef("Skipping sstx %s because it would exceed "+
				"the max number of sstx allowed in a block", tx.Hash())
			logSkippedDeps(tx, deps)
			continue
		}

		// Skip if the SStx commit value is below the value required by the
		// stake diff.
		if isSStx && (tx.MsgTx().TxOut[0].Value < best.NextStakeDiff) {
			continue
		}

		// Skip all missed tickets that we've never heard of.
		if isSSRtx {
			ticketHash := &tx.MsgTx().TxIn[0].PreviousOutPoint.Hash

			if !hashInSlice(*ticketHash, best.MissedTickets) {
				continue
			}
		}

		// Enforce maximum block size.  Also check for overflow.
		txSize := uint32(tx.MsgTx().SerializeSize())
		blockPlusTxSize := blockSize + txSize
		if blockPlusTxSize < blockSize ||
			blockPlusTxSize >= g.policy.BlockMaxSize {
			minrLog.Tracef("Skipping tx %s (size %v) because it "+
				"would exceed the max block size; cur block "+
				"size %v, cur num tx %v", tx.Hash(), txSize,
				blockSize, len(blockTxns))
			logSkippedDeps(tx, deps)
			continue
		}

		// Enforce maximum signature operations per block.  Also check
		// for overflow.
		numSigOps := int64(blockchain.CountSigOps(tx, false, isSSGen))
		if blockSigOps+numSigOps < blockSigOps ||
			blockSigOps+numSigOps > blockchain.MaxSigOpsPerBlock {
			minrLog.Tracef("Skipping tx %s because it would "+
				"exceed the maximum sigops per block", tx.Hash())
			logSkippedDeps(tx, deps)
			continue
		}

		// This isn't very expensive, but we do this check a number of times.
		// Consider caching this in the mempool in the future. - Decred
		numP2SHSigOps, err := blockchain.CountP2SHSigOps(tx, false,
			isSSGen, blockUtxos)
		if err != nil {
			minrLog.Tracef("Skipping tx %s due to error in "+
				"CountP2SHSigOps: %v", tx.Hash(), err)
			logSkippedDeps(tx, deps)
			continue
		}
		numSigOps += int64(numP2SHSigOps)
		if blockSigOps+numSigOps < blockSigOps ||
			blockSigOps+numSigOps > blockchain.MaxSigOpsPerBlock {
			minrLog.Tracef("Skipping tx %s because it would "+
				"exceed the maximum sigops per block (p2sh)",
				tx.Hash())
			logSkippedDeps(tx, deps)
			continue
		}

		// Check to see if the SSGen tx actually uses a ticket that is
		// valid for the next block.
		if isSSGen {
			if foundWinningTickets[tx.MsgTx().TxIn[1].PreviousOutPoint.Hash] {
				continue
			}
			msgTx := tx.MsgTx()
			isEligible := false
			for _, sstxHash := range best.NextWinningTickets {
				if sstxHash.IsEqual(&msgTx.TxIn[1].PreviousOutPoint.Hash) {
					isEligible = true
				}
			}

			if !isEligible {
				continue
			}
		}

		// Skip free transactions once the block is larger than the
		// minimum block size, except for stake transactions.
		if sortedByFee &&
			(prioItem.feePerKB < float64(g.policy.TxMinFreeFee)) &&
			(tx.Tree() != wire.TxTreeStake) &&
			(blockPlusTxSize >= g.policy.BlockMinSize) {

			minrLog.Tracef("Skipping tx %s with feePerKB %.2f "+
				"< TxMinFreeFee %d and block size %d >= "+
				"minBlockSize %d", tx.Hash(), prioItem.feePerKB,
				g.policy.TxMinFreeFee, blockPlusTxSize,
				g.policy.BlockMinSize)
			logSkippedDeps(tx, deps)
			continue
		}

		// Prioritize by fee per kilobyte once the block is larger than
		// the priority size or there are no more high-priority
		// transactions.
		if !sortedByFee && (blockPlusTxSize >= g.policy.BlockPrioritySize ||
			prioItem.priority <= mining.MinHighPriority) {

			minrLog.Tracef("Switching to sort by fees per "+
				"kilobyte blockSize %d >= BlockPrioritySize "+
				"%d || priority %.2f <= minHighPriority %.2f",
				blockPlusTxSize, g.policy.BlockPrioritySize,
				prioItem.priority, mining.MinHighPriority)

			sortedByFee = true
			priorityQueue.SetLessFunc(txPQByStakeAndFee)

			// Put the transaction back into the priority queue and
			// skip it so it is re-priortized by fees if it won't
			// fit into the high-priority section or the priority is
			// too low.  Otherwise this transaction will be the
			// final one in the high-priority section, so just fall
			// though to the code below so it is added now.
			if blockPlusTxSize > g.policy.BlockPrioritySize ||
				prioItem.priority < mining.MinHighPriority {

				heap.Push(priorityQueue, prioItem)
				continue
			}
		}

		// Ensure the transaction inputs pass all of the necessary
		// preconditions before allowing it to be added to the block.
		// The fraud proof is not checked because it will be filled in
		// by the miner.
		_, err = blockchain.CheckTransactionInputs(subsidyCache, tx,
			nextBlockHeight, blockUtxos, false, g.chainParams)
		if err != nil {
			minrLog.Tracef("Skipping tx %s due to error in "+
				"CheckTransactionInputs: %v", tx.Hash(), err)
			logSkippedDeps(tx, deps)
			continue
		}
		err = blockchain.ValidateTransactionScripts(tx, blockUtxos,
			scriptFlags, g.sigCache)
		if err != nil {
			minrLog.Tracef("Skipping tx %s due to error in "+
				"ValidateTransactionScripts: %v", tx.Hash(), err)
			logSkippedDeps(tx, deps)
			continue
		}

		// Spend the transaction inputs in the block utxo view and add
		// an entry for it to ensure any transactions which reference
		// this one have it available as an input and can ensure they
		// aren't double spending.
		err = spendTransaction(blockUtxos, tx, nextBlockHeight)
		if err != nil {
			minrLog.Warnf("Unable to spend transaction %v in the preliminary "+
				"UTXO view for the block template: %v",
				tx.Hash(), err)
		}

		// Add the transaction to the block, increment counters, and
		// save the fees and signature operation counts to the block
		// template.
		blockTxns = append(blockTxns, tx)
		blockSize += txSize
		blockSigOps += numSigOps

		// Accumulate the SStxs in the block, because only a certain number
		// are allowed.
		if isSStx {
			numSStx++
		}
		if isSSGen {
			foundWinningTickets[tx.MsgTx().TxIn[1].PreviousOutPoint.Hash] = true
		}

		txFeesMap[*tx.Hash()] = prioItem.fee
		txSigOpCountsMap[*tx.Hash()] = numSigOps

		minrLog.Tracef("Adding tx %s (priority %.2f, feePerKB %.2f)",
			prioItem.tx.Hash(), prioItem.priority, prioItem.feePerKB)

		// Add transactions which depend on this one (and also do not
		// have any other unsatisified dependencies) to the priority
		// queue.
		for _, item := range deps {
			// Add the transaction to the priority queue if there
			// are no more dependencies after this one.
			delete(item.dependsOn, *tx.Hash())
			if len(item.dependsOn) == 0 {
				heap.Push(priorityQueue, item)
			}
		}
	}

	// Build tx list for stake tx.
	blockTxnsStake := make([]*dcrutil.Tx, 0, len(blockTxns))

	// Stake tx ordering in stake tree:
	// 1. SSGen (votes).
	// 2. SStx (fresh stake tickets).
	// 3. SSRtx (revocations for missed tickets).

	// Get the block votes (SSGen tx) and store them and their number.
	voters := 0
	var voteBitsVoters []uint16

	for _, tx := range blockTxns {
		msgTx := tx.MsgTx()
		if nextBlockHeight < stakeValidationHeight {
			break // No SSGen should be present before this height.
		}

		if stake.IsSSGen(msgTx) {
			txCopy := dcrutil.NewTxDeepTxIns(msgTx)
			if maybeInsertStakeTx(g.blockManager, txCopy, !treeKnownInvalid) {
				vb := stake.SSGenVoteBits(txCopy.MsgTx())
				voteBitsVoters = append(voteBitsVoters, vb)
				blockTxnsStake = append(blockTxnsStake, txCopy)
				voters++
			}
		}

		// Don't let this overflow, although probably it's impossible.
		if voters >= math.MaxUint16 {
			break
		}
	}

	// Set votebits, which determines whether the TxTreeRegular of the previous
	// block is valid or not.
	var votebits uint16
	if nextBlockHeight < stakeValidationHeight {
		votebits = uint16(0x0001) // TxTreeRegular enabled pre-staking
	} else {
		// Otherwise, we need to check the votes to determine if the tx tree was
		// validated or not.
		voteYea := 0
		totalVotes := 0

		for _, vb := range voteBitsVoters {
			if dcrutil.IsFlagSet16(vb, dcrutil.BlockValid) {
				voteYea++
			}
			totalVotes++
		}

		if voteYea == 0 { // Handle zero case for div by zero error prevention.
			votebits = uint16(0x0000) // TxTreeRegular disabled
		} else if (totalVotes / voteYea) <= 1 {
			votebits = uint16(0x0001) // TxTreeRegular enabled
		} else {
			votebits = uint16(0x0000) // TxTreeRegular disabled
		}

		if votebits == uint16(0x0000) {
			// In the event TxTreeRegular is disabled, we need to remove all tx
			// in the current block that depend on tx from the TxTreeRegular of
			// the previous block.
			// DECRED WARNING: The ideal behaviour should also be that we re-add
			// all tx that we just removed from the previous block into our
			// current block template. Right now this code fails to do that;
			// these tx will then be included in the next block, which isn't
			// catastrophic but is kind of buggy.

			// Retrieve the current top block, whose TxTreeRegular was voted
			// out.
			topBlock, err := g.chain.BlockByHash(&prevHash)
			if err != nil {
				str := fmt.Sprintf("unable to get tip block %s", prevHash)
				return nil, miningRuleError(ErrGetTopBlock, str)
			}
			topBlockRegTx := topBlock.Transactions()

			tempBlockTxns := make([]*dcrutil.Tx, 0, len(sourceTxns))
			for _, tx := range blockTxns {
				if tx.Tree() == wire.TxTreeRegular {
					// Go through all the inputs and check to see if this mempool
					// tx uses outputs from the parent block. This loop is
					// probably very expensive.
					isValid := true
					for _, txIn := range tx.MsgTx().TxIn {
						for _, parentTx := range topBlockRegTx {
							if txIn.PreviousOutPoint.Hash.IsEqual(
								parentTx.Hash()) {
								isValid = false
							}
						}
					}

					if isValid {
						txCopy := dcrutil.NewTxDeepTxIns(tx.MsgTx())
						tempBlockTxns = append(tempBlockTxns, txCopy)
					}
				} else {
					txCopy := dcrutil.NewTxDeepTxIns(tx.MsgTx())
					tempBlockTxns = append(tempBlockTxns, txCopy)
				}
			}

			// Replace blockTxns with the pruned list of valid mempool tx.
			blockTxns = tempBlockTxns
		}
	}

	// Get the newly purchased tickets (SStx tx) and store them and their number.
	freshStake := 0
	for _, tx := range blockTxns {
		msgTx := tx.MsgTx()
		if tx.Tree() == wire.TxTreeStake && stake.IsSStx(msgTx) {
			// A ticket can not spend an input from TxTreeRegular, since it
			// has not yet been validated.
			if containsTxIns(blockTxns, tx) {
				continue
			}

			// Quick check for difficulty here.
			if msgTx.TxOut[0].Value >= best.NextStakeDiff {
				txCopy := dcrutil.NewTxDeepTxIns(msgTx)
				if maybeInsertStakeTx(g.blockManager, txCopy, !treeKnownInvalid) {
					blockTxnsStake = append(blockTxnsStake, txCopy)
					freshStake++
				}
			}
		}

		// Don't let this overflow.
		if freshStake >= int(g.chainParams.MaxFreshStakePerBlock) {
			break
		}
	}

	// Get the ticket revocations (SSRtx tx) and store them and their number.
	revocations := 0
	for _, tx := range blockTxns {
		if nextBlockHeight < stakeValidationHeight {
			break // No SSRtx should be present before this height.
		}

		msgTx := tx.MsgTx()
		if tx.Tree() == wire.TxTreeStake && stake.IsSSRtx(msgTx) {
			txCopy := dcrutil.NewTxDeepTxIns(msgTx)
			if maybeInsertStakeTx(g.blockManager, txCopy, !treeKnownInvalid) {
				blockTxnsStake = append(blockTxnsStake, txCopy)
				revocations++
			}
		}

		// Don't let this overflow.
		if revocations >= math.MaxUint8 {
			break
		}
	}

	// Create a standard coinbase transaction paying to the provided
	// address.  NOTE: The coinbase value will be updated to include the
	// fees from the selected transactions later after they have actually
	// been selected.  It is created here to detect any errors early
	// before potentially doing a lot of work below.  The extra nonce helps
	// ensure the transaction is not a duplicate transaction (paying the
	// same value to the same public key address would otherwise be an
	// identical transaction for block version 1).
	// Decred: We need to move this downwards because of the requirements
	// to incorporate voters and potential voters.
	coinbaseScript := []byte{0x00, 0x00}
	coinbaseScript = append(coinbaseScript, []byte(coinbaseFlags)...)

	// Add a random coinbase nonce to ensure that tx prefix hash
	// so that our merkle root is unique for lookups needed for
	// getwork, etc.
	rand, err := wire.RandomUint64()
	if err != nil {
		return nil, err
	}
	opReturnPkScript, err := standardCoinbaseOpReturn(uint32(nextBlockHeight),
		rand)
	if err != nil {
		return nil, err
	}
	coinbaseTx, err := createCoinbaseTx(subsidyCache,
		coinbaseScript,
		opReturnPkScript,
		nextBlockHeight,
		payToAddress,
		uint16(voters),
		g.chainParams)
	if err != nil {
		return nil, err
	}

	coinbaseTx.SetTree(wire.TxTreeRegular) // Coinbase only in regular tx tree
	if err != nil {
		return nil, err
	}
	numCoinbaseSigOps := int64(blockchain.CountSigOps(coinbaseTx, true, false))
	blockSize += uint32(coinbaseTx.MsgTx().SerializeSize())
	blockSigOps += numCoinbaseSigOps
	txFeesMap[*coinbaseTx.Hash()] = 0
	txSigOpCountsMap[*coinbaseTx.Hash()] = numCoinbaseSigOps

	// Build tx lists for regular tx.
	blockTxnsRegular := make([]*dcrutil.Tx, 0, len(blockTxns)+1)

	// Append coinbase.
	blockTxnsRegular = append(blockTxnsRegular, coinbaseTx)

	// Assemble the two transaction trees.
	for _, tx := range blockTxns {
		if tx.Tree() == wire.TxTreeRegular {
			blockTxnsRegular = append(blockTxnsRegular, tx)
		} else if tx.Tree() == wire.TxTreeStake {
			continue
		} else {
			minrLog.Tracef("Error adding tx %s to block; invalid tree", tx.Hash())
			continue
		}
	}

	for _, tx := range blockTxnsRegular {
		fee, ok := txFeesMap[*tx.Hash()]
		if !ok {
			return nil, fmt.Errorf("couldn't find fee for tx %v",
				*tx.Hash())
		}
		totalFees += fee
		txFees = append(txFees, fee)

		tsos, ok := txSigOpCountsMap[*tx.Hash()]
		if !ok {
			return nil, fmt.Errorf("couldn't find sig ops count for tx %v",
				*tx.Hash())
		}
		txSigOpCounts = append(txSigOpCounts, tsos)
	}

	for _, tx := range blockTxnsStake {
		fee, ok := txFeesMap[*tx.Hash()]
		if !ok {
			return nil, fmt.Errorf("couldn't find fee for stx %v",
				*tx.Hash())
		}
		totalFees += fee
		txFees = append(txFees, fee)

		tsos, ok := txSigOpCountsMap[*tx.Hash()]
		if !ok {
			return nil, fmt.Errorf("couldn't find sig ops count for stx %v",
				*tx.Hash())
		}
		txSigOpCounts = append(txSigOpCounts, tsos)
	}

	txSigOpCounts = append(txSigOpCounts, numCoinbaseSigOps)

	// If we're greater than or equal to stake validation height, scale the
	// fees according to the number of voters.
	totalFees *= int64(voters)
	totalFees /= int64(g.chainParams.TicketsPerBlock)

	// Now that the actual transactions have been selected, update the
	// block size for the real transaction count and coinbase value with
	// the total fees accordingly.
	if nextBlockHeight > 1 {
		blockSize -= wire.MaxVarIntPayload -
			uint32(wire.VarIntSerializeSize(uint64(len(blockTxnsRegular))+
				uint64(len(blockTxnsStake))))
		coinbaseTx.MsgTx().TxOut[2].Value += totalFees
		txFees[0] = -totalFees
	}

	// Calculate the required difficulty for the block.  The timestamp
	// is potentially adjusted to ensure it comes after the median time of
	// the last several blocks per the chain consensus rules.
	ts := medianAdjustedTime(best, g.timeSource)
	reqDifficulty, err := g.chain.CalcNextRequiredDifficulty(ts)
	if err != nil {
		return nil, miningRuleError(ErrGettingDifficulty, err.Error())
	}

	// Return nil if we don't yet have enough voters; sometimes it takes a
	// bit for the mempool to sync with the votes map and we end up down
	// here despite having the relevant votes available in the votes map.
	minimumVotesRequired :=
		int((g.chainParams.TicketsPerBlock / 2) + 1)
	if nextBlockHeight >= stakeValidationHeight &&
		voters < minimumVotesRequired {
		minrLog.Warnf("incongruent number of voters in mempool " +
			"vs mempool.voters; not enough voters found")
		return handleTooFewVoters(subsidyCache, nextBlockHeight, payToAddress,
			g.blockManager)
	}

	// Correct transaction index fraud proofs for any transactions that
	// are chains. maybeInsertStakeTx fills this in for stake transactions
	// already, so only do it for regular transactions.
	for i, tx := range blockTxnsRegular {
		// No need to check any of the transactions in the custom first
		// block.
		if nextBlockHeight == 1 {
			break
		}

		utxs, err := g.chain.FetchUtxoView(tx, !treeKnownInvalid)
		if err != nil {
			str := fmt.Sprintf("failed to fetch input utxs for tx %v: %s",
				tx.Hash(), err.Error())
			return nil, miningRuleError(ErrFetchTxStore, str)
		}

		// Copy the transaction and swap the pointer.
		txCopy := dcrutil.NewTxDeepTxIns(tx.MsgTx())
		blockTxnsRegular[i] = txCopy
		tx = txCopy

		for _, txIn := range tx.MsgTx().TxIn {
			originHash := &txIn.PreviousOutPoint.Hash
			utx := utxs.LookupEntry(originHash)
			if utx == nil {
				// Set a flag with the index so we can properly set
				// the fraud proof below.
				txIn.BlockIndex = wire.NullBlockIndex
			} else {
				originIdx := txIn.PreviousOutPoint.Index
				txIn.ValueIn = utx.AmountByIndex(originIdx)
				txIn.BlockHeight = uint32(utx.BlockHeight())
				txIn.BlockIndex = utx.BlockIndex()
			}
		}
	}

	// Fill in locally referenced inputs.
	for i, tx := range blockTxnsRegular {
		// Skip coinbase.
		if i == 0 {
			continue
		}

		// Copy the transaction and swap the pointer.
		txCopy := dcrutil.NewTxDeepTxIns(tx.MsgTx())
		blockTxnsRegular[i] = txCopy
		tx = txCopy

		for _, txIn := range tx.MsgTx().TxIn {
			// This tx was at some point 0-conf and now requires the
			// correct block height and index. Set it here.
			if txIn.BlockIndex == wire.NullBlockIndex {
				idx := txIndexFromTxList(txIn.PreviousOutPoint.Hash,
					blockTxnsRegular)

				// The input is in the block, set it accordingly.
				if idx != -1 {
					originIdx := txIn.PreviousOutPoint.Index
					amt := blockTxnsRegular[idx].MsgTx().TxOut[originIdx].Value
					txIn.ValueIn = amt
					txIn.BlockHeight = uint32(nextBlockHeight)
					txIn.BlockIndex = uint32(idx)
				} else {
					str := fmt.Sprintf("failed find hash in tx list "+
						"for fraud proof; tx in hash %v",
						txIn.PreviousOutPoint.Hash)
					return nil, miningRuleError(ErrFraudProofIndex, str)
				}
			}
		}
	}

	// Choose the block version to generate based on the network.
	blockVersion := int32(generatedBlockVersion)
	if g.chainParams.Net != wire.MainNet {
		blockVersion = generatedBlockVersionTest
	}

	// Figure out stake version.
	generatedStakeVersion, err :=
		g.chain.CalcStakeVersionByHash(&prevHash)
	if err != nil {
		return nil, err
	}

	// Create a new block ready to be solved.
	merkles := blockchain.BuildMerkleTreeStore(blockTxnsRegular)
	merklesStake := blockchain.BuildMerkleTreeStore(blockTxnsStake)

	var msgBlock wire.MsgBlock
	msgBlock.Header = wire.BlockHeader{
		Version:      blockVersion,
		PrevBlock:    prevHash,
		MerkleRoot:   *merkles[len(merkles)-1],
		StakeRoot:    *merklesStake[len(merklesStake)-1],
		VoteBits:     votebits,
		FinalState:   best.NextFinalState,
		Voters:       uint16(voters),
		FreshStake:   uint8(freshStake),
		Revocations:  uint8(revocations),
		PoolSize:     best.NextPoolSize,
		Timestamp:    ts,
		SBits:        best.NextStakeDiff,
		Bits:         reqDifficulty,
		StakeVersion: generatedStakeVersion,
		Height:       uint32(nextBlockHeight),
		// Size declared below
	}

	for _, tx := range blockTxnsRegular {
		if err := msgBlock.AddTransaction(tx.MsgTx()); err != nil {
			return nil, miningRuleError(ErrTransactionAppend, err.Error())
		}
	}

	for _, tx := range blockTxnsStake {
		if err := msgBlock.AddSTransaction(tx.MsgTx()); err != nil {
			return nil, miningRuleError(ErrTransactionAppend, err.Error())
		}
	}

	msgBlock.Header.Size = uint32(msgBlock.SerializeSize())

	// Finally, perform a full check on the created block against the chain
	// consensus rules to ensure it properly connects to the current best
	// chain with no issues.
	block := dcrutil.NewBlockDeepCopyCoinbase(&msgBlock)
	err = g.chain.CheckConnectBlockTemplate(block)
	if err != nil {
		str := fmt.Sprintf("failed to do final check for check connect "+
			"block when making new block template: %v",
			err.Error())
		return nil, miningRuleError(ErrCheckConnectBlock, str)
	}

	minrLog.Debugf("Created new block template (%d transactions, %d "+
		"stake transactions, %d in fees, %d signature operations, "+
		"%d bytes, target difficulty %064x, stake difficulty %v)",
		len(msgBlock.Transactions), len(msgBlock.STransactions),
		totalFees, blockSigOps, blockSize,
		blockchain.CompactToBig(msgBlock.Header.Bits),
		dcrutil.Amount(msgBlock.Header.SBits).ToCoin())

	blockTemplate := &BlockTemplate{
		Block:           &msgBlock,
		Fees:            txFees,
		SigOpCounts:     txSigOpCounts,
		Height:          nextBlockHeight,
		ValidPayAddress: payToAddress != nil,
	}

	return blockTemplate, nil
}

// UpdateBlockTime updates the timestamp in the header of the passed block to
// the current time while taking into account the median time of the last
// several blocks to ensure the new time is after that time per the chain
// consensus rules.  Finally, it will update the target difficulty if needed
// based on the new time for the test networks since their target difficulty can
// change based upon time.
func (g *BlkTmplGenerator) UpdateBlockTime(header *wire.BlockHeader) error {
	// The new timestamp is potentially adjusted to ensure it comes after
	// the median time of the last several blocks per the chain consensus
	// rules.
	newTimestamp := medianAdjustedTime(g.chain.BestSnapshot(), g.timeSource)
	header.Timestamp = newTimestamp

	// If running on a network that requires recalculating the difficulty,
	// do so now.
	if activeNetParams.ReduceMinDifficulty {
		difficulty, err := g.chain.CalcNextRequiredDifficulty(newTimestamp)
		if err != nil {
			return miningRuleError(ErrGettingDifficulty, err.Error())
		}
		header.Bits = difficulty
	}

	return nil
}