utxoviewpoint.go

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

package blockchain

import (
	"fmt"

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

// StakeViewpoint is the viewpoint of the blockchain depending on stake
// validation. There are five potential viewpoints we need to worry about.
type StakeViewpoint int8

const (
	// ViewpointPrevValidInitial is the viewpoint from the perspective of the
	// everything up the the previous block's TxTreeRegular, used to validate
	// that tx tree regular.
	ViewpointPrevValidInitial = StakeViewpoint(iota)

	// ViewpointPrevValidStake is the viewpoint from the perspective of the
	// everything up the the previous block's TxTreeRegular plus the
	// contents of the TxTreeRegular, to validate TxTreeStake.
	ViewpointPrevValidStake

	// ViewpointPrevInvalidStake is the viewpoint from the perspective of the
	// everything up the the previous block's TxTreeRegular but without the
	// contents of the TxTreeRegular, to validate TxTreeStake.
	ViewpointPrevInvalidStake

	// ViewpointPrevValidRegular is the viewpoint from the perspective of the
	// everything up the the previous block's TxTreeRegular plus the
	// contents of the TxTreeRegular and TxTreeStake of current block,
	// to validate TxTreeRegular of the current block.
	ViewpointPrevValidRegular

	// ViewpointPrevInvalidRegular is the viewpoint from the perspective of the
	// everything up the the previous block's TxTreeRegular minus the
	// contents of the TxTreeRegular and TxTreeStake of current block,
	// to validate TxTreeRegular of the current block.
	ViewpointPrevInvalidRegular
)

// utxoOutput houses details about an individual unspent transaction output such
// as whether or not it is spent, its public key script, and how much it pays.
//
// Standard public key scripts are stored in the database using a compressed
// format. Since the vast majority of scripts are of the standard form, a fairly
// significant savings is achieved by discarding the portions of the standard
// scripts that can be reconstructed.
//
// Also, since it is common for only a specific output in a given utxo entry to
// be referenced from a redeeming transaction, the script and amount for a given
// output is not uncompressed until the first time it is accessed.  This
// provides a mechanism to avoid the overhead of needlessly uncompressing all
// outputs for a given utxo entry at the time of load.
//
// The struct is aligned for memory efficiency.
type utxoOutput struct {
	pkScript      []byte // The public key script for the output.
	amount        int64  // The amount of the output.
	scriptVersion uint16 // The script version
	compressed    bool   // The public key script is compressed.
	spent         bool   // Output is spent.
}

// maybeDecompress decompresses the amount and public key script fields of the
// utxo and marks it decompressed if needed.
func (o *utxoOutput) maybeDecompress(compressionVersion uint32) {
	// Nothing to do if it's not compressed.
	if !o.compressed {
		return
	}

	o.pkScript = decompressScript(o.pkScript, compressionVersion)
	o.compressed = false
}

// UtxoEntry contains contextual information about an unspent transaction such
// as whether or not it is a coinbase transaction, which block it was found in,
// and the spent status of its outputs.
//
// The struct is aligned for memory efficiency.
type UtxoEntry struct {
	sparseOutputs map[uint32]*utxoOutput // Sparse map of unspent outputs.
	stakeExtra    []byte                 // Extra data for the staking system.

	txVersion int32        // The tx version of this tx.
	height    uint32       // Height of block containing tx.
	index     uint32       // Index of containing tx in block.
	txType    stake.TxType // The stake type of the transaction.

	isCoinBase bool // Whether entry is a coinbase tx.
	hasExpiry  bool // Whether entry has an expiry.
	modified   bool // Entry changed since load.
}

// TxVersion returns the transaction version of the transaction the
// utxo represents.
func (entry *UtxoEntry) TxVersion() int32 {
	return entry.txVersion
}

// HasExpiry returns the transaction expiry for the transaction that the utxo
// entry represents.
func (entry *UtxoEntry) HasExpiry() bool {
	return entry.hasExpiry
}

// IsCoinBase returns whether or not the transaction the utxo entry represents
// is a coinbase.
func (entry *UtxoEntry) IsCoinBase() bool {
	return entry.isCoinBase
}

// BlockHeight returns the height of the block containing the transaction the
// utxo entry represents.
func (entry *UtxoEntry) BlockHeight() int64 {
	return int64(entry.height)
}

// BlockIndex returns the height of the block containing the transaction the
// utxo entry represents.
func (entry *UtxoEntry) BlockIndex() uint32 {
	return entry.index
}

// TransactionType returns the transaction type of the transaction the utxo entry
// represents.
func (entry *UtxoEntry) TransactionType() stake.TxType {
	return entry.txType
}

// IsOutputSpent returns whether or not the provided output index has been
// spent based upon the current state of the unspent transaction output view
// the entry was obtained from.
//
// Returns true if the output index references an output that does not exist
// either due to it being invalid or because the output is not part of the view
// due to previously being spent/pruned.
func (entry *UtxoEntry) IsOutputSpent(outputIndex uint32) bool {
	output, ok := entry.sparseOutputs[outputIndex]
	if !ok {
		return true
	}

	return output.spent
}

// SpendOutput marks the output at the provided index as spent.  Specifying an
// output index that does not exist will not have any effect.
func (entry *UtxoEntry) SpendOutput(outputIndex uint32) {
	output, ok := entry.sparseOutputs[outputIndex]
	if !ok {
		return
	}

	// Nothing to do if the output is already spent.
	if output.spent {
		return
	}

	entry.modified = true
	output.spent = true
	return
}

// IsFullySpent returns whether or not the transaction the utxo entry represents
// is fully spent.
func (entry *UtxoEntry) IsFullySpent() bool {
	// The entry is not fully spent if any of the outputs are unspent.
	for _, output := range entry.sparseOutputs {
		if !output.spent {
			return false
		}
	}

	return true
}

// AmountByIndex returns the amount of the provided output index.
//
// Returns 0 if the output index references an output that does not exist
// either due to it being invalid or because the output is not part of the view
// due to previously being spent/pruned.
func (entry *UtxoEntry) AmountByIndex(outputIndex uint32) int64 {
	output, ok := entry.sparseOutputs[outputIndex]
	if !ok {
		return 0
	}

	return output.amount
}

// ScriptVersionByIndex returns the public key script for the provided output
// index.
//
// Returns 0 if the output index references an output that does not exist
// either due to it being invalid or because the output is not part of the view
// due to previously being spent/pruned.
func (entry *UtxoEntry) ScriptVersionByIndex(outputIndex uint32) uint16 {
	output, ok := entry.sparseOutputs[outputIndex]
	if !ok {
		return 0
	}

	return output.scriptVersion
}

// PkScriptByIndex returns the public key script for the provided output index.
//
// Returns nil if the output index references an output that does not exist
// either due to it being invalid or because the output is not part of the view
// due to previously being spent/pruned.
func (entry *UtxoEntry) PkScriptByIndex(outputIndex uint32) []byte {
	output, ok := entry.sparseOutputs[outputIndex]
	if !ok {
		return nil
	}

	// Ensure the output is decompressed before returning the script.
	output.maybeDecompress(currentCompressionVersion)
	return output.pkScript
}

// newUtxoEntry returns a new unspent transaction output entry with the provided
// coinbase flag and block height ready to have unspent outputs added.
func newUtxoEntry(txVersion int32, height uint32, index uint32, isCoinBase bool,
	hasExpiry bool, tt stake.TxType) *UtxoEntry {
	return &UtxoEntry{
		sparseOutputs: make(map[uint32]*utxoOutput),
		txVersion:     txVersion,
		height:        height,
		index:         index,
		isCoinBase:    isCoinBase,
		hasExpiry:     hasExpiry,
		txType:        tt,
	}
}

// UtxoViewpoint represents a view into the set of unspent transaction outputs
// from a specific point of view in the chain.  For example, it could be for
// the end of the main chain, some point in the history of the main chain, or
// down a side chain.
//
// The unspent outputs are needed by other transactions for things such as
// script validation and double spend prevention.
type UtxoViewpoint struct {
	entries   map[chainhash.Hash]*UtxoEntry
	bestHash  chainhash.Hash
	stakeView StakeViewpoint
}

// BestHash returns the hash of the best block in the chain the view currently
// respresents.
func (view *UtxoViewpoint) BestHash() *chainhash.Hash {
	return &view.bestHash
}

// SetBestHash sets the hash of the best block in the chain the view currently
// respresents.
func (view *UtxoViewpoint) SetBestHash(hash *chainhash.Hash) {
	view.bestHash = *hash
}

// StakeViewpoint returns the stake viewpoint of the current UTXO state.
func (view *UtxoViewpoint) StakeViewpoint() StakeViewpoint {
	return view.stakeView
}

// SetStakeViewpoint sets the stake viewpoint of the current UTXO state.
func (view *UtxoViewpoint) SetStakeViewpoint(sv StakeViewpoint) {
	view.stakeView = sv
}

// LookupEntry returns information about a given transaction according to the
// current state of the view.  It will return nil if the passed transaction
// hash does not exist in the view or is otherwise not available such as when
// it has been disconnected during a reorg.
func (view *UtxoViewpoint) LookupEntry(txHash *chainhash.Hash) *UtxoEntry {
	entry, ok := view.entries[*txHash]
	if !ok {
		return nil
	}

	return entry
}

// AddTxOuts adds all outputs in the passed transaction which are not provably
// unspendable to the view.  When the view already has entries for any of the
// outputs, they are simply marked unspent.  All fields will be updated for
// existing entries since it's possible it has changed during a reorg.
func (view *UtxoViewpoint) AddTxOuts(tx *dcrutil.Tx, blockHeight int64,
	blockIndex uint32) {
	msgTx := tx.MsgTx()
	// When there are not already any utxos associated with the transaction,
	// add a new entry for it to the view.
	entry := view.LookupEntry(tx.Sha())
	if entry == nil {
		txType := stake.DetermineTxType(msgTx)
		entry = newUtxoEntry(msgTx.Version, uint32(blockHeight),
			blockIndex, IsCoinBaseTx(msgTx), msgTx.Expiry != 0, txType)
		if txType == stake.TxTypeSStx {
			stakeExtra := make([]byte, serializeSizeForMinimalOutputs(tx))
			putTxToMinimalOutputs(stakeExtra, tx)
			entry.stakeExtra = stakeExtra
		}
		view.entries[*tx.Sha()] = entry
	} else {
		entry.height = uint32(blockHeight)
		entry.index = uint32(blockIndex)
	}
	entry.modified = true

	// Loop all of the transaction outputs and add those which are not
	// provably unspendable.
	for txOutIdx, txOut := range tx.MsgTx().TxOut {
		// TODO allow pruning of stake utxs after all other outputs are spent
		if txscript.IsUnspendable(txOut.Value, txOut.PkScript) {
			continue
		}

		// Update existing entries.  All fields are updated because it's
		// possible (although extremely unlikely) that the existing
		// entry is being replaced by a different transaction with the
		// same hash.  This is allowed so long as the previous
		// transaction is fully spent.
		if output, ok := entry.sparseOutputs[uint32(txOutIdx)]; ok {
			output.spent = false
			output.amount = txOut.Value
			output.scriptVersion = txOut.Version
			output.pkScript = txOut.PkScript
			output.compressed = false
			continue
		}

		// Add the unspent transaction output.
		entry.sparseOutputs[uint32(txOutIdx)] = &utxoOutput{
			spent:         false,
			amount:        txOut.Value,
			scriptVersion: txOut.Version,
			pkScript:      txOut.PkScript,
			compressed:    false,
		}
	}
	return
}

// connectTransaction updates the view by adding all new utxos created by the
// passed transaction and marking all utxos that the transactions spend as
// spent.  In addition, when the 'stxos' argument is not nil, it will be updated
// to append an entry for each spent txout.  An error will be returned if the
// view does not contain the required utxos.
func (view *UtxoViewpoint) connectTransaction(tx *dcrutil.Tx, blockHeight int64,
	blockIndex uint32, stxos *[]spentTxOut) error {
	msgTx := tx.MsgTx()
	// Coinbase transactions don't have any inputs to spend.
	if IsCoinBaseTx(msgTx) {
		// Add the transaction's outputs as available utxos.
		view.AddTxOuts(tx, blockHeight, blockIndex)
		return nil
	}

	// Spend the referenced utxos by marking them spent in the view and,
	// if a slice was provided for the spent txout details, append an entry
	// to it.
	txType := stake.DetermineTxType(msgTx)
	for i, txIn := range msgTx.TxIn {
		if i == 0 && (txType == stake.TxTypeSSGen) {
			continue
		}

		originIndex := txIn.PreviousOutPoint.Index
		entry := view.entries[txIn.PreviousOutPoint.Hash]

		// Ensure the referenced utxo exists in the view.  This should
		// never happen unless there is a bug is introduced in the code.
		if entry == nil {
			return AssertError(fmt.Sprintf("view missing input %v",
				txIn.PreviousOutPoint))
		}
		entry.SpendOutput(originIndex)

		// Don't create the stxo details if not requested.
		if stxos == nil {
			continue
		}

		// Populate the stxo details using the utxo entry.  When the
		// transaction is fully spent, set the additional stxo fields
		// accordingly since those details will no longer be available
		// in the utxo set.
		var stxo = spentTxOut{
			compressed:    false,
			amount:        txIn.ValueIn,
			scriptVersion: entry.ScriptVersionByIndex(originIndex),
			pkScript:      entry.PkScriptByIndex(originIndex),
		}
		if entry.IsFullySpent() {
			stxo.txVersion = entry.TxVersion()
			stxo.height = uint32(entry.BlockHeight())
			stxo.index = entry.BlockIndex()
			stxo.isCoinBase = entry.IsCoinBase()
			stxo.hasExpiry = entry.HasExpiry()
			stxo.txType = entry.txType
			stxo.txFullySpent = true

			if entry.txType == stake.TxTypeSStx {
				stxo.stakeExtra = entry.stakeExtra
			}
		}

		// Append the entry to the provided spent txouts slice.
		*stxos = append(*stxos, stxo)
	}

	// Add the transaction's outputs as available utxos.
	view.AddTxOuts(tx, blockHeight, blockIndex)

	return nil
}

// connectTransactions updates the view by adding all new utxos created by all
// of the transactions in the passed block, marking all utxos the transactions
// spend as spent, and setting the best hash for the view to the passed block.
// In addition, when the 'stxos' argument is not nil, it will be updated to
// append an entry for each spent txout.
func (b *BlockChain) connectTransactions(view *UtxoViewpoint, block *dcrutil.Block,
	parent *dcrutil.Block, stxos *[]spentTxOut) error {
	regularTxTreeValid := dcrutil.IsFlagSet16(block.MsgBlock().Header.VoteBits,
		dcrutil.BlockValid)
	thisNodeStakeViewpoint := ViewpointPrevInvalidStake
	if regularTxTreeValid {
		thisNodeStakeViewpoint = ViewpointPrevValidStake
	}

	if parent != nil && block.Height() != 0 {
		view.SetStakeViewpoint(ViewpointPrevValidInitial)
		err := view.fetchInputUtxos(b.db, block, parent)
		if err != nil {
			return err
		}
		mBlock := block.MsgBlock()
		votebits := mBlock.Header.VoteBits
		regularTxTreeValid := dcrutil.IsFlagSet16(votebits, dcrutil.BlockValid)
		if regularTxTreeValid {
			for i, tx := range parent.Transactions() {
				err := view.connectTransaction(tx, parent.Height(), uint32(i),
					stxos)
				if err != nil {
					return err
				}
			}
		}
	}

	for i, stx := range block.STransactions() {
		view.SetStakeViewpoint(thisNodeStakeViewpoint)
		err := view.fetchInputUtxos(b.db, block, parent)
		if err != nil {
			return err
		}
		err = view.connectTransaction(stx, block.Height(), uint32(i), stxos)
		if err != nil {
			return err
		}
	}

	// Update the best hash for view to include this block since all of its
	// transactions have been connected.
	view.SetBestHash(block.Sha())
	return nil
}

// disconnectTransactions updates the view by removing all of the transactions
// created by the passed block, restoring all utxos the transactions spent by
// using the provided spent txo information, and setting the best hash for the
// view to the block before the passed block.
//
// This function will ONLY work correctly for a single transaction tree at a
// time because of index tracking.
func (b *BlockChain) disconnectTransactions(view *UtxoViewpoint,
	block *dcrutil.Block, parent *dcrutil.Block, stxos []spentTxOut) error {
	// Sanity check the correct number of stxos are provided.
	if len(stxos) != countSpentOutputs(block, parent) {
		return AssertError(fmt.Sprintf("disconnectTransactions "+
			"called with bad spent transaction out information "+
			"(len stxos %v, count is %v)", len(stxos),
			countSpentOutputs(block, parent)))
	}

	// Loop backwards through all transactions so everything is unspent in
	// reverse order.  This is necessary since transactions later in a block
	// can spend from previous ones.
	regularTxTreeValid := dcrutil.IsFlagSet16(block.MsgBlock().Header.VoteBits,
		dcrutil.BlockValid)
	thisNodeStakeViewpoint := ViewpointPrevInvalidStake
	if regularTxTreeValid {
		thisNodeStakeViewpoint = ViewpointPrevValidStake
	}
	view.SetStakeViewpoint(thisNodeStakeViewpoint)
	err := view.fetchInputUtxos(b.db, block, parent)
	if err != nil {
		return err
	}
	stxoIdx := len(stxos) - 1
	transactions := block.STransactions()
	for txIdx := len(transactions) - 1; txIdx > -1; txIdx-- {
		tx := transactions[txIdx]
		msgTx := tx.MsgTx()
		tt := stake.DetermineTxType(msgTx)

		// Clear this transaction from the view if it already exists or
		// create a new empty entry for when it does not.  This is done
		// because the code relies on its existence in the view in order
		// to signal modifications have happened.
		entry := view.entries[*tx.Sha()]
		if entry == nil {
			entry = newUtxoEntry(msgTx.Version, uint32(block.Height()),
				uint32(txIdx), IsCoinBaseTx(msgTx), msgTx.Expiry != 0, tt)
			if tt == stake.TxTypeSStx {
				stakeExtra := make([]byte, serializeSizeForMinimalOutputs(tx))
				putTxToMinimalOutputs(stakeExtra, tx)
				entry.stakeExtra = stakeExtra
			}
			view.entries[*tx.Sha()] = entry
		}
		entry.modified = true
		entry.sparseOutputs = make(map[uint32]*utxoOutput)

		// Loop backwards through all of the transaction inputs (except
		// for the coinbase which has no inputs) and unspend the
		// referenced txos.  This is necessary to match the order of the
		// spent txout entries.
		for txInIdx := len(tx.MsgTx().TxIn) - 1; txInIdx > -1; txInIdx-- {
			// Skip empty vote stakebases.
			if txInIdx == 0 && (tt == stake.TxTypeSSGen) {
				continue
			}

			// Ensure the spent txout index is decremented to stay
			// in sync with the transaction input.
			stxo := &stxos[stxoIdx]
			stxoIdx--

			// When there is not already an entry for the referenced
			// transaction in the view, it means it was fully spent,
			// so create a new utxo entry in order to resurrect it.
			txIn := tx.MsgTx().TxIn[txInIdx]
			originHash := &txIn.PreviousOutPoint.Hash
			originIndex := txIn.PreviousOutPoint.Index
			entry := view.LookupEntry(originHash)
			if entry == nil {
				if !stxo.txFullySpent {
					return AssertError(fmt.Sprintf("tried to revive utx %v from "+
						"non-fully spent stx entry", originHash))
				}

				entry = newUtxoEntry(tx.MsgTx().Version, stxo.height,
					stxo.index, stxo.isCoinBase, stxo.hasExpiry,
					stxo.txType)
				if stxo.txType == stake.TxTypeSStx {
					entry.stakeExtra = stxo.stakeExtra
				}
				view.entries[*originHash] = entry
			}

			// Mark the entry as modified since it is either new
			// or will be changed below.
			entry.modified = true

			// Restore the specific utxo using the stxo data from
			// the spend journal if it doesn't already exist in the
			// view.
			output, ok := entry.sparseOutputs[originIndex]
			if !ok {
				// Add the unspent transaction output.
				entry.sparseOutputs[originIndex] = &utxoOutput{
					compressed:    stxo.compressed,
					spent:         false,
					amount:        txIn.ValueIn,
					scriptVersion: stxo.scriptVersion,
					pkScript:      stxo.pkScript,
				}
				continue
			}

			// Mark the existing referenced transaction output as
			// unspent.
			output.spent = false
		}
	}

	// There is no regular tx from before the genesis block, so ignore the genesis
	// block for the next step.
	if parent != nil && block.Height() != 0 {
		// Only bother to unspend transactions if the parent's tx tree was
		// validated. Otherwise, these transactions were never in the blockchain's
		// history in the first place.
		if regularTxTreeValid {
			view.SetStakeViewpoint(ViewpointPrevValidInitial)
			err = view.fetchInputUtxos(b.db, block, parent)
			if err != nil {
				return err
			}

			transactions := parent.Transactions()
			for txIdx := len(transactions) - 1; txIdx > -1; txIdx-- {
				tx := transactions[txIdx]

				// Clear this transaction from the view if it already exists or
				// create a new empty entry for when it does not.  This is done
				// because the code relies on its existence in the view in order
				// to signal modifications have happened.
				isCoinbase := txIdx == 0
				entry := view.entries[*tx.Sha()]
				if entry == nil {
					entry = newUtxoEntry(tx.MsgTx().Version,
						uint32(parent.Height()), uint32(txIdx), isCoinbase,
						tx.MsgTx().Expiry != 0, stake.TxTypeRegular)
					view.entries[*tx.Sha()] = entry
				}
				entry.modified = true
				entry.sparseOutputs = make(map[uint32]*utxoOutput)

				// Loop backwards through all of the transaction inputs (except
				// for the coinbase which has no inputs) and unspend the
				// referenced txos.  This is necessary to match the order of the
				// spent txout entries.
				if isCoinbase {
					continue
				}
				for txInIdx := len(tx.MsgTx().TxIn) - 1; txInIdx > -1; txInIdx-- {
					// Ensure the spent txout index is decremented to stay
					// in sync with the transaction input.
					stxo := &stxos[stxoIdx]
					stxoIdx--

					// When there is not already an entry for the referenced
					// transaction in the view, it means it was fully spent,
					// so create a new utxo entry in order to resurrect it.
					txIn := tx.MsgTx().TxIn[txInIdx]
					originHash := &txIn.PreviousOutPoint.Hash
					originIndex := txIn.PreviousOutPoint.Index
					entry := view.entries[*originHash]
					if entry == nil {
						if !stxo.txFullySpent {
							return AssertError(fmt.Sprintf("tried to "+
								"revive utx %v from non-fully spent stx entry",
								originHash))
						}
						entry = newUtxoEntry(tx.MsgTx().Version,
							stxo.height, stxo.index, stxo.isCoinBase,
							stxo.hasExpiry, stxo.txType)
						if stxo.txType == stake.TxTypeSStx {
							entry.stakeExtra = stxo.stakeExtra
						}
						view.entries[*originHash] = entry
					}

					// Mark the entry as modified since it is either new
					// or will be changed below.
					entry.modified = true

					// Restore the specific utxo using the stxo data from
					// the spend journal if it doesn't already exist in the
					// view.
					output, ok := entry.sparseOutputs[originIndex]
					if !ok {
						// Add the unspent transaction output.
						entry.sparseOutputs[originIndex] = &utxoOutput{
							compressed:    stxo.compressed,
							spent:         false,
							amount:        txIn.ValueIn,
							scriptVersion: stxo.scriptVersion,
							pkScript:      stxo.pkScript,
						}
						continue
					}

					// Mark the existing referenced transaction output as
					// unspent.
					output.spent = false
				}
			}
		}
	}

	// Update the best hash for view to the previous block since all of the
	// transactions for the current block have been disconnected.
	view.SetBestHash(parent.Sha())
	return nil
}

// disconnectTransactionSlice updates the view by removing all of the transactions
// created by the passed slice of transactions, restoring all utxos the
// transactions spent by using the provided spent txo information, and setting
// the best hash for the view to the block before the passed block.
func (view *UtxoViewpoint) disconnectTransactionSlice(transactions []*dcrutil.Tx,
	height int64, stxosPtr *[]spentTxOut) (int, error) {
	if stxosPtr == nil {
		return 0, AssertError("passed pointer to non-existing stxos slice")
	}

	stxos := *stxosPtr
	stxoIdx := len(stxos) - 1
	if stxoIdx == -1 {
		return 0, nil
	}
	for txIdx := len(transactions) - 1; txIdx > -1; txIdx-- {
		tx := transactions[txIdx]
		msgTx := tx.MsgTx()
		txType := stake.DetermineTxType(msgTx)

		// Clear this transaction from the view if it already exists or
		// create a new empty entry for when it does not.  This is done
		// because the code relies on its existence in the view in order
		// to signal modifications have happened.
		isCoinbase := txIdx == 0
		entry := view.entries[*tx.Sha()]
		if entry == nil {
			entry = newUtxoEntry(msgTx.Version, uint32(height),
				uint32(txIdx), IsCoinBaseTx(msgTx), msgTx.Expiry != 0, txType)
			if txType == stake.TxTypeSStx {
				stakeExtra := make([]byte, serializeSizeForMinimalOutputs(tx))
				putTxToMinimalOutputs(stakeExtra, tx)
				entry.stakeExtra = stakeExtra
			}
			view.entries[*tx.Sha()] = entry
		}
		entry.modified = true
		entry.sparseOutputs = make(map[uint32]*utxoOutput)

		// Loop backwards through all of the transaction inputs (except
		// for the coinbase which has no inputs) and unspend the
		// referenced txos.  This is necessary to match the order of the
		// spent txout entries.
		if isCoinbase {
			continue
		}
		for txInIdx := len(msgTx.TxIn) - 1; txInIdx > -1; txInIdx-- {
			// Ensure the spent txout index is decremented to stay
			// in sync with the transaction input.
			stxo := &stxos[stxoIdx]
			stxoIdx--

			// When there is not already an entry for the referenced
			// transaction in the view, it means it was fully spent,
			// so create a new utxo entry in order to resurrect it.
			txIn := msgTx.TxIn[txInIdx]
			originHash := &txIn.PreviousOutPoint.Hash
			originInIndex := txIn.PreviousOutPoint.Index
			//originHeight := txIn.BlockHeight
			// originIndex := txIn.BlockIndex
			entry := view.entries[*originHash]
			if entry == nil {
				entry = newUtxoEntry(stxo.txVersion, stxo.height,
					stxo.index, stxo.isCoinBase, stxo.hasExpiry,
					stxo.txType)
				if txType == stake.TxTypeSStx {
					//stakeExtra := make([]byte,
					//	serializeSizeForMinimalOutputs(tx))
					// putTxToMinimalOutputs(stakeExtra, tx)
					// entry.stakeExtra = stakeExtra
					entry.stakeExtra = stxo.stakeExtra
				}
				view.entries[*originHash] = entry
			}

			// Mark the entry as modified since it is either new
			// or will be changed below.
			entry.modified = true

			// Restore the specific utxo using the stxo data from
			// the spend journal if it doesn't already exist in the
			// view.
			output, ok := entry.sparseOutputs[originInIndex]
			if !ok {
				// Add the unspent transaction output.
				entry.sparseOutputs[originInIndex] = &utxoOutput{
					compressed:    stxo.compressed,
					spent:         false,
					amount:        txIn.ValueIn,
					scriptVersion: stxo.scriptVersion,
					pkScript:      stxo.pkScript,
				}
				continue
			}

			// Mark the existing referenced transaction output as
			// unspent.
			output.spent = false
		}
	}

	return stxoIdx + 1, nil
}

// Entries returns the underlying map that stores of all the utxo entries.
func (view *UtxoViewpoint) Entries() map[chainhash.Hash]*UtxoEntry {
	return view.entries
}

// commit prunes all entries marked modified that are now fully spent and marks
// all entries as unmodified.
func (view *UtxoViewpoint) commit() {
	for txHash, entry := range view.entries {
		if entry == nil || (entry.modified && entry.IsFullySpent()) {
			delete(view.entries, txHash)
			continue
		}

		entry.modified = false
	}
}

// fetchUtxosMain fetches unspent transaction output data about the provided
// set of transactions from the point of view of the end of the main chain at
// the time of the call.
//
// Upon completion of this function, the view will contain an entry for each
// requested transaction.  Fully spent transactions, or those which otherwise
// don't exist, will result in a nil entry in the view.
func (view *UtxoViewpoint) fetchUtxosMain(db database.DB,
	txSet map[chainhash.Hash]struct{}) error {
	// Nothing to do if there are no requested hashes.
	if len(txSet) == 0 {
		return nil
	}

	// Load the unspent transaction output information for the requested set
	// of transactions from the point of view of the end of the main chain.
	//
	// NOTE: Missing entries are not considered an error here and instead
	// will result in nil entries in the view.  This is intentionally done
	// since other code uses the presence of an entry in the store as a way
	// to optimize spend and unspend updates to apply only to the specific
	// utxos that the caller needs access to.
	err := db.View(func(dbTx database.Tx) error {
		for hash := range txSet {
			hashCopy := hash
			// If the UTX already exists in the view, skip adding it.
			if _, ok := view.entries[hashCopy]; ok {
				continue
			}
			entry, err := dbFetchUtxoEntry(dbTx, &hashCopy)
			if err != nil {
				return err
			}

			view.entries[hash] = entry
		}

		return nil
	})
	if err != nil {
		return err
	}

	return nil
}

// fetchUtxos loads utxo details about provided set of transaction hashes into
// the view from the database as needed unless they already exist in the view in
// which case they are ignored.
func (view *UtxoViewpoint) fetchUtxos(db database.DB,
	txSet map[chainhash.Hash]struct{}) error {
	// Nothing to do if there are no requested hashes.
	if len(txSet) == 0 {
		return nil
	}

	// Filter entries that are already in the view.
	txNeededSet := make(map[chainhash.Hash]struct{})
	for hash := range txSet {
		// Already loaded into the current view.
		if _, ok := view.entries[hash]; ok {
			continue
		}

		txNeededSet[hash] = struct{}{}
	}

	// Request the input utxos from the database.
	err := view.fetchUtxosMain(db, txNeededSet)
	if err != nil {
		return err
	}

	// Connect up to the stake viewpoint that was requested
	// if the flag to do so is given.
	return nil
}

// fetchInputUtxos loads utxo details about the input transactions referenced
// by the transactions in the given block into the view from the database as
// needed.  In particular, referenced entries that are earlier in the block are
// added to the view and entries that are already in the view are not modified.
func (view *UtxoViewpoint) fetchInputUtxos(db database.DB,
	block, parent *dcrutil.Block) error {
	viewpoint := view.StakeViewpoint()

	// Build a map of in-flight transactions because some of the inputs in
	// this block could be referencing other transactions earlier in this
	// block which are not yet in the chain.
	txInFlight := map[chainhash.Hash]int{}
	txNeededSet := make(map[chainhash.Hash]struct{})

	// Case 1: ViewpointPrevValidInitial. We need the viewpoint of the
	// current chain without the TxTreeRegular of the previous block
	// added so we can validate that.
	if viewpoint == ViewpointPrevValidInitial {
		transactions := parent.Transactions()
		for i, tx := range transactions {
			txInFlight[*tx.Sha()] = i
		}

		// Loop through all of the transaction inputs (except for the coinbase
		// which has no inputs) collecting them into sets of what is needed and
		// what is already known (in-flight).
		for i, tx := range transactions[1:] {
			for _, txIn := range tx.MsgTx().TxIn {
				// It is acceptable for a transaction input to reference
				// the output of another transaction in this block only
				// if the referenced transaction comes before the
				// current one in this block.  Add the outputs of the
				// referenced transaction as available utxos when this
				// is the case.  Otherwise, the utxo details are still
				// needed.
				//
				// NOTE: The >= is correct here because i is one less
				// than the actual position of the transaction within
				// the block due to skipping the coinbase.
				originHash := &txIn.PreviousOutPoint.Hash
				if inFlightIndex, ok := txInFlight[*originHash]; ok &&
					i >= inFlightIndex {

					originTx := transactions[inFlightIndex]
					view.AddTxOuts(originTx, block.Height(), uint32(i))
					continue
				}

				// Don't request entries that are already in the view
				// from the database.
				if _, ok := view.entries[*originHash]; ok {
					continue
				}

				txNeededSet[*originHash] = struct{}{}
			}
		}

		// Request the input utxos from the database.
		return view.fetchUtxosMain(db, txNeededSet)
	}

	// Case 2+3: ViewpointPrevValidStake and ViewpointPrevValidStake.
	// For ViewpointPrevValidStake, we need the viewpoint of the
	// current chain with the TxTreeRegular of the previous block
	// added so we can validate the TxTreeStake of the current block.
	// For ViewpointPrevInvalidStake, we need the viewpoint of the
	// current chain with the TxTreeRegular of the previous block
	// missing so we can validate the TxTreeStake of the current block.
	if viewpoint == ViewpointPrevValidStake ||
		viewpoint == ViewpointPrevInvalidStake {
		// We need all of the stake tx txins. None of these are considered
		// in-flight in relation to the regular tx tree or to other tx in
		// the stake tx tree, so don't do any of those expensive checks and
		// just append it to the tx slice.
		for _, tx := range block.MsgBlock().STransactions {
			isSSGen, _ := stake.IsSSGen(tx)

			for i, txIn := range tx.TxIn {
				// Ignore stakebases.
				if isSSGen && i == 0 {
					continue
				}

				// Add an entry to the transaction store for the needed
				// transaction with it set to missing by default.
				originHash := &txIn.PreviousOutPoint.Hash

				// Don't request entries that are already in the view
				// from the database.
				if _, ok := view.entries[*originHash]; ok {
					continue
				}

				txNeededSet[*originHash] = struct{}{}
			}
		}

		// Request the input utxos from the database.
		return view.fetchUtxosMain(db, txNeededSet)
	}

	// Case 4+5: ViewpointPrevValidRegular and
	// ViewpointPrevInvalidRegular.
	// For ViewpointPrevValidRegular, we need the viewpoint of the
	// current chain with the TxTreeRegular of the previous block
	// and the TxTreeStake of the current block added so we can
	// validate the TxTreeRegular of the current block.
	// For ViewpointPrevInvalidRegular, we need the viewpoint of the
	// current chain with the TxTreeRegular of the previous block
	// missing and the TxTreeStake of the current block added so we
	// can validate the TxTreeRegular of the current block.
	if viewpoint == ViewpointPrevValidRegular ||
		viewpoint == ViewpointPrevInvalidRegular {
		transactions := block.Transactions()
		for i, tx := range transactions {
			txInFlight[*tx.Sha()] = i
		}

		// Loop through all of the transaction inputs (except for the coinbase
		// which has no inputs) collecting them into sets of what is needed and
		// what is already known (in-flight).
		txNeededSet := make(map[chainhash.Hash]struct{})
		for i, tx := range transactions[1:] {
			for _, txIn := range tx.MsgTx().TxIn {
				// It is acceptable for a transaction input to reference
				// the output of another transaction in this block only
				// if the referenced transaction comes before the
				// current one in this block.  Add the outputs of the
				// referenced transaction as available utxos when this
				// is the case.  Otherwise, the utxo details are still
				// needed.
				//
				// NOTE: The >= is correct here because i is one less
				// than the actual position of the transaction within
				// the block due to skipping the coinbase.
				originHash := &txIn.PreviousOutPoint.Hash
				if inFlightIndex, ok := txInFlight[*originHash]; ok &&
					i >= inFlightIndex {

					originTx := transactions[inFlightIndex]
					view.AddTxOuts(originTx, block.Height(), uint32(i))
					continue
				}

				// Don't request entries that are already in the view
				// from the database.
				if _, ok := view.entries[*originHash]; ok {
					continue
				}

				txNeededSet[*originHash] = struct{}{}
			}
		}

		// Request the input utxos from the database.
		return view.fetchUtxosMain(db, txNeededSet)
	}

	// TODO actual blockchain error
	return fmt.Errorf("invalid stake viewpoint")
}

// NewUtxoViewpoint returns a new empty unspent transaction output view.
func NewUtxoViewpoint() *UtxoViewpoint {
	return &UtxoViewpoint{
		entries: make(map[chainhash.Hash]*UtxoEntry),
	}
}

// FetchUtxoView loads utxo details about the input transactions referenced by
// the passed transaction from the point of view of the end of the main chain.
// It also attempts to fetch the utxo details for the transaction itself so the
// returned view can be examined for duplicate unspent transaction outputs.
//
// This function is safe for concurrent access however the returned view is NOT.
func (b *BlockChain) FetchUtxoView(tx *dcrutil.Tx, treeValid bool) (*UtxoViewpoint,
	error) {
	b.chainLock.RLock()
	defer b.chainLock.RUnlock()

	// Request the utxos from the point of view of the end of the main
	// chain.
	view := NewUtxoViewpoint()
	if treeValid {
		view.SetStakeViewpoint(ViewpointPrevValidRegular)
		block, err := b.fetchBlockFromHash(&b.bestNode.hash)
		if err != nil {
			return nil, err
		}
		parent, err := b.fetchBlockFromHash(&b.bestNode.header.PrevBlock)
		if err != nil {
			return nil, err
		}
		err = view.fetchInputUtxos(b.db, block, parent)
		if err != nil {
			return nil, err
		}
		for i, blockTx := range block.Transactions() {
			err := view.connectTransaction(blockTx, b.bestNode.height,
				uint32(i), nil)
			if err != nil {
				return nil, err
			}
		}
	}
	view.SetBestHash(&b.bestNode.hash)

	// Create a set of needed transactions based on those referenced by the
	// inputs of the passed transaction.  Also, add the passed transaction
	// itself as a way for the caller to detect duplicates that are not
	// fully spent.
	txNeededSet := make(map[chainhash.Hash]struct{})
	txNeededSet[*tx.Sha()] = struct{}{}
	msgTx := tx.MsgTx()
	isSSGen, _ := stake.IsSSGen(msgTx)
	if !IsCoinBaseTx(msgTx) {
		for i, txIn := range msgTx.TxIn {
			if isSSGen && i == 0 {
				continue
			}
			txNeededSet[txIn.PreviousOutPoint.Hash] = struct{}{}
		}
	}

	err := view.fetchUtxosMain(b.db, txNeededSet)

	return view, err
}

// FetchUtxoEntry loads and returns the unspent transaction output entry for the
// passed hash from the point of view of the end of the main chain.
//
// NOTE: Requesting a hash for which there is no data will NOT return an error.
// Instead both the entry and the error will be nil.  This is done to allow
// pruning of fully spent transactions.  In practice this means the caller must
// check if the returned entry is nil before invoking methods on it.
//
// This function is safe for concurrent access however the returned entry (if
// any) is NOT.
func (b *BlockChain) FetchUtxoEntry(txHash *chainhash.Hash) (*UtxoEntry, error) {
	b.chainLock.RLock()
	defer b.chainLock.RUnlock()

	var entry *UtxoEntry
	err := b.db.View(func(dbTx database.Tx) error {
		var err error
		entry, err = dbFetchUtxoEntry(dbTx, txHash)
		return err
	})
	if err != nil {
		return nil, err
	}

	return entry, nil
}