package blockchain

import (
	"fmt"
	"github.com/p9c/p9/pkg/block"
	
	"github.com/p9c/p9/pkg/chainhash"
	"github.com/p9c/p9/pkg/database"
	"github.com/p9c/p9/pkg/txscript"
	"github.com/p9c/p9/pkg/util"
	"github.com/p9c/p9/pkg/wire"
)

// txoFlags is a bitmask defining additional information and state for a transaction output in a utxo view.
type txoFlags uint8

const (
	// tfCoinBase indicates that a txout was contained in a coinbase tx.
	tfCoinBase txoFlags = 1 << iota
	// tfSpent indicates that a txout is spent.
	tfSpent
	// tfModified indicates that a txout has been modified since it was loaded.
	tfModified
)

// UtxoEntry houses details about an individual transaction output in a utxo view such as whether or not it was
// contained in a coinbase tx, the height of the block that contains the tx, whether or not it is spent, its public key
// script, and how much it pays.
type UtxoEntry struct {
	// NOTE: Additions, deletions, or modifications to the order of the definitions in this struct should not be changed
	// without considering how it affects alignment on 64-bit platforms. The current order is specifically crafted to
	// result in minimal padding. There will be a lot of these in memory, so a few extra bytes of padding adds up.
	amount      int64
	pkScript    []byte // The public key script for the output.
	blockHeight int32  // Height of block containing tx.
	// packedFlags contains additional info about output such as whether it is a coinbase, whether it is spent, and
	// whether it has been modified since it was loaded. This approach is used in order to reduce memory usage since
	// there will be a lot of these in memory.
	packedFlags txoFlags
}

// isModified returns whether or not the output has been modified since it was loaded.
func (entry *UtxoEntry) isModified() bool {
	return entry.packedFlags&tfModified == tfModified
}

// IsCoinBase returns whether or not the output was contained in a coinbase transaction.
func (entry *UtxoEntry) IsCoinBase() bool {
	return entry.packedFlags&tfCoinBase == tfCoinBase
}

// BlockHeight returns the height of the block containing the output.
func (entry *UtxoEntry) BlockHeight() int32 {
	return entry.blockHeight
}

// IsSpent returns whether or not the output has been spent based upon the current state of the unspent transaction
// output view it was obtained from.
func (entry *UtxoEntry) IsSpent() bool {
	return entry.packedFlags&tfSpent == tfSpent
}

// Spend marks the output as spent. Spending an output that is already spent has no effect.
func (entry *UtxoEntry) Spend() {
	// Nothing to do if the output is already spent.
	if entry.IsSpent() {
		return
	}
	// Mark the output as spent and modified.
	entry.packedFlags |= tfSpent | tfModified
}

// Amount returns the amount of the output.
func (entry *UtxoEntry) Amount() int64 {
	return entry.amount
}

// PkScript returns the public key script for the output.
func (entry *UtxoEntry) PkScript() []byte {
	return entry.pkScript
}

// Clone returns a shallow copy of the utxo entry.
func (entry *UtxoEntry) Clone() *UtxoEntry {
	if entry == nil {
		return nil
	}
	return &UtxoEntry{
		amount:      entry.amount,
		pkScript:    entry.pkScript,
		blockHeight: entry.blockHeight,
		packedFlags: entry.packedFlags,
	}
}

// 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[wire.OutPoint]*UtxoEntry
	bestHash chainhash.Hash
}

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

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

// LookupEntry returns information about a given transaction output according to the current state of the view. It will
// return nil if the passed output 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(outpoint wire.OutPoint) *UtxoEntry {
	return view.entries[outpoint]
}

// addTxOut adds the specified output to the view if it is not provably unspendable. When the view already has an entry
// for the output, it will be marked unspent. All fields will be updated for existing entries since it's possible it has
// changed during a reorg.
func (view *UtxoViewpoint) addTxOut(outpoint wire.OutPoint, txOut *wire.TxOut, isCoinBase bool, blockHeight int32) {
	// Don't add provably unspendable outputs.
	if txscript.IsUnspendable(txOut.PkScript) {
		return
	}
	// 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.
	entry := view.LookupEntry(outpoint)
	if entry == nil {
		entry = new(UtxoEntry)
		view.entries[outpoint] = entry
	}
	entry.amount = txOut.Value
	entry.pkScript = txOut.PkScript
	entry.blockHeight = blockHeight
	entry.packedFlags = tfModified
	if isCoinBase {
		entry.packedFlags |= tfCoinBase
	}
}

// AddTxOut adds the specified output of the passed transaction to the view if it exists and is not provably
// unspendable. When the view already has an entry for the output, it will be marked unspent. All fields will be updated
// for existing entries since it's possible it has changed during a reorg.
func (view *UtxoViewpoint) AddTxOut(tx *util.Tx, txOutIdx uint32, blockHeight int32) {
	// Can't add an output for an out of bounds index.
	if txOutIdx >= uint32(len(tx.MsgTx().TxOut)) {
		return
	}
	// 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.
	prevOut := wire.OutPoint{Hash: *tx.Hash(), Index: txOutIdx}
	txOut := tx.MsgTx().TxOut[txOutIdx]
	view.addTxOut(prevOut, txOut, IsCoinBase(tx), blockHeight)
}

// 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 *util.Tx, blockHeight int32) {
	// Loop all of the transaction outputs and add those which are not
	// provably unspendable.
	isCoinBase := IsCoinBase(tx)
	prevOut := wire.OutPoint{Hash: *tx.Hash()}
	for txOutIdx, txOut := range tx.MsgTx().TxOut {
		// 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.
		prevOut.Index = uint32(txOutIdx)
		view.addTxOut(prevOut, txOut, isCoinBase, blockHeight)
	}
}

// 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 *util.Tx, blockHeight int32, stxos *[]SpentTxOut) (e error) {
	// Coinbase transactions don't have any inputs to spend.
	if IsCoinBase(tx) {
		// Add the transaction's outputs as available utxos.
		view.AddTxOuts(tx, blockHeight)
		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.
	for _, txIn := range tx.MsgTx().TxIn {
		// Ensure the referenced utxo exists in the view. This should never happen unless there is a bug is introduced
		// in the code.
		entry := view.entries[txIn.PreviousOutPoint]
		if entry == nil {
			return AssertError(
				fmt.Sprintf(
					"view missing input %v",
					txIn.PreviousOutPoint,
				),
			)
		}
		// Only create the stxo details if requested.
		if stxos != nil {
			// Populate the stxo details using the utxo entry.
			var stxo = SpentTxOut{
				Amount:     entry.Amount(),
				PkScript:   entry.PkScript(),
				Height:     entry.BlockHeight(),
				IsCoinBase: entry.IsCoinBase(),
			}
			*stxos = append(*stxos, stxo)
		}
		// Mark the entry as spent. This is not done until after the relevant details have been accessed since spending
		// it might clear the fields from memory in the future.
		entry.Spend()
	}
	// Add the transaction's outputs as available utxos.
	view.AddTxOuts(tx, blockHeight)
	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 (view *UtxoViewpoint) connectTransactions(block *block.Block, stxos *[]SpentTxOut) (e error) {
	for _, tx := range block.Transactions() {
		e = view.connectTransaction(tx, block.Height(), stxos)
		if e != nil {
			return e
		}
	}
	// Update the best hash for view to include this block since all of its transactions have been connected.
	view.SetBestHash(block.Hash())
	return nil
}

// fetchEntryByHash attempts to find any available utxo for the given hash by searching the entire set of possible
// outputs for the given hash. It checks the view first and then falls back to the database if needed.
func (view *UtxoViewpoint) fetchEntryByHash(db database.DB, hash *chainhash.Hash) (entry *UtxoEntry, e error) {
	// First attempt to find a utxo with the provided hash in the view.
	prevOut := wire.OutPoint{Hash: *hash}
	for idx := uint32(0); idx < MaxOutputsPerBlock; idx++ {
		prevOut.Index = idx
		entry = view.LookupEntry(prevOut)
		if entry != nil {
			return entry, nil
		}
	}
	// Chk the database since it doesn't exist in the view. This will often by the case since only specifically
	// referenced utxos are loaded into the view.
	e = db.View(
		func(dbTx database.Tx) (e error) {
			entry, e = dbFetchUtxoEntryByHash(dbTx, hash)
			return e
		},
	)
	return entry, e
}

// 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.
func (view *UtxoViewpoint) disconnectTransactions(db database.DB, block *block.Block, stxos []SpentTxOut) (e error) {
	// Sanity check the correct number of stxos are provided.
	if len(stxos) != countSpentOutputs(block) {
		return AssertError(
			"disconnectTransactions called with bad " +
				"spent transaction out information",
		)
	}
	// 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.
	stxoIdx := len(stxos) - 1
	transactions := block.Transactions()
	for txIdx := len(transactions) - 1; txIdx > -1; txIdx-- {
		tx := transactions[txIdx]
		// All entries will need to potentially be marked as a coinbase.
		var packedFlags txoFlags
		isCoinBase := txIdx == 0
		if isCoinBase {
			packedFlags |= tfCoinBase
		}
		// Mark all of the spendable outputs originally created by the transaction as spent. It is instructive to note
		// that while the outputs aren't actually being spent here, rather they no longer exist, since a pruned utxo set
		// is used, there is no practical difference between a utxo that does not exist and one that has been spent.
		//
		// When the utxo does not already exist in the view, add an entry for it and then mark it spent. This is done
		// because the code relies on its existence in the view in order to signal modifications have happened.
		txHash := tx.Hash()
		prevOut := wire.OutPoint{Hash: *txHash}
		for txOutIdx, txOut := range tx.MsgTx().TxOut {
			if txscript.IsUnspendable(txOut.PkScript) {
				continue
			}
			prevOut.Index = uint32(txOutIdx)
			entry := view.entries[prevOut]
			if entry == nil {
				entry = &UtxoEntry{
					amount:      txOut.Value,
					pkScript:    txOut.PkScript,
					blockHeight: block.Height(),
					packedFlags: packedFlags,
				}
				view.entries[prevOut] = entry
			}
			entry.Spend()
		}
		// 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 output in the view, it means it was previously
			// spent, so create a new utxo entry in order to resurrect it.
			originOut := &tx.MsgTx().TxIn[txInIdx].PreviousOutPoint
			entry := view.entries[*originOut]
			if entry == nil {
				entry = new(UtxoEntry)
				view.entries[*originOut] = entry
			}
			// The legacy v1 spend journal format only stored the coinbase flag and height when the output was the last
			// unspent output of the transaction. As a result, when the information is missing, search for it by
			// scanning all possible outputs of the transaction since it must be in one of them.
			//
			// It should be noted that this is quite inefficient, but it realistically will almost never run since all
			// new entries include the information for all outputs and thus the only way this will be hit is if a long
			// enough reorg happens such that a block with the old spend data is being disconnected. The probability of
			// that in practice is extremely low to begin with and becomes vanishingly small the more new blocks are
			// connected. In the case of a fresh database that has only ever run with the new v2 format, this code path
			// will never run.
			if stxo.Height == 0 {
				utxo, e := view.fetchEntryByHash(db, txHash)
				if e != nil {
					return e
				}
				if utxo == nil {
					return AssertError(
						fmt.Sprintf(
							"unable "+
								"to resurrect legacy stxo %v",
							*originOut,
						),
					)
				}
				stxo.Height = utxo.BlockHeight()
				stxo.IsCoinBase = utxo.IsCoinBase()
			}
			// Restore the utxo using the stxo data from the spend journal and mark it as modified.
			entry.amount = stxo.Amount
			entry.pkScript = stxo.PkScript
			entry.blockHeight = stxo.Height
			entry.packedFlags = tfModified
			if stxo.IsCoinBase {
				entry.packedFlags |= tfCoinBase
			}
		}
	}
	// Update the best hash for view to the previous block since all of the transactions for the current block have been
	// disconnected.
	view.SetBestHash(&block.WireBlock().Header.PrevBlock)
	return nil
}

// RemoveEntry removes the given transaction output from the current state of the view. It will have no effect if the
// passed output does not exist in the view.
func (view *UtxoViewpoint) RemoveEntry(outpoint wire.OutPoint) {
	delete(view.entries, outpoint)
}

// Entries returns the underlying map that stores of all the utxo entries.
func (view *UtxoViewpoint) Entries() map[wire.OutPoint]*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 outpoint, entry := range view.entries {
		if entry == nil || (entry.isModified() && entry.IsSpent()) {
			delete(view.entries, outpoint)
			continue
		}
		entry.packedFlags ^= tfModified
	}
}

// fetchUtxosMain fetches unspent transaction output data about the provided set of outpoints 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 outpoint. Spent outputs, or those
// which otherwise don't exist, will result in a nil entry in the view.
func (view *UtxoViewpoint) fetchUtxosMain(db database.DB, outpoints map[wire.OutPoint]struct{}) (e error) {
	// Nothing to do if there are no requested outputs.
	if len(outpoints) == 0 {
		return nil
	}
	// Load the requested set of unspent transaction outputs 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 so other code can use the presence of an entry in the store as a way to unnecessarily avoid
	// attempting to reload it from the database.
	return db.View(
		func(dbTx database.Tx) (e error) {
			for outpoint := range outpoints {
				entry, e := dbFetchUtxoEntry(dbTx, outpoint)
				if e != nil {
					return e
				}
				view.entries[outpoint] = entry
			}
			return nil
		},
	)
}

// fetchUtxos loads the unspent transaction outputs for the provided set of outputs 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, outpoints map[wire.OutPoint]struct{}) (e error) {
	// Nothing to do if there are no requested outputs.
	if len(outpoints) == 0 {
		return nil
	}
	// Filter entries that are already in the view.
	neededSet := make(map[wire.OutPoint]struct{})
	for outpoint := range outpoints {
		// Already loaded into the current view.
		if _, ok := view.entries[outpoint]; ok {
			continue
		}
		neededSet[outpoint] = struct{}{}
	}
	// Request the input utxos from the database.
	return view.fetchUtxosMain(db, neededSet)
}

// fetchInputUtxos loads the unspent transaction outputs for the inputs 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 *block.Block) (e error) {
	// 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{}
	transactions := block.Transactions()
	for i, tx := range transactions {
		txInFlight[*tx.Hash()] = 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).
	neededSet := make(map[wire.OutPoint]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())
				continue
			}
			// Don't request entries that are already in the view from the database.
			if _, ok := view.entries[txIn.PreviousOutPoint]; ok {
				continue
			}
			neededSet[txIn.PreviousOutPoint] = struct{}{}
		}
	}
	// Request the input utxos from the database.
	return view.fetchUtxosMain(db, neededSet)
}

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

// FetchUtxoView loads unspent transaction outputs for the inputs referenced by the passed transaction from the point of
// view of the end of the main chain.
//
// It also attempts to fetch the utxos for the outputs of the transaction itself so the returned view can be examined
// for duplicate transactions.
//
// This function is safe for concurrent access however the returned view is NOT.
func (b *BlockChain) FetchUtxoView(tx *util.Tx) (view *UtxoViewpoint, e error) {
	// Create a set of needed outputs based on those referenced by the inputs of the passed transaction and the outputs
	// of the transaction itself.
	neededSet := make(map[wire.OutPoint]struct{})
	prevOut := wire.OutPoint{Hash: *tx.Hash()}
	for txOutIdx := range tx.MsgTx().TxOut {
		prevOut.Index = uint32(txOutIdx)
		neededSet[prevOut] = struct{}{}
	}
	if !IsCoinBase(tx) {
		for _, txIn := range tx.MsgTx().TxIn {
			neededSet[txIn.PreviousOutPoint] = struct{}{}
		}
	}
	// Request the utxos from the point of view of the end of the main
	// chain.
	view = NewUtxoViewpoint()
	b.ChainLock.RLock()
	e = view.fetchUtxosMain(b.db, neededSet)
	b.ChainLock.RUnlock()
	return view, e
}

// FetchUtxoEntry loads and returns the requested unspent transaction output from the point of view of the end of the
// main chain.
//
// NOTE: Requesting an output 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 spent transaction outputs. 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.
// TODO: concurrent unsafe and copy creation should be universal and able to be expected
func (b *BlockChain) FetchUtxoEntry(outpoint wire.OutPoint) (*UtxoEntry, error) {
	b.ChainLock.RLock()
	defer b.ChainLock.RUnlock()
	var entry *UtxoEntry
	e := b.db.View(
		func(dbTx database.Tx) (e error) {
			entry, e = dbFetchUtxoEntry(dbTx, outpoint)
			return e
		},
	)
	if e != nil {
		return nil, e
	}
	return entry, nil
}