!h.Less(j, i) for 0 <= i < h.Len() and 2*i+1 <= j <= 2*i+2 and j < h.Len()
// 把数组看成是一颗满的二叉树，第一个元素是树根，第二和第三个元素是树根的两个树枝，
// 这样依次推下去 那么如果树根是  i 那么它的两个树枝就是 2*i+2 和 2*i + 2。
// 最小堆的定义是 任意的树根不能比它的两个树枝大。 也就是上面的代码描述的定义。
heap.Interface的定义

我们只需要定义满足下面接口的数据结构，就能够使用heap的一些方法来实现为堆结构。
type Interface interface {
	sort.Interface
	Push(x interface{}) // add x as element Len() 把x增加到最后
	Pop() interface{}   //  remove and return element Len() - 1. 移除并返回最后的一个元素
}

// nonceHeap is a heap.Interface implementation over 64bit unsigned integers for
// retrieving sorted transactions from the possibly gapped future queue.
type nonceHeap []uint64

func (h nonceHeap) Len() int           { return len(h) }
func (h nonceHeap) Less(i, j int) bool { return h[i] < h[j] }
func (h nonceHeap) Swap(i, j int)      { h[i], h[j] = h[j], h[i] }

func (h *nonceHeap) Push(x interface{}) {
	*h = append(*h, x.(uint64))
}

func (h *nonceHeap) Pop() interface{} {
	old := *h
	n := len(old)
	x := old[n-1]
	*h = old[0 : n-1]
	return x
}

// txSortedMap is a nonce->transaction hash map with a heap based index to allow
// iterating over the contents in a nonce-incrementing way.
// txSortedMap是一个具有基于堆的索引的nonce->交易 的hashmap，
// 允许以nonce递增的方式迭代内容。

type Transactions []*Transaction 

type txSortedMap struct {
	items map[uint64]*types.Transaction // Hash map storing the transaction data
	index *nonceHeap                    // Heap of nonces of all the stored transactions (non-strict mode)
	cache types.Transactions            // Cache of the transactions already sorted 用来缓存已经排好序的交易。
}

// Get retrieves the current transactions associated with the given nonce.
func (m *txSortedMap) Get(nonce uint64) *types.Transaction {
	return m.items[nonce]
}

// Put inserts a new transaction into the map, also updating the map's nonce
// index. If a transaction already exists with the same nonce, it's overwritten.
// 把一个新的事务插入到map中，同时更新map的nonce索引。 如果一个事务已经存在，就把它覆盖。 同时任何缓存的数据会被删除。
func (m *txSortedMap) Put(tx *types.Transaction) {
	nonce := tx.Nonce()
	if m.items[nonce] == nil {
		heap.Push(m.index, nonce)
	}
	m.items[nonce], m.cache = tx, nil
}

// Forward removes all transactions from the map with a nonce lower than the
// provided threshold. Every removed transaction is returned for any post-removal
// maintenance.
func (m *txSortedMap) Forward(threshold uint64) types.Transactions {
	var removed types.Transactions

	// Pop off heap items until the threshold is reached
	for m.index.Len() > 0 && (*m.index)[0] < threshold {
		nonce := heap.Pop(m.index).(uint64)
		removed = append(removed, m.items[nonce])
		delete(m.items, nonce)
	}
	// If we had a cached order, shift the front
	// cache是排好序的交易。 
	if m.cache != nil {
		m.cache = m.cache[len(removed):]
	}
	return removed
}

// Filter iterates over the list of transactions and removes all of them for which
// the specified function evaluates to true.
func (m *txSortedMap) Filter(filter func(*types.Transaction) bool) types.Transactions {
	var removed types.Transactions

	// Collect all the transactions to filter out
	for nonce, tx := range m.items {
		if filter(tx) {
			removed = append(removed, tx)
			delete(m.items, nonce)
		}
	}
	// If transactions were removed, the heap and cache are ruined
	// 如果事务被删除，堆和缓存被毁坏
	if len(removed) > 0 {
		*m.index = make([]uint64, 0, len(m.items))
		for nonce := range m.items {
			*m.index = append(*m.index, nonce)
		}
		// 需要重建堆
		heap.Init(m.index)
		// 设置cache为nil
		m.cache = nil
	}
	return removed
}

// Cap places a hard limit on the number of items, returning all transactions
// exceeding that limit.
// Cap 对items里面的数量有限制，返回超过限制的所有交易。
func (m *txSortedMap) Cap(threshold int) types.Transactions {
	// Short circuit if the number of items is under the limit
	if len(m.items) <= threshold {
		return nil
	}
	// Otherwise gather and drop the highest nonce'd transactions
	var drops types.Transactions

	sort.Sort(*m.index) //从小到大排序 从尾部删除。
	for size := len(m.items); size > threshold; size-- {
		drops = append(drops, m.items[(*m.index)[size-1]])
		delete(m.items, (*m.index)[size-1])
	}
	*m.index = (*m.index)[:threshold]
	// 重建堆
	heap.Init(m.index)

	// If we had a cache, shift the back
	if m.cache != nil {
		m.cache = m.cache[:len(m.cache)-len(drops)]
	}
	return drops
}

// Remove deletes a transaction from the maintained map, returning whether the
// transaction was found.
// 
func (m *txSortedMap) Remove(nonce uint64) bool {
	// Short circuit if no transaction is present
	_, ok := m.items[nonce]
	if !ok {
		return false
	}
	// Otherwise delete the transaction and fix the heap index
	for i := 0; i < m.index.Len(); i++ {
		if (*m.index)[i] == nonce {
			heap.Remove(m.index, i)
			break
		}
	}
	delete(m.items, nonce)
	m.cache = nil

	return true
}

// Ready retrieves a sequentially increasing list of transactions starting at the
// provided nonce that is ready for processing. The returned transactions will be
// removed from the list.
// Ready 返回一个从指定nonce开始，连续的交易。 返回的交易会被删除。
// Note, all transactions with nonces lower than start will also be returned to
// prevent getting into and invalid state. This is not something that should ever
// happen but better to be self correcting than failing!
// 注意，请注意，所有具有低于start的nonce的交易也将被返回，以防止进入和无效状态。 
// 这不是应该发生的事情，而是自我纠正而不是失败！
func (m *txSortedMap) Ready(start uint64) types.Transactions {
	// Short circuit if no transactions are available
	if m.index.Len() == 0 || (*m.index)[0] > start {
		return nil
	}
	// Otherwise start accumulating incremental transactions
	var ready types.Transactions
	// 从最小的开始，一个一个的增加，
	for next := (*m.index)[0]; m.index.Len() > 0 && (*m.index)[0] == next; next++ {
		ready = append(ready, m.items[next])
		delete(m.items, next)
		heap.Pop(m.index)
	}
	m.cache = nil

	return ready
}

// Len returns the length of the transaction map.
func (m *txSortedMap) Len() int {
	return len(m.items)
}

// Flatten creates a nonce-sorted slice of transactions based on the loosely
// sorted internal representation. The result of the sorting is cached in case
// it's requested again before any modifications are made to the contents.
func (m *txSortedMap) Flatten() types.Transactions {
	// If the sorting was not cached yet, create and cache it
	if m.cache == nil {
		m.cache = make(types.Transactions, 0, len(m.items))
		for _, tx := range m.items {
			m.cache = append(m.cache, tx)
		}
		sort.Sort(types.TxByNonce(m.cache))
	}
	// Copy the cache to prevent accidental modifications
	txs := make(types.Transactions, len(m.cache))
	copy(txs, m.cache)
	return txs
}

// txList is a "list" of transactions belonging to an account, sorted by account
// nonce. The same type can be used both for storing contiguous transactions for
// the executable/pending queue; and for storing gapped transactions for the non-
// executable/future queue, with minor behavioral changes.
type txList struct {
	strict bool         // Whether nonces are strictly continuous or not nonces是严格连续的还是非连续的
	txs    *txSortedMap // Heap indexed sorted hash map of the transactions 基于堆索引的交易的hashmap

	costcap *big.Int // Price of the highest costing transaction (reset only if exceeds balance)  所有交易里面，GasPrice * GasLimit最高的值
	gascap  *big.Int // Gas limit of the highest spending transaction (reset only if exceeds block limit) 所有交易里面， GasPrice最高的值
}

// Overlaps returns whether the transaction specified has the same nonce as one
// already contained within the list.
// 
func (l *txList) Overlaps(tx *types.Transaction) bool {
	return l.txs.Get(tx.Nonce()) != nil
}

// Add tries to insert a new transaction into the list, returning whether the
// transaction was accepted, and if yes, any previous transaction it replaced.
// Add 尝试插入一个新的交易，返回交易是否被接收，如果被接收，那么任意之前的交易会被替换。
// If the new transaction is accepted into the list, the lists' cost and gas
// thresholds are also potentially updated.
// 如果新的交易被接收，那么总的cost和gas限制会被更新。
func (l *txList) Add(tx *types.Transaction, priceBump uint64) (bool, *types.Transaction) {
	// If there's an older better transaction, abort
	// 如果存在老的交易。 而且新的交易的价格比老的高出一定的数量。那么替换。
	old := l.txs.Get(tx.Nonce())
	if old != nil {
		threshold := new(big.Int).Div(new(big.Int).Mul(old.GasPrice(), big.NewInt(100+int64(priceBump))), big.NewInt(100))
		if threshold.Cmp(tx.GasPrice()) >= 0 {
			return false, nil
		}
	}
	// Otherwise overwrite the old transaction with the current one
	l.txs.Put(tx)
	if cost := tx.Cost(); l.costcap.Cmp(cost) < 0 {
		l.costcap = cost
	}
	if gas := tx.Gas(); l.gascap.Cmp(gas) < 0 {
		l.gascap = gas
	}
	return true, old
}

// Forward removes all transactions from the list with a nonce lower than the
// provided threshold. Every removed transaction is returned for any post-removal
// maintenance.
func (l *txList) Forward(threshold uint64) types.Transactions {
	return l.txs.Forward(threshold)
}

// Filter removes all transactions from the list with a cost or gas limit higher
// than the provided thresholds. Every removed transaction is returned for any
// post-removal maintenance. Strict-mode invalidated transactions are also
// returned.
// Filter 移除所有比提供的cost或者gasLimit的值更高的交易。 被移除的交易会返回以便进一步处理。 在严格模式下，所有无效的交易同样被返回。
// 
// This method uses the cached costcap and gascap to quickly decide if there's even
// a point in calculating all the costs or if the balance covers all. If the threshold
// is lower than the costgas cap, the caps will be reset to a new high after removing
// the newly invalidated transactions.
// 这个方法会使用缓存的costcap和gascap以便快速的决定是否需要遍历所有的交易。如果限制小于缓存的costcap和gascap，那么在移除不合法的交易之后会更新costcap和gascap的值。

func (l *txList) Filter(costLimit, gasLimit *big.Int) (types.Transactions, types.Transactions) {
	// If all transactions are below the threshold, short circuit
	// 如果所有的交易都小于限制，那么直接返回。
	if l.costcap.Cmp(costLimit) <= 0 && l.gascap.Cmp(gasLimit) <= 0 {
		return nil, nil
	}
	l.costcap = new(big.Int).Set(costLimit) // Lower the caps to the thresholds
	l.gascap = new(big.Int).Set(gasLimit)

	// Filter out all the transactions above the account's funds
	removed := l.txs.Filter(func(tx *types.Transaction) bool { return tx.Cost().Cmp(costLimit) > 0 || tx.Gas().Cmp(gasLimit) > 0 })

	// If the list was strict, filter anything above the lowest nonce
	var invalids types.Transactions

	if l.strict && len(removed) > 0 {
		// 所有的nonce大于 最小的被移除的nonce的交易都被任务是无效的。
		// 在严格模式下，这种交易也被移除。
		lowest := uint64(math.MaxUint64)
		for _, tx := range removed {
			if nonce := tx.Nonce(); lowest > nonce {
				lowest = nonce
			}
		}
		invalids = l.txs.Filter(func(tx *types.Transaction) bool { return tx.Nonce() > lowest })
	}
	return removed, invalids
}

// Cap places a hard limit on the number of items, returning all transactions
// exceeding that limit.
func (l *txList) Cap(threshold int) types.Transactions {
	return l.txs.Cap(threshold)
}

// Remove deletes a transaction from the maintained list, returning whether the
// transaction was found, and also returning any transaction invalidated due to
// the deletion (strict mode only).
func (l *txList) Remove(tx *types.Transaction) (bool, types.Transactions) {
	// Remove the transaction from the set
	nonce := tx.Nonce()
	if removed := l.txs.Remove(nonce); !removed {
		return false, nil
	}
	// In strict mode, filter out non-executable transactions
	if l.strict {
		return true, l.txs.Filter(func(tx *types.Transaction) bool { return tx.Nonce() > nonce })
	}
	return true, nil
}

// Ready retrieves a sequentially increasing list of transactions starting at the
// provided nonce that is ready for processing. The returned transactions will be
// removed from the list.
//
// Note, all transactions with nonces lower than start will also be returned to
// prevent getting into and invalid state. This is not something that should ever
// happen but better to be self correcting than failing!
func (l *txList) Ready(start uint64) types.Transactions {
	return l.txs.Ready(start)
}

// Len returns the length of the transaction list.
func (l *txList) Len() int {
	return l.txs.Len()
}

// Empty returns whether the list of transactions is empty or not.
func (l *txList) Empty() bool {
	return l.Len() == 0
}

// Flatten creates a nonce-sorted slice of transactions based on the loosely
// sorted internal representation. The result of the sorting is cached in case
// it's requested again before any modifications are made to the contents.
func (l *txList) Flatten() types.Transactions {
	return l.txs.Flatten()
}

// priceHeap is a heap.Interface implementation over transactions for retrieving
// price-sorted transactions to discard when the pool fills up.
type priceHeap []*types.Transaction

func (h priceHeap) Len() int           { return len(h) }
func (h priceHeap) Less(i, j int) bool { return h[i].GasPrice().Cmp(h[j].GasPrice()) < 0 }
func (h priceHeap) Swap(i, j int)      { h[i], h[j] = h[j], h[i] }

func (h *priceHeap) Push(x interface{}) {
	*h = append(*h, x.(*types.Transaction))
}

func (h *priceHeap) Pop() interface{} {
	old := *h
	n := len(old)
	x := old[n-1]
	*h = old[0 : n-1]
	return x
}

// txPricedList is a price-sorted heap to allow operating on transactions pool
// contents in a price-incrementing way.
type txPricedList struct {
	all    *map[common.Hash]*types.Transaction // Pointer to the map of all transactions 这是一个指针，指向了所有交易的map
	items  *priceHeap                          // Heap of prices of all the stored transactions
	stales int                                 // Number of stale price points to (re-heap trigger)
}

// newTxPricedList creates a new price-sorted transaction heap.
func newTxPricedList(all *map[common.Hash]*types.Transaction) *txPricedList {
	return &txPricedList{
		all:   all,
		items: new(priceHeap),
	}
}

// Put inserts a new transaction into the heap.
func (l *txPricedList) Put(tx *types.Transaction) {
	heap.Push(l.items, tx)
}

// Removed notifies the prices transaction list that an old transaction dropped
// from the pool. The list will just keep a counter of stale objects and update
// the heap if a large enough ratio of transactions go stale.
// Removed 用来通知txPricedList有一个老的交易被删除. txPricedList使用一个计数器来决定何时更新堆信息.
func (l *txPricedList) Removed() {
	// Bump the stale counter, but exit if still too low (< 25%)
	l.stales++
	if l.stales <= len(*l.items)/4 {
		return
	}
	// Seems we've reached a critical number of stale transactions, reheap
	reheap := make(priceHeap, 0, len(*l.all))

	l.stales, l.items = 0, &reheap
	for _, tx := range *l.all {
		*l.items = append(*l.items, tx)
	}
	heap.Init(l.items)
}

// Cap finds all the transactions below the given price threshold, drops them
// from the priced list and returs them for further removal from the entire pool.
func (l *txPricedList) Cap(threshold *big.Int, local *accountSet) types.Transactions {
	drop := make(types.Transactions, 0, 128) // Remote underpriced transactions to drop
	save := make(types.Transactions, 0, 64)  // Local underpriced transactions to keep

	for len(*l.items) > 0 {
		// Discard stale transactions if found during cleanup
		tx := heap.Pop(l.items).(*types.Transaction)
		if _, ok := (*l.all)[tx.Hash()]; !ok {
			// 如果发现一个已经删除的,那么更新states计数器
			l.stales--
			continue
		}
		// Stop the discards if we've reached the threshold
		if tx.GasPrice().Cmp(threshold) >= 0 {
			// 如果价格不小于阈值, 那么退出
			save = append(save, tx)
			break
		}
		// Non stale transaction found, discard unless local
		if local.containsTx(tx) {  //本地的交易不会删除
			save = append(save, tx)
		} else {
			drop = append(drop, tx)
		}
	}
	for _, tx := range save {
		heap.Push(l.items, tx)
	}
	return drop
}

// Underpriced checks whether a transaction is cheaper than (or as cheap as) the
// lowest priced transaction currently being tracked.
func (l *txPricedList) Underpriced(tx *types.Transaction, local *accountSet) bool {
	// Local transactions cannot be underpriced
	if local.containsTx(tx) {
		return false
	}
	// Discard stale price points if found at the heap start
	for len(*l.items) > 0 {
		head := []*types.Transaction(*l.items)[0]
		if _, ok := (*l.all)[head.Hash()]; !ok {
			l.stales--
			heap.Pop(l.items)
			continue
		}
		break
	}
	// Check if the transaction is underpriced or not
	if len(*l.items) == 0 {
		log.Error("Pricing query for empty pool") // This cannot happen, print to catch programming errors
		return false
	}
	cheapest := []*types.Transaction(*l.items)[0]
	return cheapest.GasPrice().Cmp(tx.GasPrice()) >= 0
}

// Discard finds a number of most underpriced transactions, removes them from the
// priced list and returns them for further removal from the entire pool.
func (l *txPricedList) Discard(count int, local *accountSet) types.Transactions {
	drop := make(types.Transactions, 0, count) // Remote underpriced transactions to drop
	save := make(types.Transactions, 0, 64)    // Local underpriced transactions to keep

	for len(*l.items) > 0 && count > 0 {
		// Discard stale transactions if found during cleanup
		tx := heap.Pop(l.items).(*types.Transaction)
		if _, ok := (*l.all)[tx.Hash()]; !ok {
			l.stales--
			continue
		}
		// Non stale transaction found, discard unless local
		if local.containsTx(tx) {
			save = append(save, tx)
		} else {
			drop = append(drop, tx)
			count--
		}
	}
	for _, tx := range save {
		heap.Push(l.items, tx)
	}
	return drop
}

// accountSet is simply a set of addresses to check for existence, and a signer
// capable of deriving addresses from transactions.
type accountSet struct {
	accounts map[common.Address]struct{}
	signer   types.Signer
}

// newAccountSet creates a new address set with an associated signer for sender
// derivations.
func newAccountSet(signer types.Signer) *accountSet {
	return &accountSet{
		accounts: make(map[common.Address]struct{}),
		signer:   signer,
	}
}

// contains checks if a given address is contained within the set.
func (as *accountSet) contains(addr common.Address) bool {
	_, exist := as.accounts[addr]
	return exist
}

// containsTx checks if the sender of a given tx is within the set. If the sender
// cannot be derived, this method returns false.
// containsTx检查给定tx的发送者是否在集合内。 如果发件人无法被计算出，则此方法返回false。
func (as *accountSet) containsTx(tx *types.Transaction) bool {
	if addr, err := types.Sender(as.signer, tx); err == nil {
		return as.contains(addr)
	}
	return false
}

// add inserts a new address into the set to track.
func (as *accountSet) add(addr common.Address) {
	as.accounts[addr] = struct{}{}
}

// txJournal is a rotating log of transactions with the aim of storing locally
// created transactions to allow non-executed ones to survive node restarts.
type txJournal struct {
	path   string         // Filesystem path to store the transactions at 用来存储交易的文件系统路径.
	writer io.WriteCloser // Output stream to write new transactions into 用来写入新交易的输出流.
}

// newTxJournal creates a new transaction journal to
func newTxJournal(path string) *txJournal {
	return &txJournal{
		path: path,
	}
}

// load parses a transaction journal dump from disk, loading its contents into
// the specified pool.
func (journal *txJournal) load(add func(*types.Transaction) error) error {
	// Skip the parsing if the journal file doens't exist at all
	if _, err := os.Stat(journal.path); os.IsNotExist(err) {
		return nil
	}
	// Open the journal for loading any past transactions
	input, err := os.Open(journal.path)
	if err != nil {
		return err
	}
	defer input.Close()

	// Inject all transactions from the journal into the pool
	stream := rlp.NewStream(input, 0)
	total, dropped := 0, 0

	var failure error
	for {
		// Parse the next transaction and terminate on error
		tx := new(types.Transaction)
		if err = stream.Decode(tx); err != nil {
			if err != io.EOF {
				failure = err
			}
			break
		}
		// Import the transaction and bump the appropriate progress counters
		total++
		if err = add(tx); err != nil {
			log.Debug("Failed to add journaled transaction", "err", err)
			dropped++
			continue
		}
	}
	log.Info("Loaded local transaction journal", "transactions", total, "dropped", dropped)

	return failure
}

// insert adds the specified transaction to the local disk journal.
func (journal *txJournal) insert(tx *types.Transaction) error {
	if journal.writer == nil {
		return errNoActiveJournal
	}
	if err := rlp.Encode(journal.writer, tx); err != nil {
		return err
	}
	return nil
}

// rotate regenerates the transaction journal based on the current contents of
// the transaction pool.
func (journal *txJournal) rotate(all map[common.Address]types.Transactions) error {
	// Close the current journal (if any is open)
	if journal.writer != nil {
		if err := journal.writer.Close(); err != nil {
			return err
		}
		journal.writer = nil
	}
	// Generate a new journal with the contents of the current pool
	replacement, err := os.OpenFile(journal.path+".new", os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0755)
	if err != nil {
		return err
	}
	journaled := 0
	for _, txs := range all {
		for _, tx := range txs {
			if err = rlp.Encode(replacement, tx); err != nil {
				replacement.Close()
				return err
			}
		}
		journaled += len(txs)
	}
	replacement.Close()

	// Replace the live journal with the newly generated one
	if err = os.Rename(journal.path+".new", journal.path); err != nil {
		return err
	}
	sink, err := os.OpenFile(journal.path, os.O_WRONLY|os.O_APPEND, 0755)
	if err != nil {
		return err
	}
	journal.writer = sink
	log.Info("Regenerated local transaction journal", "transactions", journaled, "accounts", len(all))

	return nil
}

// close flushes the transaction journal contents to disk and closes the file.
func (journal *txJournal) close() error {
	var err error

	if journal.writer != nil {
		err = journal.writer.Close()
		journal.writer = nil
	}
	return err
}