// peerConnection represents an active peer from which hashes and blocks are retrieved.
type peerConnection struct {
	id string // Unique identifier of the peer

	headerIdle  int32 // Current header activity state of the peer (idle = 0, active = 1) 当前的header获取的工作状态。
	blockIdle   int32 // Current block activity state of the peer (idle = 0, active = 1)	当前的区块获取的工作状态
	receiptIdle int32 // Current receipt activity state of the peer (idle = 0, active = 1) 当前的收据获取的工作状态
	stateIdle   int32 // Current node data activity state of the peer (idle = 0, active = 1) 当前节点状态的工作状态

	headerThroughput  float64 // Number of headers measured to be retrievable per second	//记录每秒能够接收多少个区块头的度量值
	blockThroughput   float64 // Number of blocks (bodies) measured to be retrievable per second  //记录每秒能够接收多少个区块的度量值
	receiptThroughput float64 // Number of receipts measured to be retrievable per second 记录每秒能够接收多少个收据的度量值
	stateThroughput   float64 // Number of node data pieces measured to be retrievable per second  记录每秒能够接收多少个账户状态的度量值

	rtt time.Duration // Request round trip time to track responsiveness (QoS)  请求回应时间

	headerStarted  time.Time // Time instance when the last header fetch was started	记录最后一个header fetch的请求时间
	blockStarted   time.Time // Time instance when the last block (body) fetch was started
	receiptStarted time.Time // Time instance when the last receipt fetch was started
	stateStarted   time.Time // Time instance when the last node data fetch was started
	
	lacking map[common.Hash]struct{} // Set of hashes not to request (didn't have previously)  记录的Hash值不会去请求，一般是因为之前的请求失败

	peer Peer			// eth的peer

	version int        // Eth protocol version number to switch strategies
	log     log.Logger // Contextual logger to add extra infos to peer logs
	lock    sync.RWMutex
}

// FetchHeaders sends a header retrieval request to the remote peer.
func (p *peerConnection) FetchHeaders(from uint64, count int) error {
	// Sanity check the protocol version
	if p.version < 62 {
		panic(fmt.Sprintf("header fetch [eth/62+] requested on eth/%d", p.version))
	}
	// Short circuit if the peer is already fetching
	if !atomic.CompareAndSwapInt32(&p.headerIdle, 0, 1) {
		return errAlreadyFetching
	}
	p.headerStarted = time.Now()

	// Issue the header retrieval request (absolut upwards without gaps)
	go p.peer.RequestHeadersByNumber(from, count, 0, false)

	return nil
}

// SetHeadersIdle sets the peer to idle, allowing it to execute new header retrieval
// requests. Its estimated header retrieval throughput is updated with that measured
// just now.
func (p *peerConnection) SetHeadersIdle(delivered int) {
	p.setIdle(p.headerStarted, delivered, &p.headerThroughput, &p.headerIdle)
}

// setIdle sets the peer to idle, allowing it to execute new retrieval requests.
// Its estimated retrieval throughput is updated with that measured just now.
func (p *peerConnection) setIdle(started time.Time, delivered int, throughput *float64, idle *int32) {
	// Irrelevant of the scaling, make sure the peer ends up idle
	defer atomic.StoreInt32(idle, 0)

	p.lock.Lock()
	defer p.lock.Unlock()

	// If nothing was delivered (hard timeout / unavailable data), reduce throughput to minimum
	if delivered == 0 {
		*throughput = 0
		return
	}
	// Otherwise update the throughput with a new measurement
	elapsed := time.Since(started) + 1 // +1 (ns) to ensure non-zero divisor
	measured := float64(delivered) / (float64(elapsed) / float64(time.Second))
	
	// measurementImpact = 0.1 , 新的吞吐量=老的吞吐量*0.9 + 这次的吞吐量*0.1
	*throughput = (1-measurementImpact)*(*throughput) + measurementImpact*measured	
	// 更新RTT
	p.rtt = time.Duration((1-measurementImpact)*float64(p.rtt) + measurementImpact*float64(elapsed))

	p.log.Trace("Peer throughput measurements updated",
		"hps", p.headerThroughput, "bps", p.blockThroughput,
		"rps", p.receiptThroughput, "sps", p.stateThroughput,
		"miss", len(p.lacking), "rtt", p.rtt)
}

// HeaderCapacity retrieves the peers header download allowance based on its
// previously discovered throughput.
func (p *peerConnection) HeaderCapacity(targetRTT time.Duration) int {
	p.lock.RLock()
	defer p.lock.RUnlock()
	// 这里有点奇怪，targetRTT越大，请求的数量就越多。
	return int(math.Min(1+math.Max(1, p.headerThroughput*float64(targetRTT)/float64(time.Second)), float64(MaxHeaderFetch)))
}

	// MarkLacking appends a new entity to the set of items (blocks, receipts, states)
	// that a peer is known not to have (i.e. have been requested before). If the
	// set reaches its maximum allowed capacity, items are randomly dropped off.
	func (p *peerConnection) MarkLacking(hash common.Hash) {
		p.lock.Lock()
		defer p.lock.Unlock()
	
		for len(p.lacking) >= maxLackingHashes {
			for drop := range p.lacking {
				delete(p.lacking, drop)
				break
			}
		}
		p.lacking[hash] = struct{}{}
	}
	
	// Lacks retrieves whether the hash of a blockchain item is on the peers lacking
	// list (i.e. whether we know that the peer does not have it).
	func (p *peerConnection) Lacks(hash common.Hash) bool {
		p.lock.RLock()
		defer p.lock.RUnlock()
	
		_, ok := p.lacking[hash]
		return ok
	}

// peerSet represents the collection of active peer participating in the chain
// download procedure.
type peerSet struct {
	peers        map[string]*peerConnection
	newPeerFeed  event.Feed
	peerDropFeed event.Feed
	lock         sync.RWMutex
}

// Register injects a new peer into the working set, or returns an error if the
// peer is already known.
//
// The method also sets the starting throughput values of the new peer to the
// average of all existing peers, to give it a realistic chance of being used
// for data retrievals.
func (ps *peerSet) Register(p *peerConnection) error {
	// Retrieve the current median RTT as a sane default
	p.rtt = ps.medianRTT()

	// Register the new peer with some meaningful defaults
	ps.lock.Lock()
	if _, ok := ps.peers[p.id]; ok {
		ps.lock.Unlock()
		return errAlreadyRegistered
	}
	if len(ps.peers) > 0 {
		p.headerThroughput, p.blockThroughput, p.receiptThroughput, p.stateThroughput = 0, 0, 0, 0

		for _, peer := range ps.peers {
			peer.lock.RLock()
			p.headerThroughput += peer.headerThroughput
			p.blockThroughput += peer.blockThroughput
			p.receiptThroughput += peer.receiptThroughput
			p.stateThroughput += peer.stateThroughput
			peer.lock.RUnlock()
		}
		p.headerThroughput /= float64(len(ps.peers))
		p.blockThroughput /= float64(len(ps.peers))
		p.receiptThroughput /= float64(len(ps.peers))
		p.stateThroughput /= float64(len(ps.peers))
	}
	ps.peers[p.id] = p
	ps.lock.Unlock()

	ps.newPeerFeed.Send(p)
	return nil
}

// Unregister removes a remote peer from the active set, disabling any further
// actions to/from that particular entity.
func (ps *peerSet) Unregister(id string) error {
	ps.lock.Lock()
	p, ok := ps.peers[id]
	if !ok {
		defer ps.lock.Unlock()
		return errNotRegistered
	}
	delete(ps.peers, id)
	ps.lock.Unlock()

	ps.peerDropFeed.Send(p)
	return nil
}

// HeaderIdlePeers retrieves a flat list of all the currently header-idle peers
// within the active peer set, ordered by their reputation.
func (ps *peerSet) HeaderIdlePeers() ([]*peerConnection, int) {
	idle := func(p *peerConnection) bool {
		return atomic.LoadInt32(&p.headerIdle) == 0
	}
	throughput := func(p *peerConnection) float64 {
		p.lock.RLock()
		defer p.lock.RUnlock()
		return p.headerThroughput
	}
	return ps.idlePeers(62, 64, idle, throughput)
}

// idlePeers retrieves a flat list of all currently idle peers satisfying the
// protocol version constraints, using the provided function to check idleness.
// The resulting set of peers are sorted by their measure throughput.
func (ps *peerSet) idlePeers(minProtocol, maxProtocol int, idleCheck func(*peerConnection) bool, throughput func(*peerConnection) float64) ([]*peerConnection, int) {
	ps.lock.RLock()
	defer ps.lock.RUnlock()

	idle, total := make([]*peerConnection, 0, len(ps.peers)), 0
	for _, p := range ps.peers { //首先抽取idle的peer
		if p.version >= minProtocol && p.version <= maxProtocol {
			if idleCheck(p) {
				idle = append(idle, p)
			}
			total++
		}
	}
	for i := 0; i < len(idle); i++ { // 冒泡排序， 从吞吐量大到吞吐量小。
		for j := i + 1; j < len(idle); j++ {
			if throughput(idle[i]) < throughput(idle[j]) {
				idle[i], idle[j] = idle[j], idle[i]
			}
		}
	}
	return idle, total
}

// medianRTT returns the median RTT of te peerset, considering only the tuning
// peers if there are more peers available.
func (ps *peerSet) medianRTT() time.Duration {
	// Gather all the currnetly measured round trip times
	ps.lock.RLock()
	defer ps.lock.RUnlock()

	rtts := make([]float64, 0, len(ps.peers))
	for _, p := range ps.peers {
		p.lock.RLock()
		rtts = append(rtts, float64(p.rtt))
		p.lock.RUnlock()
	}
	sort.Float64s(rtts)

	median := rttMaxEstimate
	if qosTuningPeers <= len(rtts) {
		median = time.Duration(rtts[qosTuningPeers/2]) // Median of our tuning peers
	} else if len(rtts) > 0 {
		median = time.Duration(rtts[len(rtts)/2]) // Median of our connected peers (maintain even like this some baseline qos)
	}
	// Restrict the RTT into some QoS defaults, irrelevant of true RTT
	if median < rttMinEstimate {
		median = rttMinEstimate
	}
	if median > rttMaxEstimate {
		median = rttMaxEstimate
	}
	return median
}