package connmgr

import (
	"context"
	"sort"
	"sync"
	"sync/atomic"
	"time"

	"github.com/libp2p/go-libp2p-core/connmgr"
	"github.com/libp2p/go-libp2p-core/network"
	"github.com/libp2p/go-libp2p-core/peer"

	logging "github.com/ipfs/go-log"
	ma "github.com/multiformats/go-multiaddr"
)

var SilencePeriod = 10 * time.Second

var log = logging.Logger("connmgr")

// BasicConnMgr is a ConnManager that trims connections whenever the count exceeds the
// high watermark. New connections are given a grace period before they're subject
// to trimming. Trims are automatically run on demand, only if the time from the
// previous trim is higher than 10 seconds. Furthermore, trims can be explicitly
// requested through the public interface of this struct (see TrimOpenConns).
//
// See configuration parameters in NewConnManager.
type BasicConnMgr struct {
	highWater   int
	lowWater    int
	connCount   int32
	gracePeriod time.Duration
	segments    segments

	plk       sync.RWMutex
	protected map[peer.ID]map[string]struct{}

	// channel-based semaphore that enforces only a single trim is in progress
	trimRunningCh chan struct{}
	lastTrim      time.Time
	silencePeriod time.Duration

	ctx    context.Context
	cancel func()
}

var _ connmgr.ConnManager = (*BasicConnMgr)(nil)

type segment struct {
	sync.Mutex
	peers map[peer.ID]*peerInfo
}

type segments [256]*segment

func (ss *segments) get(p peer.ID) *segment {
	return ss[byte(p[len(p)-1])]
}

func (ss *segments) countPeers() (count int) {
	for _, seg := range ss {
		seg.Lock()
		count += len(seg.peers)
		seg.Unlock()
	}
	return count
}

func (s *segment) tagInfoFor(p peer.ID) *peerInfo {
	pi, ok := s.peers[p]
	if ok {
		return pi
	}
	// create a temporary peer to buffer early tags before the Connected notification arrives.
	pi = &peerInfo{
		id:        p,
		firstSeen: time.Now(), // this timestamp will be updated when the first Connected notification arrives.
		temp:      true,
		tags:      make(map[string]int),
		conns:     make(map[network.Conn]time.Time),
	}
	s.peers[p] = pi
	return pi
}

// NewConnManager creates a new BasicConnMgr with the provided params:
// * lo and hi are watermarks governing the number of connections that'll be maintained.
//   When the peer count exceeds the 'high watermark', as many peers will be pruned (and
//   their connections terminated) until 'low watermark' peers remain.
// * grace is the amount of time a newly opened connection is given before it becomes
//   subject to pruning.
func NewConnManager(low, hi int, grace time.Duration) *BasicConnMgr {
	ctx, cancel := context.WithCancel(context.Background())
	cm := &BasicConnMgr{
		highWater:     hi,
		lowWater:      low,
		gracePeriod:   grace,
		trimRunningCh: make(chan struct{}, 1),
		protected:     make(map[peer.ID]map[string]struct{}, 16),
		silencePeriod: SilencePeriod,
		ctx:           ctx,
		cancel:        cancel,
		segments: func() (ret segments) {
			for i := range ret {
				ret[i] = &segment{
					peers: make(map[peer.ID]*peerInfo),
				}
			}
			return ret
		}(),
	}

	go cm.background()
	return cm
}

func (cm *BasicConnMgr) Close() error {
	cm.cancel()
	return nil
}

func (cm *BasicConnMgr) Protect(id peer.ID, tag string) {
	cm.plk.Lock()
	defer cm.plk.Unlock()

	tags, ok := cm.protected[id]
	if !ok {
		tags = make(map[string]struct{}, 2)
		cm.protected[id] = tags
	}
	tags[tag] = struct{}{}
}

func (cm *BasicConnMgr) Unprotect(id peer.ID, tag string) (protected bool) {
	cm.plk.Lock()
	defer cm.plk.Unlock()

	tags, ok := cm.protected[id]
	if !ok {
		return false
	}
	if delete(tags, tag); len(tags) == 0 {
		delete(cm.protected, id)
		return false
	}
	return true
}

// peerInfo stores metadata for a given peer.
type peerInfo struct {
	id    peer.ID
	tags  map[string]int // value for each tag
	value int            // cached sum of all tag values
	temp  bool           // this is a temporary entry holding early tags, and awaiting connections

	conns map[network.Conn]time.Time // start time of each connection

	firstSeen time.Time // timestamp when we began tracking this peer.
}

// TrimOpenConns closes the connections of as many peers as needed to make the peer count
// equal the low watermark. Peers are sorted in ascending order based on their total value,
// pruning those peers with the lowest scores first, as long as they are not within their
// grace period.
//
// TODO: error return value so we can cleanly signal we are aborting because:
// (a) there's another trim in progress, or (b) the silence period is in effect.
func (cm *BasicConnMgr) TrimOpenConns(ctx context.Context) {
	select {
	case cm.trimRunningCh <- struct{}{}:
	default:
		return
	}
	defer func() { <-cm.trimRunningCh }()
	if time.Since(cm.lastTrim) < cm.silencePeriod {
		// skip this attempt to trim as the last one just took place.
		return
	}

	defer log.EventBegin(ctx, "connCleanup").Done()
	for _, c := range cm.getConnsToClose(ctx) {
		log.Info("closing conn: ", c.RemotePeer())
		log.Event(ctx, "closeConn", c.RemotePeer())
		c.Close()
	}

	cm.lastTrim = time.Now()
}

func (cm *BasicConnMgr) background() {
	ticker := time.NewTicker(time.Minute)
	defer ticker.Stop()

	for {
		select {
		case <-ticker.C:
			if atomic.LoadInt32(&cm.connCount) > int32(cm.highWater) {
				cm.TrimOpenConns(cm.ctx)
			}

		case <-cm.ctx.Done():
			return
		}
	}
}

// getConnsToClose runs the heuristics described in TrimOpenConns and returns the
// connections to close.
func (cm *BasicConnMgr) getConnsToClose(ctx context.Context) []network.Conn {
	if cm.lowWater == 0 || cm.highWater == 0 {
		// disabled
		return nil
	}
	now := time.Now()
	nconns := int(atomic.LoadInt32(&cm.connCount))
	if nconns <= cm.lowWater {
		log.Info("open connection count below limit")
		return nil
	}

	npeers := cm.segments.countPeers()
	candidates := make([]*peerInfo, 0, npeers)
	cm.plk.RLock()
	for _, s := range cm.segments {
		s.Lock()
		for id, inf := range s.peers {
			if _, ok := cm.protected[id]; ok {
				// skip over protected peer.
				continue
			}
			candidates = append(candidates, inf)
		}
		s.Unlock()
	}
	cm.plk.RUnlock()

	// Sort peers according to their value.
	sort.Slice(candidates, func(i, j int) bool {
		left, right := candidates[i], candidates[j]
		// temporary peers are preferred for pruning.
		if left.temp != right.temp {
			return left.temp
		}
		// otherwise, compare by value.
		return left.value < right.value
	})

	target := nconns - cm.lowWater

	// slightly overallocate because we may have more than one conns per peer
	selected := make([]network.Conn, 0, target+10)

	for _, inf := range candidates {
		if target <= 0 {
			break
		}
		// TODO: should we be using firstSeen or the time associated with the connection itself?
		if inf.firstSeen.Add(cm.gracePeriod).After(now) {
			continue
		}

		// lock this to protect from concurrent modifications from connect/disconnect events
		s := cm.segments.get(inf.id)
		s.Lock()

		if len(inf.conns) == 0 && inf.temp {
			// handle temporary entries for early tags -- this entry has gone past the grace period
			// and still holds no connections, so prune it.
			delete(s.peers, inf.id)
		} else {
			for c := range inf.conns {
				selected = append(selected, c)
			}
		}
		target -= len(inf.conns)
		s.Unlock()
	}

	return selected
}

// GetTagInfo is called to fetch the tag information associated with a given
// peer, nil is returned if p refers to an unknown peer.
func (cm *BasicConnMgr) GetTagInfo(p peer.ID) *connmgr.TagInfo {
	s := cm.segments.get(p)
	s.Lock()
	defer s.Unlock()

	pi, ok := s.peers[p]
	if !ok {
		return nil
	}

	out := &connmgr.TagInfo{
		FirstSeen: pi.firstSeen,
		Value:     pi.value,
		Tags:      make(map[string]int),
		Conns:     make(map[string]time.Time),
	}

	for t, v := range pi.tags {
		out.Tags[t] = v
	}
	for c, t := range pi.conns {
		out.Conns[c.RemoteMultiaddr().String()] = t
	}

	return out
}

// TagPeer is called to associate a string and integer with a given peer.
func (cm *BasicConnMgr) TagPeer(p peer.ID, tag string, val int) {
	s := cm.segments.get(p)
	s.Lock()
	defer s.Unlock()

	pi := s.tagInfoFor(p)

	// Update the total value of the peer.
	pi.value += val - pi.tags[tag]
	pi.tags[tag] = val
}

// UntagPeer is called to disassociate a string and integer from a given peer.
func (cm *BasicConnMgr) UntagPeer(p peer.ID, tag string) {
	s := cm.segments.get(p)
	s.Lock()
	defer s.Unlock()

	pi, ok := s.peers[p]
	if !ok {
		log.Info("tried to remove tag from untracked peer: ", p)
		return
	}

	// Update the total value of the peer.
	pi.value -= pi.tags[tag]
	delete(pi.tags, tag)
}

// UpsertTag is called to insert/update a peer tag
func (cm *BasicConnMgr) UpsertTag(p peer.ID, tag string, upsert func(int) int) {
	s := cm.segments.get(p)
	s.Lock()
	defer s.Unlock()

	pi := s.tagInfoFor(p)

	oldval := pi.tags[tag]
	newval := upsert(oldval)
	pi.value += newval - oldval
	pi.tags[tag] = newval
}

// CMInfo holds the configuration for BasicConnMgr, as well as status data.
type CMInfo struct {
	// The low watermark, as described in NewConnManager.
	LowWater int

	// The high watermark, as described in NewConnManager.
	HighWater int

	// The timestamp when the last trim was triggered.
	LastTrim time.Time

	// The configured grace period, as described in NewConnManager.
	GracePeriod time.Duration

	// The current connection count.
	ConnCount int
}

// GetInfo returns the configuration and status data for this connection manager.
func (cm *BasicConnMgr) GetInfo() CMInfo {
	return CMInfo{
		HighWater:   cm.highWater,
		LowWater:    cm.lowWater,
		LastTrim:    cm.lastTrim,
		GracePeriod: cm.gracePeriod,
		ConnCount:   int(atomic.LoadInt32(&cm.connCount)),
	}
}

// Notifee returns a sink through which Notifiers can inform the BasicConnMgr when
// events occur. Currently, the notifee only reacts upon connection events
// {Connected, Disconnected}.
func (cm *BasicConnMgr) Notifee() network.Notifiee {
	return (*cmNotifee)(cm)
}

type cmNotifee BasicConnMgr

func (nn *cmNotifee) cm() *BasicConnMgr {
	return (*BasicConnMgr)(nn)
}

// Connected is called by notifiers to inform that a new connection has been established.
// The notifee updates the BasicConnMgr to start tracking the connection. If the new connection
// count exceeds the high watermark, a trim may be triggered.
func (nn *cmNotifee) Connected(n network.Network, c network.Conn) {
	cm := nn.cm()

	p := c.RemotePeer()
	s := cm.segments.get(p)
	s.Lock()
	defer s.Unlock()

	id := c.RemotePeer()
	pinfo, ok := s.peers[id]
	if !ok {
		pinfo = &peerInfo{
			id:        id,
			firstSeen: time.Now(),
			tags:      make(map[string]int),
			conns:     make(map[network.Conn]time.Time),
		}
		s.peers[id] = pinfo
	} else if pinfo.temp {
		// we had created a temporary entry for this peer to buffer early tags before the
		// Connected notification arrived: flip the temporary flag, and update the firstSeen
		// timestamp to the real one.
		pinfo.temp = false
		pinfo.firstSeen = time.Now()
	}

	_, ok = pinfo.conns[c]
	if ok {
		log.Error("received connected notification for conn we are already tracking: ", p)
		return
	}

	pinfo.conns[c] = time.Now()
	atomic.AddInt32(&cm.connCount, 1)
}

// Disconnected is called by notifiers to inform that an existing connection has been closed or terminated.
// The notifee updates the BasicConnMgr accordingly to stop tracking the connection, and performs housekeeping.
func (nn *cmNotifee) Disconnected(n network.Network, c network.Conn) {
	cm := nn.cm()

	p := c.RemotePeer()
	s := cm.segments.get(p)
	s.Lock()
	defer s.Unlock()

	cinf, ok := s.peers[p]
	if !ok {
		log.Error("received disconnected notification for peer we are not tracking: ", p)
		return
	}

	_, ok = cinf.conns[c]
	if !ok {
		log.Error("received disconnected notification for conn we are not tracking: ", p)
		return
	}

	delete(cinf.conns, c)
	if len(cinf.conns) == 0 {
		delete(s.peers, p)
	}
	atomic.AddInt32(&cm.connCount, -1)
}

// Listen is no-op in this implementation.
func (nn *cmNotifee) Listen(n network.Network, addr ma.Multiaddr) {}

// ListenClose is no-op in this implementation.
func (nn *cmNotifee) ListenClose(n network.Network, addr ma.Multiaddr) {}

// OpenedStream is no-op in this implementation.
func (nn *cmNotifee) OpenedStream(network.Network, network.Stream) {}

// ClosedStream is no-op in this implementation.
func (nn *cmNotifee) ClosedStream(network.Network, network.Stream) {}