package hscontrol

import (
	"cmp"
	"context"
	"fmt"
	"math/rand/v2"
	"net/http"
	"net/netip"
	"sort"
	"strings"
	"sync"
	"time"

	"github.com/juanfont/headscale/hscontrol/db"
	"github.com/juanfont/headscale/hscontrol/mapper"
	"github.com/juanfont/headscale/hscontrol/types"
	"github.com/rs/zerolog/log"
	"github.com/sasha-s/go-deadlock"
	xslices "golang.org/x/exp/slices"
	"gorm.io/gorm"
	"tailscale.com/tailcfg"
)

const (
	keepAliveInterval = 50 * time.Second
)

type contextKey string

const nodeNameContextKey = contextKey("nodeName")

type sessionManager struct {
	mu   sync.RWMutex
	sess map[types.NodeID]*mapSession
}

type mapSession struct {
	h      *Headscale
	req    tailcfg.MapRequest
	ctx    context.Context
	capVer tailcfg.CapabilityVersion
	mapper *mapper.Mapper

	serving   bool
	servingMu deadlock.Mutex

	ch       chan types.StateUpdate
	cancelCh chan struct{}

	node *types.Node
	w    http.ResponseWriter

	warnf  func(string, ...any)
	infof  func(string, ...any)
	tracef func(string, ...any)
	errf   func(error, string, ...any)
}

func (h *Headscale) newMapSession(
	ctx context.Context,
	req tailcfg.MapRequest,
	w http.ResponseWriter,
	node *types.Node,
) *mapSession {
	warnf, tracef, infof, errf := logPollFunc(req, node)

	// Use a buffered channel in case a node is not fully ready
	// to receive a message to make sure we dont block the entire
	// notifier.
	updateChan := make(chan types.StateUpdate, h.cfg.Tuning.NodeMapSessionBufferedChanSize)

	return &mapSession{
		h:      h,
		ctx:    ctx,
		req:    req,
		w:      w,
		node:   node,
		capVer: req.Version,
		mapper: h.mapper,

		// serving indicates if a client is being served.
		serving: false,

		ch:       updateChan,
		cancelCh: make(chan struct{}),

		// Loggers
		warnf:  warnf,
		infof:  infof,
		tracef: tracef,
		errf:   errf,
	}
}

func (m *mapSession) close() {
	m.servingMu.Lock()
	defer m.servingMu.Unlock()
	if !m.serving {
		return
	}

	select {
	case m.cancelCh <- struct{}{}:
	default:
	}
}

func (m *mapSession) isStreaming() bool {
	return m.req.Stream && !m.req.ReadOnly
}

func (m *mapSession) isEndpointUpdate() bool {
	return !m.req.Stream && !m.req.ReadOnly && m.req.OmitPeers
}

func (m *mapSession) isReadOnlyUpdate() bool {
	return !m.req.Stream && m.req.OmitPeers && m.req.ReadOnly
}

func (m *mapSession) flush200() {
	m.w.WriteHeader(http.StatusOK)
	if f, ok := m.w.(http.Flusher); ok {
		f.Flush()
	}
}

// handlePoll ensures the node gets the appropriate updates from either
// polling or immediate responses.
//
//nolint:gocyclo
func (m *mapSession) serve() {
	// Register with the notifier if this is a streaming
	// session
	if m.isStreaming() {
		// defers are called in reverse order,
		// so top one is executed last.

		// Failover the node's routes if any.
		defer m.infof("node has disconnected, mapSession: %p", m)
		defer m.pollFailoverRoutes("node closing connection", m.node)

		defer m.h.updateNodeOnlineStatus(false, m.node)
		defer m.h.nodeNotifier.RemoveNode(m.node.ID)

		defer func() {
			m.servingMu.Lock()
			defer m.servingMu.Unlock()

			m.serving = false
			close(m.cancelCh)
		}()

		m.serving = true

		m.h.nodeNotifier.AddNode(m.node.ID, m.ch)
		m.h.updateNodeOnlineStatus(true, m.node)

		m.infof("node has connected, mapSession: %p", m)
	}

	// TODO(kradalby): A set todos to harden:
	// - func to tell the stream to die, readonly -> false, !stream && omitpeers -> false, true

	// This is the mechanism where the node gives us information about its
	// current configuration.
	//
	// If OmitPeers is true, Stream is false, and ReadOnly is false,
	// then te server will let clients update their endpoints without
	// breaking existing long-polling (Stream == true) connections.
	// In this case, the server can omit the entire response; the client
	// only checks the HTTP response status code.
	//
	// This is what Tailscale calls a Lite update, the client ignores
	// the response and just wants a 200.
	// !req.stream && !req.ReadOnly && req.OmitPeers
	//
	// TODO(kradalby): remove ReadOnly when we only support capVer 68+
	if m.isEndpointUpdate() {
		m.handleEndpointUpdate()

		return
	}

	// ReadOnly is whether the client just wants to fetch the
	// MapResponse, without updating their Endpoints. The
	// Endpoints field will be ignored and LastSeen will not be
	// updated and peers will not be notified of changes.
	//
	// The intended use is for clients to discover the DERP map at
	// start-up before their first real endpoint update.
	if m.isReadOnlyUpdate() {
		m.handleReadOnlyRequest()

		return
	}

	// From version 68, all streaming requests can be treated as read only.
	if m.capVer < 68 {
		// Error has been handled/written to client in the func
		// return
		err := m.handleSaveNode()
		if err != nil {
			return
		}
	}

	// Set up the client stream
	m.h.pollNetMapStreamWG.Add(1)
	defer m.h.pollNetMapStreamWG.Done()

	m.pollFailoverRoutes("node connected", m.node)

	keepAliveTicker := time.NewTicker(keepAliveInterval + (time.Duration(rand.IntN(9000)) * time.Millisecond))

	ctx, cancel := context.WithCancel(context.WithValue(m.ctx, nodeNameContextKey, m.node.Hostname))
	defer cancel()

	// TODO(kradalby): Make this available through a tuning envvar
	wait := time.Second

	// Add a circuit breaker, if the loop is not interrupted
	// inbetween listening for the channels, some updates
	// might get stale and stucked in the "changed" map
	// defined below.
	blockBreaker := time.NewTicker(wait)

	// true means changed, false means removed
	var changed map[types.NodeID]bool
	var patches []*tailcfg.PeerChange
	var derp bool

	// Set full to true to immediatly send a full mapresponse
	full := true
	prev := time.Now()
	lastMessage := ""

	// Loop through updates and continuously send them to the
	// client.
	for {
		// If a full update has been requested or there are patches, then send it immediately
		// otherwise wait for the "batching" of changes or patches
		if full || patches != nil || (changed != nil && time.Since(prev) > wait) {
			var data []byte
			var err error

			// Ensure the node object is updated, for example, there
			// might have been a hostinfo update in a sidechannel
			// which contains data needed to generate a map response.
			m.node, err = m.h.db.GetNodeByID(m.node.ID)
			if err != nil {
				m.errf(err, "Could not get machine from db")

				return
			}

			// If there are patches _and_ fully changed nodes, filter the
			// patches and remove all patches that are present for the full
			// changes updates. This allows us to send them as part of the
			// PeerChange update, but only for nodes that are not fully changed.
			// The fully changed nodes will be updated from the database and
			// have all the updates needed.
			// This means that the patches left are for nodes that has no
			// updates that requires a full update.
			// Patches are not suppose to be mixed in, but can be.
			//
			// From tailcfg docs:
			// These are applied after Peers* above, but in practice the
			// control server should only send these on their own, without
			//
			// Currently, there is no effort to merge patch updates, they
			// are all sent, and the client will apply them in order.
			// TODO(kradalby): Merge Patches for the same IDs to send less
			// data and give the client less work.
			if patches != nil && changed != nil {
				var filteredPatches []*tailcfg.PeerChange

				for _, patch := range patches {
					if _, ok := changed[types.NodeID(patch.NodeID)]; !ok {
						filteredPatches = append(filteredPatches, patch)
					}
				}

				patches = filteredPatches
			}

			// When deciding what update to send, the following is considered,
			// Full is a superset of all updates, when a full update is requested,
			// send only that and move on, all other updates will be present in
			// a full map response.
			//
			// If a map of changed nodes exists, prefer sending that as it will
			// contain all the updates for the node, including patches, as it
			// is fetched freshly from the database when building the response.
			//
			// If there is full changes registered, but we have patches for individual
			// nodes, send them.
			//
			// Finally, if a DERP map is the only request, send that alone.
			if full {
				m.tracef("Sending Full MapResponse")
				data, err = m.mapper.FullMapResponse(m.req, m.node, m.h.ACLPolicy, fmt.Sprintf("from mapSession: %p, stream: %t", m, m.isStreaming()))
			} else if changed != nil {
				m.tracef(fmt.Sprintf("Sending Changed MapResponse: %v", lastMessage))
				data, err = m.mapper.PeerChangedResponse(m.req, m.node, changed, patches, m.h.ACLPolicy, lastMessage)
			} else if patches != nil {
				m.tracef(fmt.Sprintf("Sending Changed Patch MapResponse: %v", lastMessage))
				data, err = m.mapper.PeerChangedPatchResponse(m.req, m.node, patches, m.h.ACLPolicy)
			} else if derp {
				m.tracef("Sending DERPUpdate MapResponse")
				data, err = m.mapper.DERPMapResponse(m.req, m.node, m.h.DERPMap)
			}

			if err != nil {
				m.errf(err, "Could not get the create map update")

				return
			}

			// log.Trace().Str("node", m.node.Hostname).TimeDiff("timeSpent", time.Now(), startMapResp).Str("mkey", m.node.MachineKey.String()).Int("type", int(update.Type)).Msg("finished making map response")

			// Only send update if there is change
			if data != nil {
				startWrite := time.Now()
				_, err = m.w.Write(data)
				if err != nil {
					m.errf(err, "Could not write the map response, for mapSession: %p, stream: %t", m, m.isStreaming())

					return
				}

				if flusher, ok := m.w.(http.Flusher); ok {
					flusher.Flush()
				} else {
					log.Error().Msg("Failed to create http flusher")

					return
				}
				log.Trace().Str("node", m.node.Hostname).TimeDiff("timeSpent", time.Now(), startWrite).Str("mkey", m.node.MachineKey.String()).Msg("finished writing mapresp to node")

				m.infof("update sent")
			}

			// reset
			changed = nil
			patches = nil
			lastMessage = ""
			full = false
			derp = false
			prev = time.Now()
		}

		// consume channels with update, keep alives or "batch" blocking signals
		select {
		case <-m.cancelCh:
			m.tracef("poll cancelled received")
			return
		case <-ctx.Done():
			m.tracef("poll context done")
			return

			// Avoid infinite block that would potentially leave
		// some updates in the changed map.
		case <-blockBreaker.C:
			continue

		// Consume all updates sent to node
		case update := <-m.ch:
			m.tracef("received stream update: %d %s", update.Type, update.Message)

			switch update.Type {
			case types.StateFullUpdate:
				full = true
			case types.StatePeerChanged:
				if changed == nil {
					changed = make(map[types.NodeID]bool)
				}

				for _, nodeID := range update.ChangeNodes {
					changed[nodeID] = true
				}

				lastMessage = update.Message
			case types.StatePeerChangedPatch:
				patches = append(patches, update.ChangePatches...)
			case types.StatePeerRemoved:
				if changed == nil {
					changed = make(map[types.NodeID]bool)
				}

				for _, nodeID := range update.Removed {
					changed[nodeID] = false
				}
			case types.StateSelfUpdate:
				// create the map so an empty (self) update is sent
				if changed == nil {
					changed = make(map[types.NodeID]bool)
				}

				lastMessage = update.Message
			case types.StateDERPUpdated:
				derp = true
			}

		case <-keepAliveTicker.C:
			data, err := m.mapper.KeepAliveResponse(m.req, m.node)
			if err != nil {
				m.errf(err, "Error generating the keep alive msg")

				return
			}
			_, err = m.w.Write(data)
			if err != nil {
				m.errf(err, "Cannot write keep alive message")

				return
			}
			if flusher, ok := m.w.(http.Flusher); ok {
				flusher.Flush()
			} else {
				log.Error().Msg("Failed to create http flusher")

				return
			}
		}
	}
}

func (m *mapSession) pollFailoverRoutes(where string, node *types.Node) {
	update, err := db.Write(m.h.db.DB, func(tx *gorm.DB) (*types.StateUpdate, error) {
		return db.FailoverRouteIfAvailable(tx, m.h.nodeNotifier.ConnectedMap(), node)
	})
	if err != nil {
		m.errf(err, fmt.Sprintf("failed to ensure failover routes, %s", where))

		return
	}

	if update != nil && !update.Empty() {
		ctx := types.NotifyCtx(context.Background(), fmt.Sprintf("poll-%s-routes-ensurefailover", strings.ReplaceAll(where, " ", "-")), node.Hostname)
		m.h.nodeNotifier.NotifyWithIgnore(ctx, *update, node.ID)
	}
}

// updateNodeOnlineStatus records the last seen status of a node and notifies peers
// about change in their online/offline status.
// It takes a StateUpdateType of either StatePeerOnlineChanged or StatePeerOfflineChanged.
func (h *Headscale) updateNodeOnlineStatus(online bool, node *types.Node) {
	change := &tailcfg.PeerChange{
		NodeID: tailcfg.NodeID(node.ID),
		Online: &online,
	}

	if !online {
		now := time.Now()

		// lastSeen is only relevant if the node is disconnected.
		node.LastSeen = &now
		change.LastSeen = &now

		err := h.db.DB.Transaction(func(tx *gorm.DB) error {
			return db.SetLastSeen(tx, node.ID, *node.LastSeen)
		})
		if err != nil {
			log.Error().Err(err).Msg("Cannot update node LastSeen")

			return
		}
	}

	ctx := types.NotifyCtx(context.Background(), "poll-nodeupdate-onlinestatus", node.Hostname)
	h.nodeNotifier.NotifyWithIgnore(ctx, types.StateUpdate{
		Type: types.StatePeerChangedPatch,
		ChangePatches: []*tailcfg.PeerChange{
			change,
		},
	}, node.ID)
}

func closeChanWithLog[C chan []byte | chan struct{} | chan types.StateUpdate](channel C, node, name string) {
	log.Trace().
		Str("handler", "PollNetMap").
		Str("node", node).
		Str("channel", "Done").
		Msg(fmt.Sprintf("Closing %s channel", name))

	close(channel)
}

func (m *mapSession) handleEndpointUpdate() {
	m.tracef("received endpoint update")

	change := m.node.PeerChangeFromMapRequest(m.req)

	online := m.h.nodeNotifier.IsLikelyConnected(m.node.ID)
	change.Online = &online

	m.node.ApplyPeerChange(&change)

	sendUpdate, routesChanged := hostInfoChanged(m.node.Hostinfo, m.req.Hostinfo)
	m.node.Hostinfo = m.req.Hostinfo

	logTracePeerChange(m.node.Hostname, sendUpdate, &change)

	// If there is no changes and nothing to save,
	// return early.
	if peerChangeEmpty(change) && !sendUpdate {
		return
	}

	// Check if the Hostinfo of the node has changed.
	// If it has changed, check if there has been a change to
	// the routable IPs of the host and update update them in
	// the database. Then send a Changed update
	// (containing the whole node object) to peers to inform about
	// the route change.
	// If the hostinfo has changed, but not the routes, just update
	// hostinfo and let the function continue.
	if routesChanged {
		var err error
		_, err = m.h.db.SaveNodeRoutes(m.node)
		if err != nil {
			m.errf(err, "Error processing node routes")
			http.Error(m.w, "", http.StatusInternalServerError)

			return
		}

		if m.h.ACLPolicy != nil {
			// update routes with peer information
			err := m.h.db.EnableAutoApprovedRoutes(m.h.ACLPolicy, m.node)
			if err != nil {
				m.errf(err, "Error running auto approved routes")
			}
		}

		// Send an update to the node itself with to ensure it
		// has an updated packetfilter allowing the new route
		// if it is defined in the ACL.
		ctx := types.NotifyCtx(context.Background(), "poll-nodeupdate-self-hostinfochange", m.node.Hostname)
		m.h.nodeNotifier.NotifyByMachineKey(
			ctx,
			types.StateUpdate{
				Type:        types.StateSelfUpdate,
				ChangeNodes: []types.NodeID{m.node.ID},
			},
			m.node.ID)

	}

	if err := m.h.db.DB.Save(m.node).Error; err != nil {
		m.errf(err, "Failed to persist/update node in the database")
		http.Error(m.w, "", http.StatusInternalServerError)

		return
	}

	ctx := types.NotifyCtx(context.Background(), "poll-nodeupdate-peers-patch", m.node.Hostname)
	m.h.nodeNotifier.NotifyWithIgnore(
		ctx,
		types.StateUpdate{
			Type:        types.StatePeerChanged,
			ChangeNodes: []types.NodeID{m.node.ID},
			Message:     "called from handlePoll -> update",
		},
		m.node.ID)

	m.flush200()

	return
}

// handleSaveNode saves node updates in the maprequest _streaming_
// path and is mostly the same code as in handleEndpointUpdate.
// It is not attempted to be deduplicated since it will go away
// when we stop supporting older than 68 which removes updates
// when the node is streaming.
func (m *mapSession) handleSaveNode() error {
	m.tracef("saving node update from stream session")

	change := m.node.PeerChangeFromMapRequest(m.req)

	// A stream is being set up, the node is Online
	online := true
	change.Online = &online

	m.node.ApplyPeerChange(&change)

	sendUpdate, routesChanged := hostInfoChanged(m.node.Hostinfo, m.req.Hostinfo)
	m.node.Hostinfo = m.req.Hostinfo

	// If there is no changes and nothing to save,
	// return early.
	if peerChangeEmpty(change) || !sendUpdate {
		return nil
	}

	// Check if the Hostinfo of the node has changed.
	// If it has changed, check if there has been a change to
	// the routable IPs of the host and update update them in
	// the database. Then send a Changed update
	// (containing the whole node object) to peers to inform about
	// the route change.
	// If the hostinfo has changed, but not the routes, just update
	// hostinfo and let the function continue.
	if routesChanged {
		var err error
		_, err = m.h.db.SaveNodeRoutes(m.node)
		if err != nil {
			return err
		}

		if m.h.ACLPolicy != nil {
			// update routes with peer information
			err := m.h.db.EnableAutoApprovedRoutes(m.h.ACLPolicy, m.node)
			if err != nil {
				return err
			}
		}
	}

	if err := m.h.db.DB.Save(m.node).Error; err != nil {
		return err
	}

	ctx := types.NotifyCtx(context.Background(), "pre-68-update-while-stream", m.node.Hostname)
	m.h.nodeNotifier.NotifyWithIgnore(
		ctx,
		types.StateUpdate{
			Type:        types.StatePeerChanged,
			ChangeNodes: []types.NodeID{m.node.ID},
			Message:     "called from handlePoll -> pre-68-update-while-stream",
		},
		m.node.ID)

	return nil
}

func (m *mapSession) handleReadOnlyRequest() {
	m.tracef("Client asked for a lite update, responding without peers")

	mapResp, err := m.mapper.ReadOnlyMapResponse(m.req, m.node, m.h.ACLPolicy)
	if err != nil {
		m.errf(err, "Failed to create MapResponse")
		http.Error(m.w, "", http.StatusInternalServerError)

		return
	}

	m.w.Header().Set("Content-Type", "application/json; charset=utf-8")
	m.w.WriteHeader(http.StatusOK)
	_, err = m.w.Write(mapResp)
	if err != nil {
		m.errf(err, "Failed to write response")
	}

	m.flush200()
}

func logTracePeerChange(hostname string, hostinfoChange bool, change *tailcfg.PeerChange) {
	trace := log.Trace().Uint64("node.id", uint64(change.NodeID)).Str("hostname", hostname)

	if change.Key != nil {
		trace = trace.Str("node_key", change.Key.ShortString())
	}

	if change.DiscoKey != nil {
		trace = trace.Str("disco_key", change.DiscoKey.ShortString())
	}

	if change.Online != nil {
		trace = trace.Bool("online", *change.Online)
	}

	if change.Endpoints != nil {
		eps := make([]string, len(change.Endpoints))
		for idx, ep := range change.Endpoints {
			eps[idx] = ep.String()
		}

		trace = trace.Strs("endpoints", eps)
	}

	if hostinfoChange {
		trace = trace.Bool("hostinfo_changed", hostinfoChange)
	}

	if change.DERPRegion != 0 {
		trace = trace.Int("derp_region", change.DERPRegion)
	}

	trace.Time("last_seen", *change.LastSeen).Msg("PeerChange received")
}

func peerChangeEmpty(chng tailcfg.PeerChange) bool {
	return chng.Key == nil &&
		chng.DiscoKey == nil &&
		chng.Online == nil &&
		chng.Endpoints == nil &&
		chng.DERPRegion == 0 &&
		chng.LastSeen == nil &&
		chng.KeyExpiry == nil
}

func logPollFunc(
	mapRequest tailcfg.MapRequest,
	node *types.Node,
) (func(string, ...any), func(string, ...any), func(string, ...any), func(error, string, ...any)) {
	return func(msg string, a ...any) {
			log.Warn().
				Caller().
				Bool("readOnly", mapRequest.ReadOnly).
				Bool("omitPeers", mapRequest.OmitPeers).
				Bool("stream", mapRequest.Stream).
				Uint64("node.id", node.ID.Uint64()).
				Str("node", node.Hostname).
				Msgf(msg, a...)
		},
		func(msg string, a ...any) {
			log.Info().
				Caller().
				Bool("readOnly", mapRequest.ReadOnly).
				Bool("omitPeers", mapRequest.OmitPeers).
				Bool("stream", mapRequest.Stream).
				Uint64("node.id", node.ID.Uint64()).
				Str("node", node.Hostname).
				Msgf(msg, a...)
		},
		func(msg string, a ...any) {
			log.Trace().
				Caller().
				Bool("readOnly", mapRequest.ReadOnly).
				Bool("omitPeers", mapRequest.OmitPeers).
				Bool("stream", mapRequest.Stream).
				Uint64("node.id", node.ID.Uint64()).
				Str("node", node.Hostname).
				Msgf(msg, a...)
		},
		func(err error, msg string, a ...any) {
			log.Error().
				Caller().
				Bool("readOnly", mapRequest.ReadOnly).
				Bool("omitPeers", mapRequest.OmitPeers).
				Bool("stream", mapRequest.Stream).
				Uint64("node.id", node.ID.Uint64()).
				Str("node", node.Hostname).
				Err(err).
				Msgf(msg, a...)
		}
}

// hostInfoChanged reports if hostInfo has changed in two ways,
// - first bool reports if an update needs to be sent to nodes
// - second reports if there has been changes to routes
// the caller can then use this info to save and update nodes
// and routes as needed.
func hostInfoChanged(old, new *tailcfg.Hostinfo) (bool, bool) {
	if old.Equal(new) {
		return false, false
	}

	// Routes
	oldRoutes := old.RoutableIPs
	newRoutes := new.RoutableIPs

	sort.Slice(oldRoutes, func(i, j int) bool {
		return comparePrefix(oldRoutes[i], oldRoutes[j]) > 0
	})
	sort.Slice(newRoutes, func(i, j int) bool {
		return comparePrefix(newRoutes[i], newRoutes[j]) > 0
	})

	if !xslices.Equal(oldRoutes, newRoutes) {
		return true, true
	}

	// Services is mostly useful for discovery and not critical,
	// except for peerapi, which is how nodes talk to eachother.
	// If peerapi was not part of the initial mapresponse, we
	// need to make sure its sent out later as it is needed for
	// Taildrop.
	// TODO(kradalby): Length comparison is a bit naive, replace.
	if len(old.Services) != len(new.Services) {
		return true, false
	}

	return false, false
}

// TODO(kradalby): Remove after go 1.23, will be in stdlib.
// Compare returns an integer comparing two prefixes.
// The result will be 0 if p == p2, -1 if p < p2, and +1 if p > p2.
// Prefixes sort first by validity (invalid before valid), then
// address family (IPv4 before IPv6), then prefix length, then
// address.
func comparePrefix(p, p2 netip.Prefix) int {
	if c := cmp.Compare(p.Addr().BitLen(), p2.Addr().BitLen()); c != 0 {
		return c
	}
	if c := cmp.Compare(p.Bits(), p2.Bits()); c != 0 {
		return c
	}
	return p.Addr().Compare(p2.Addr())
}