package peermanager

import (
	"context"
	"encoding/json"
	"errors"

	"github.com/libp2p/go-libp2p/core/peer"
	"github.com/libp2p/go-libp2p/core/protocol"
	wps "github.com/waku-org/go-waku/waku/v2/peerstore"
	waku_proto "github.com/waku-org/go-waku/waku/v2/protocol"
	"go.uber.org/zap"
	"golang.org/x/exp/maps"
)

type PeerSet map[peer.ID]struct{}

func PeerInSet(peers PeerSet, peer peer.ID) bool {
	if len(peers) > 0 {
		if _, ok := peers[peer]; ok {
			return true
		}
	}
	return false
}

// SelectPeerByContentTopic is used to return a random peer that supports a given protocol for given contentTopic.
// If a list of specific peers is passed, the peer will be chosen from that list assuming
// it supports the chosen protocol and contentTopic, otherwise it will chose a peer from the service slot.
// If a peer cannot be found in the service slot, a peer will be selected from node peerstore
func (pm *PeerManager) SelectPeerByContentTopics(proto protocol.ID, contentTopics []string, specificPeers ...peer.ID) (peer.ID, error) {
	pubsubTopics := []string{}
	for _, cTopic := range contentTopics {
		pubsubTopic, err := waku_proto.GetPubSubTopicFromContentTopic(cTopic)
		if err != nil {
			pm.logger.Debug("selectPeer: failed to get contentTopic from pubsubTopic", zap.String("contentTopic", cTopic))
			return "", err
		}
		pubsubTopics = append(pubsubTopics, pubsubTopic)
	}
	peers, err := pm.SelectPeers(PeerSelectionCriteria{PubsubTopics: pubsubTopics, Proto: proto, SpecificPeers: specificPeers})
	if err != nil {
		return "", err
	}
	return peers[0], nil
}

// SelectRandomPeer is used to return a random peer that supports a given protocol.
// If a list of specific peers is passed, the peer will be chosen from that list assuming
// it supports the chosen protocol, otherwise it will chose a peer from the service slot.
// If a peer cannot be found in the service slot, a peer will be selected from node peerstore
// if pubSubTopic is specified, peer is selected from list that support the pubSubTopic
func (pm *PeerManager) SelectRandom(criteria PeerSelectionCriteria) (peer.IDSlice, error) {
	// @TODO We need to be more strategic about which peers we dial. Right now we just set one on the service.
	// Ideally depending on the query and our set  of peers we take a subset of ideal peers.
	// This will require us to check for various factors such as:
	//  - which topics they track
	//  - latency?
	peerIDs, err := pm.selectServicePeer(criteria)
	if err == nil && len(peerIDs) == criteria.MaxPeers {
		return maps.Keys(peerIDs), nil
	} else if !errors.Is(err, ErrNoPeersAvailable) {
		pm.logger.Debug("could not retrieve random peer from slot", zap.String("protocol", string(criteria.Proto)),
			zap.Strings("pubsubTopics", criteria.PubsubTopics), zap.Error(err))
		return nil, err
	} else if len(peerIDs) == 0 {
		peerIDs = make(PeerSet)
	}
	// if not found in serviceSlots or proto == WakuRelayIDv200
	filteredPeers, err := pm.FilterPeersByProto(criteria.SpecificPeers, criteria.ExcludePeers, criteria.Proto)
	if err != nil {
		return nil, err
	}
	if len(criteria.PubsubTopics) > 0 {
		filteredPeers = pm.host.Peerstore().(wps.WakuPeerstore).PeersByPubSubTopics(criteria.PubsubTopics, filteredPeers...)
	}
	//Not passing excludePeers as filterPeers are already considering excluded ones.
	randomPeers, err := selectRandomPeers(filteredPeers, nil, criteria.MaxPeers-len(peerIDs))
	if err != nil && len(peerIDs) == 0 {
		return nil, err
	}

	for tmpPeer := range randomPeers {
		peerIDs[tmpPeer] = struct{}{}
	}
	return maps.Keys(peerIDs), nil
}

func getRandom(filter PeerSet, count int, excludePeers PeerSet) (PeerSet, error) {
	i := 0
	selectedPeers := make(PeerSet)
	for pID := range filter {
		if PeerInSet(excludePeers, pID) {
			continue
		}
		//Map's iterator in golang works using randomness and hence not random function is being used.
		selectedPeers[pID] = struct{}{}
		i++
		if i == count {
			break
		}
	}
	if len(selectedPeers) == 0 {
		return nil, ErrNoPeersAvailable
	}
	return selectedPeers, nil
}

// selects count random peers from list of peers
func selectRandomPeers(peers peer.IDSlice, excludePeers PeerSet, count int) (PeerSet, error) {
	filteredPeerMap := PeerSliceToMap(peers)
	return getRandom(filteredPeerMap, count, excludePeers)
}

func PeerSliceToMap(peers peer.IDSlice) PeerSet {
	peerSet := make(PeerSet, peers.Len())
	for _, peer := range peers {
		peerSet[peer] = struct{}{}
	}
	return peerSet
}

func (pm *PeerManager) selectServicePeer(criteria PeerSelectionCriteria) (PeerSet, error) {
	peers := make(PeerSet)
	var err error
	for retryCnt := 0; retryCnt < 1; retryCnt++ {
		//Try to fetch from serviceSlot
		if slot := pm.serviceSlots.getPeers(criteria.Proto); slot != nil {
			if len(criteria.PubsubTopics) == 0 || (len(criteria.PubsubTopics) == 1 && criteria.PubsubTopics[0] == "") {
				return slot.getRandom(criteria.MaxPeers, criteria.ExcludePeers)
			} else { //PubsubTopic based selection
				keys := make([]peer.ID, 0, len(slot.m))
				for i := range slot.m {
					if PeerInSet(criteria.ExcludePeers, i) {
						continue
					}
					keys = append(keys, i)
				}
				selectedPeers := pm.host.Peerstore().(wps.WakuPeerstore).PeersByPubSubTopics(criteria.PubsubTopics, keys...)
				tmpPeers, err := selectRandomPeers(selectedPeers, criteria.ExcludePeers, criteria.MaxPeers)
				for tmpPeer := range tmpPeers {
					peers[tmpPeer] = struct{}{}
				}
				if err == nil && len(peers) == criteria.MaxPeers {
					return peers, nil
				} else {
					pm.logger.Debug("discovering peers by pubsubTopic", zap.Strings("pubsubTopics", criteria.PubsubTopics))
					//Trigger on-demand discovery for this topic and connect to peer immediately.
					//For now discover atleast 1 peer for the criteria
					pm.discoverPeersByPubsubTopics(criteria.PubsubTopics, criteria.Proto, criteria.Ctx, 1)
					//Try to fetch peers again.
					continue
				}
			}
		}
	}
	if len(peers) == 0 {
		pm.logger.Debug("could not retrieve random peer from slot", zap.Error(err))
	}
	return peers, ErrNoPeersAvailable
}

// PeerSelectionCriteria is the selection Criteria that is used by PeerManager to select peers.
type PeerSelectionCriteria struct {
	SelectionType PeerSelection   `json:"selectionType"`
	Proto         protocol.ID     `json:"protocolId"`
	PubsubTopics  []string        `json:"pubsubTopics"`
	SpecificPeers peer.IDSlice    `json:"specificPeers"`
	MaxPeers      int             `json:"maxPeerCount"`
	Ctx           context.Context `json:"-"`
	ExcludePeers  PeerSet         `json:"excludePeers"`
}

func (psc PeerSelectionCriteria) String() string {
	pscJson, err := json.Marshal(psc)
	if err != nil {
		return ""
	}
	return string(pscJson)
}

// SelectPeers selects a peer based on selectionType specified.
// Context is required only in case of selectionType set to LowestRTT
func (pm *PeerManager) SelectPeers(criteria PeerSelectionCriteria) (peer.IDSlice, error) {
	if criteria.MaxPeers == 0 {
		criteria.MaxPeers = 1
	}
	excPeers := maps.Keys(criteria.ExcludePeers)
	var excPeer peer.ID
	if len(excPeers) > 0 {
		excPeer = excPeers[0]
	}
	pm.logger.Debug("Select Peers", zap.Stringer("selectionCriteria", criteria), zap.Stringer("excludedPeers", excPeer))
	switch criteria.SelectionType {
	case Automatic:
		return pm.SelectRandom(criteria)
	case LowestRTT:
		peerID, err := pm.SelectPeerWithLowestRTT(criteria)
		if err != nil {
			return nil, err
		}
		//TODO: Update this once peer Ping cache PR is merged into this code.
		return []peer.ID{peerID}, nil
	default:
		return nil, errors.New("unknown peer selection type specified")
	}
}

// SelectPeerWithLowestRTT will select a peer that supports a specific protocol with the lowest reply time
// If a list of specific peers is passed, the peer will be chosen from that list assuming
// it supports the chosen protocol, otherwise it will chose a peer from the node peerstore
// TO OPTIMIZE: As of now the peer with lowest RTT is identified when select is called, this should be optimized
// to maintain the RTT as part of peer-scoring and just select based on that.
func (pm *PeerManager) SelectPeerWithLowestRTT(criteria PeerSelectionCriteria) (peer.ID, error) {
	var peers peer.IDSlice
	var err error
	if criteria.Ctx == nil {
		pm.logger.Warn("context is not passed for peerSelectionwithRTT, using background context")
		criteria.Ctx = context.Background()
	}

	if len(criteria.PubsubTopics) == 0 || (len(criteria.PubsubTopics) == 1 && criteria.PubsubTopics[0] == "") {
		peers = pm.host.Peerstore().(wps.WakuPeerstore).PeersByPubSubTopics(criteria.PubsubTopics, criteria.SpecificPeers...)
	}

	peers, err = pm.FilterPeersByProto(peers, criteria.ExcludePeers, criteria.Proto)
	if err != nil {
		return "", err
	}

	return pm.rttCache.FastestPeer(criteria.Ctx, peers)
}

// FilterPeersByProto filters list of peers that support specified protocols.
// If specificPeers is nil, all peers in the host's peerStore are considered for filtering.
func (pm *PeerManager) FilterPeersByProto(specificPeers peer.IDSlice, excludePeers PeerSet, proto ...protocol.ID) (peer.IDSlice, error) {
	peerSet := specificPeers
	if len(peerSet) == 0 {
		peerSet = pm.host.Peerstore().Peers()
	}
	var peers peer.IDSlice
	for _, peer := range peerSet {
		protocols, err := pm.host.Peerstore().SupportsProtocols(peer, proto...)
		if err != nil {
			return nil, err
		}
		if len(protocols) > 0 {
			//Maybe we can optimize below set of statements a better way??
			if PeerInSet(excludePeers, peer) {
				continue
			}
			peers = append(peers, peer)
		}
	}
	return peers, nil
}