package mesh

import (
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"strings"
	"time"
)

type VPNPacketBatchPayload struct {
	SchemaVersion   string    `json:"schema_version"`
	VPNConnectionID string    `json:"vpn_connection_id"`
	TunnelID        string    `json:"tunnel_id,omitempty"`
	PoolID          string    `json:"pool_id,omitempty"`
	ServiceID       string    `json:"service_id,omitempty"`
	LocalServiceID  string    `json:"local_service_id,omitempty"`
	RemoteServiceID string    `json:"remote_service_id,omitempty"`
	ServiceKind     string    `json:"service_kind,omitempty"`
	ServiceClass    string    `json:"service_class,omitempty"`
	ServiceRole     string    `json:"service_role,omitempty"`
	RouteLeaseID    string    `json:"route_lease_id,omitempty"`
	RouteGeneration string    `json:"route_generation,omitempty"`
	DataPlane       string    `json:"data_plane,omitempty"`
	TransportOwner  string    `json:"transport_owner,omitempty"`
	RouteVisibility string    `json:"route_visibility,omitempty"`
	TrafficClasses  []string  `json:"traffic_classes,omitempty"`
	StreamShards    int       `json:"stream_shards,omitempty"`
	Direction       string    `json:"direction"`
	Packets         [][]byte  `json:"packets"`
	SentAt          time.Time `json:"sent_at"`
}

type ProductionVPNPacketEnvelopeInput struct {
	MessageID         string
	RouteID           string
	ClusterID         string
	SourceNodeID      string
	DestinationNodeID string
	CurrentHopNodeID  string
	NextHopNodeID     string
	RoutePath         []string
	TTL               int
	HopCount          int
	ExpiresAt         time.Time
	VPNConnectionID   string
	Direction         string
	Packets           [][]byte
	Now               time.Time
}

func NewProductionVPNPacketBatchEnvelope(input ProductionVPNPacketEnvelopeInput) (ProductionEnvelope, error) {
	now := input.Now.UTC()
	if now.IsZero() {
		now = time.Now().UTC()
	}
	packets := cleanProductionVPNPacketBatch(input.Packets)
	if len(packets) == 0 {
		return ProductionEnvelope{}, fmt.Errorf("%w: empty vpn packet batch", ErrForwardEnvelopeInvalid)
	}
	if input.MessageID == "" {
		input.MessageID = fmt.Sprintf("vpnpkt-%d", now.UnixNano())
	}
	if input.TTL <= 0 {
		input.TTL = 8
	}
	if input.ExpiresAt.IsZero() {
		input.ExpiresAt = now.Add(15 * time.Second)
	}
	payload, err := json.Marshal(VPNPacketBatchPayload{
		SchemaVersion:   "rap.vpn_packet_batch.v1",
		VPNConnectionID: input.VPNConnectionID,
		Direction:       input.Direction,
		Packets:         packets,
		SentAt:          now,
	})
	if err != nil {
		return ProductionEnvelope{}, err
	}
	if len(payload) > MaxProductionVPNPacketPayloadBytes {
		return ProductionEnvelope{}, fmt.Errorf("%w: vpn packet batch exceeds channel limit", ErrForwardEnvelopeInvalid)
	}
	sum := sha256.Sum256(payload)
	return ProductionEnvelope{
		FabricProtocolVersion: ProtocolVersion,
		MessageID:             input.MessageID,
		RouteID:               input.RouteID,
		ClusterID:             input.ClusterID,
		SourceNodeID:          input.SourceNodeID,
		DestinationNodeID:     input.DestinationNodeID,
		CurrentHopNodeID:      input.CurrentHopNodeID,
		NextHopNodeID:         input.NextHopNodeID,
		RoutePath:             append([]string{}, input.RoutePath...),
		ChannelClass:          ProductionChannelVPNPacket,
		MessageType:           ProductionMessageVPNPacketBatch,
		TTL:                   input.TTL,
		HopCount:              input.HopCount,
		CreatedAt:             now,
		ExpiresAt:             input.ExpiresAt.UTC(),
		PayloadLength:         len(payload),
		PayloadHash:           hex.EncodeToString(sum[:]),
		Payload:               payload,
	}, nil
}

func DecodeProductionVPNPacketBatch(envelope ProductionEnvelope) (VPNPacketBatchPayload, error) {
	if envelope.ChannelClass != ProductionChannelVPNPacket || envelope.MessageType != ProductionMessageVPNPacketBatch {
		return VPNPacketBatchPayload{}, ErrUnauthorizedChannel
	}
	var payload VPNPacketBatchPayload
	if err := json.Unmarshal(envelope.Payload, &payload); err != nil {
		return VPNPacketBatchPayload{}, err
	}
	if payload.SchemaVersion != "rap.vpn_packet_batch.v1" || payload.VPNConnectionID == "" {
		return VPNPacketBatchPayload{}, fmt.Errorf("%w: invalid vpn packet batch payload", ErrForwardEnvelopeInvalid)
	}
	payload.Packets = cleanProductionVPNPacketBatch(payload.Packets)
	if len(payload.Packets) == 0 {
		return VPNPacketBatchPayload{}, fmt.Errorf("%w: empty vpn packet batch payload", ErrForwardEnvelopeInvalid)
	}
	return payload, nil
}

func cleanProductionVPNPacketBatch(packets [][]byte) [][]byte {
	if len(packets) == 0 {
		return nil
	}
	cleaned := make([][]byte, 0, len(packets))
	for _, packet := range packets {
		if len(packet) == 0 {
			continue
		}
		cleaned = append(cleaned, append([]byte(nil), packet...))
	}
	return cleaned
}

func inferVPNPacketTrafficClass(explicit string, packets [][]byte) string {
	explicit = strings.TrimSpace(strings.ToLower(explicit))
	if explicit != "" && explicit != "bulk" {
		return explicit
	}
	for _, packet := range packets {
		if isVPNPacketTCPControl(packet) {
			return "interactive"
		}
	}
	return explicit
}

func isVPNPacketTCPControl(packet []byte) bool {
	if len(packet) < 20 || packet[0]>>4 != 4 {
		return false
	}
	ihl := int(packet[0]&0x0f) * 4
	if ihl < 20 || len(packet) < ihl+20 || packet[9] != 6 {
		return false
	}
	flags := packet[ihl+13]
	return flags&0x17 != 0
}