var (
    ErrBadScheme = errors.New("unknown scheme")
)

type Action int32

const (
    ActionConnect Action = iota
    ActionAnnounce
    ActionScrape
    ActionError
)

type Announce struct {
    TrackerUrl string
    Request    AnnounceRequest
    HostHeader string
    HTTPProxy  func(*http.Request) (*url.URL, error)
    ServerName string
    UserAgent  string
    UdpNetwork string
    // If the port is zero, it's assumed to be the same as the Request.Port.
    ClientIp4 krpc.NodeAddr
    // If the port is zero, it's assumed to be the same as the Request.Port.
    ClientIp6 krpc.NodeAddr
    Context   context.Context
}

type AnnounceEvent int32

const (
    None      AnnounceEvent = iota
    Completed               // The local peer just completed the torrent.
    Started                 // The local peer has just resumed this torrent.
    Stopped                 // The local peer is leaving the swarm.
)

type AnnounceRequest struct {
    InfoHash   [20]byte
    PeerId     [20]byte
    Downloaded int64
    Left       int64 // If less than 0, math.MaxInt64 will be used for HTTP trackers instead.
    Uploaded   int64
    // Apparently this is optional. None can be used for announces done at
    // regular intervals.
    Event     AnnounceEvent
    IPAddress uint32
    Key       int32
    NumWant   int32 // How many peer addresses are desired. -1 for default.
    Port      uint16
}   // 82 bytes

type AnnounceResponse struct {
    Interval int32 // Minimum seconds the local peer should wait before next announce.
    Leechers int32
    Seeders  int32
    Peers    []Peer
}

type AnnounceResponseHeader struct {
    Interval int32
    Leechers int32
    Seeders  int32
}

type ConnectionRequest struct {
    ConnectionId int64
    Action       int32
    TransctionId int32
}

type ConnectionResponse struct {
    ConnectionId int64
}

type HttpResponse struct {
    FailureReason string `bencode:"failure reason"`
    Interval      int32  `bencode:"interval"`
    TrackerId     string `bencode:"tracker id"`
    Complete      int32  `bencode:"complete"`
    Incomplete    int32  `bencode:"incomplete"`
    Peers         Peers  `bencode:"peers"`
    // BEP 7
    Peers6 krpc.CompactIPv6NodeAddrs `bencode:"peers6"`
}

type Peer struct {
    IP   net.IP
    Port int
    ID   []byte
}

type Peers []Peer

type RequestHeader struct {
    ConnectionId  int64
    Action        Action
    TransactionId int32
}   // 16 bytes

type ResponseHeader struct {
    Action        Action
    TransactionId int32
}