package mtglib

import (
	"context"
	"errors"
	"net"
	"net/http"
	"time"
)

var (
	ErrSecretEmpty                    = errors.New("secret is empty")
	ErrSecretInvalid                  = errors.New("secret is invalid")
	ErrNetworkIsNotDefined            = errors.New("network is not defined")
	ErrAntiReplayCacheIsNotDefined    = errors.New("anti-replay cache is not defined")
	ErrTimeAttackDetectorIsNotDefined = errors.New("time attack detector is not defined")
	ErrIPBlocklistIsNotDefined        = errors.New("ip blocklist is not defined")
	ErrEventStreamIsNotDefined        = errors.New("event stream is not defined")
	ErrLoggerIsNotDefined             = errors.New("logger is not defined")
)

const (
	DefaultConcurrency        = 4096
	DefaultBufferSize         = 16 * 1024 // 16 kib
	DefaultDomainFrontingPort = 443
	DefaultIdleTimeout        = time.Minute
	DefaultPreferIP           = "prefer-ipv6"
)

// Network defines a knowledge how to work with a network. It may sound
// fun but it encapsulates all the knowledge how to properly establish
// connections to remote hosts and configure HTTP clients.
//
// For example, if you want to use SOCKS5 proxy, you probably want to
// have all traffic routed to this proxy: telegram connections, http
// requests and so on. This knowledge is encapsulated into instances of
// such interface.
//
// mtglib uses Network for:
//
// 1. Dialing to Telegram
//
// 2. Dialing to front domain
//
// 3. Doing HTTP requests (for example, for FireHOL ipblocklist).
type Network interface {
	// Dial establishes context-free TCP connections.
	Dial(network, address string) (net.Conn, error)

	// DialContext dials using a context. This is a preferrable
	// way of establishing TCP connections.
	DialContext(ctx context.Context, network, address string) (net.Conn, error)

	// MakeHTTPClient build an HTTP client with given dial function. If
	// nothing is provided, then DialContext of this interface is going
	// to be used.
	MakeHTTPClient(func(ctx context.Context, network, address string) (net.Conn, error)) *http.Client
}

// AntiReplayCache is an interface that is used to detect replay attacks
// based on some traffic fingerprints.
//
// Replay attacks are probe attacks whose main goal is to identify if
// server software can be classified in some way. For example, if you
// send some HTTP request to a web server, then you can expect that this
// server will respond with HTTP response back.
//
// There is a problem though. Let's imagine, that connection is
// encrypted. Let's imagine, that it is encrypted with some static key
// like ShadowSocks (https://shadowsocks.org/assets/whitepaper.pdf).
// In that case, in theory, if you repeat the same bytes, you can get
// the same responses. Let's imagine, that you've cracked the key. then
// if you send the same bytes, you can decrypt a response and see its
// structure. Based on its structure you can identify if this server is
// SOCKS5, MTPROTO proxy etc.
//
// This is just one example, maybe not the best or not the most
// relevant. In real life, different organizations use such replay
// attacks to perform some reverse engineering of the proxy, do some
// statical analysis to identify server software.
//
// There are many ways how to protect your proxy against them. One
// is domain fronting which is a core part of mtg. Another one is to
// collect some 'handshake fingerprints' and forbid duplication.
//
// So, it one is sending the same byte flow right after you (or a couple
// of hours after), mtg should detect that and reject this connection
// (or redirect to fronting domain).
type AntiReplayCache interface {
	// Seen before checks if this set of bytes was observed before or
	// not. If it is required to store this information somewhere else,
	// then it has to do that.
	SeenBefore(data []byte) bool
}

// IPBlocklist filters requests based on IP address.
//
// If this filter has an IP address, then mtg closes a request without
// reading anything from a socket. It also does not give such request to
// a worker pool, so in worst cases you can expect that you invoke this
// object more frequent than defined proxy concurrency.
type IPBlocklist interface {
	// Contains checks if given IP address belongs to this blocklist If.
	// it is, a connection is terminated                               .
	Contains(net.IP) bool
}

type Event interface {
	StreamID() string
	Timestamp() time.Time
}

// EventStream is an abstraction that accepts a set of events produced
// by mtg. Its main goal is to inject your logging or monitoring system.
//
// The idea is simple. When mtg works, it emits a set of events during
// a lifecycle of the requestor: EventStart, EventFinish etc. mtg is a
// producer which puts these events into a stream. Responsibility of
// the stream is to deliver this event to consumers/observers. There
// might be many different observers (for example, you want to have both
// statsd and prometheus), mtg should know nothing about them.
type EventStream interface {
	// Send delivers an event to observers. Given context has to be
	// respected. If the context is closed, all blocking operations should
	// be released ASAP.
	//
	// It is possible that context is closed but the message is delivered.
	// EventStream implementations should solve this issue somehow.
	Send(context.Context, Event)
}

// TimeAttackDetector is an abstraction that checks a time, taken from
// the faketls client hello message. This timestamp is encoded into
// client-generated random bytes and can be extracted after some client
// hello verification.
//
// This is mostly to prevent replay attacks.
type TimeAttackDetector interface {
	// Valid returns an error if timestamp is invalid or should not be
	// accepted.
	Valid(time.Time) error
}

// Logger defines an interface of the logger used by mtglib.
//
// Each logger has a name. It is possible to stack names to organize
// poor-man namespaces. Also, each logger must be able to bind
// parameters to avoid pushing them all the time.
//
// Example
//
//     logger := SomeLogger{}
//     logger = logger.BindStr("ip", net.IP{127, 0, 0, 1})
//     logger.Info("Hello")
//
// In that case, ip is bound as a parameter. It is a great idea to
// put this parameter somewhere in a log message.
//
//     logger1 = logger.BindStr("param1", "11")
//     logger2 = logger.BindInt("param2", 11)
//
// logger1 should see no param2 and vice versa, logger2 should not see param1
// If you attach a parameter to a logger, parents should not know about that.
type Logger interface {
	Named(name string) Logger

	BindInt(name string, value int) Logger
	BindStr(name, value string) Logger

	Printf(format string, args ...interface{})
	Info(msg string)
	InfoError(msg string, err error)
	Warning(msg string)
	WarningError(msg string, err error)
	Debug(msg string)
	DebugError(msg string, err error)
}