// mtglib defines a package with MTPROTO proxy.
//
// Since mtg itself is build as an example of how to work with mtglib,
// it worth to telling a couple of words about a project organization.
//
// A core object of the project is mtglib.Proxy. This is a proxy you
// expect: that one which you configure, set to serve on a listener
// and/or shutdown on application termination.
//
// But it also has a core logic unrelated to Telegram per se: anti
// replay cache, network connectivity (who knows, maybe you want to have
// a native VMESS integration) and so on.
//
// You can supply such parts to a proxy with interfaces. The rest of
// the packages in mtg define some default implementations of these
// interfaces. But if you want to integrate it with, let say, influxdb,
// you can do it easily.
package mtglib

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

var (
	// ErrSecretEmpty is returned if you are trying to create a proxy
	// but do not provide a secret.
	ErrSecretEmpty = errors.New("secret is empty")

	// ErrSecretInvalid is returned if you are trying to create a proxy
	// but secret value is invalid (no host or payload are zeroes).
	ErrSecretInvalid = errors.New("secret is invalid")

	// ErrNetworkIsNotDefined is returned if you are trying to create a
	// proxy but network value is undefined.
	ErrNetworkIsNotDefined = errors.New("network is not defined")

	// ErrAntiReplayCacheIsNotDefined is returned if you are trying to
	// create a proxy but anti replay cache value is undefined.
	ErrAntiReplayCacheIsNotDefined = errors.New("anti-replay cache is not defined")

	// ErrIPBlocklistIsNotDefined is returned if you are trying to
	// create a proxy but ip blocklist instance is not defined.
	ErrIPBlocklistIsNotDefined = errors.New("ip blocklist is not defined")

	// ErrEventStreamIsNotDefined is returned if you are trying to create a
	// proxy but event stream instance is not defined.
	ErrEventStreamIsNotDefined = errors.New("event stream is not defined")

	// ErrLoggerIsNotDefined is returned if you are trying to
	// create a proxy but logger is not defined.
	ErrLoggerIsNotDefined = errors.New("logger is not defined")
)

const (
	// DefaultConcurrency is a default max count of simultaneously
	// connected clients.
	DefaultConcurrency = 4096

	// DefaultBufferSize is a default size of a copy buffer.
	DefaultBufferSize = 16 * 1024 // 16 kib

	// DefaultDomainFrontingPort is a default port (HTTPS) to connect to in
	// case of probe-resistance activity.
	DefaultDomainFrontingPort = 443

	// DefaultIdleTimeout is a default timeout for closing a connection
	// in case of idling.
	//
	// Deprecated: no longer in use because of changed TCP relay
	// algorithm.
	DefaultIdleTimeout = time.Minute

	// DefaultTolerateTimeSkewness is a default timeout for time
	// skewness on a faketls timeout verification.
	DefaultTolerateTimeSkewness = 3 * time.Second

	// DefaultPreferIP is a default value for Telegram IP connectivity
	// preference.
	DefaultPreferIP = "prefer-ipv6"

	// SecretKeyLength defines a length of the secret bytes used
	// by Telegram and a proxy.
	SecretKeyLength = 16

	// ConnectionIDBytesLength defines a count of random bytes used to generate
	// a stream/connection ids.
	ConnectionIDBytesLength = 16

	// TCPRelayReadTimeout defines a max time period between two consecuitive
	// reads from Telegram after which connection will be terminated. This is
	// required to abort stale connections.
	TCPRelayReadTimeout = 20 * time.Second
)

// 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
}

// Event is a data structure which is populated during mtg request
// processing lifecycle. Each request popluates many events:
//
// 1. Client connected
//
// 2. Request is finished
//
// 3. Connection to Telegram server is established
//
// and so on. All these events are data structures but all of them
// must conform the same interface.
type Event interface {
	// StreamID returns an identifier of the stream, connection,
	// request, you name it. All events within the same stream returns
	// the same stream id.
	StreamID() string

	// Timestamp returns a timestamp when this event was generated.
	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)
}

// 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 returns a new logger with a bound name. Name chaining is
	// allowed and appreciated.
	Named(name string) Logger

	// BindInt binds new integer parameter to a new logger instance.
	BindInt(name string, value int) Logger

	// BindStr binds new string parameter to a new logger instance.
	BindStr(name, value string) Logger

	// Printf is to support log.Logger behavior.
	Printf(format string, args ...interface{})

	// Info puts a message about some normal situation.
	Info(msg string)

	// InfoError puts a message about some normal situation but this
	// situation is related to a given error.
	InfoError(msg string, err error)

	// Warning puts a message about some extraordinary situation
	// worth to look at.
	Warning(msg string)

	// WarningError puts a message about some extraordinary situation
	// worth to look at. This situation is related to a given error.
	WarningError(msg string, err error)

	// Debug puts a message useful for debugging only.
	Debug(msg string)

	// Debug puts a message useful for debugging only. This message is
	// related to a given error.
	DebugError(msg string, err error)
}