// Network contains a default implementation of the network.
//
// Please see mtglib.Network interface to get some basic idea behind
// this abstraction.
//
// Some notable feature of this implementation:
//
// 1. It detaches dialer from a network. Dialer is something which
// implements a real dialer and network completes it with more higher
// level details.
//
// 2. It uses only TCP connections. Even for DNS it uses DNS-Over-HTTPS
//
// 3. It has some simple implementation of DNS cache which is good
// enough for our purpose.
//
// 4. It sets uses SO_REUSEPORT port if applicable.
package network

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

const (
	// DefaultTimeout is a default timeout for establishing TCP
	// connection.
	DefaultTimeout = 10 * time.Second

	// DefaultHTTPTimeout defines a default timeout for making HTTP
	// request.
	DefaultHTTPTimeout = 10 * time.Second

	// DefaultBufferSize defines a TCP buffer size. Both read and write, so
	// for real size, please multiply this number by 2.
	DefaultBufferSize = 16 * 1024 // 16 kib

	// ProxyDialerOpenThreshold is used for load balancing SOCKS5 dialer
	// only.
	//
	// This dialer uses circuit breaker with of 3 stages: OPEN,
	// HALF_OPEN and CLOSED. If state is CLOSED, all requests go in
	// a normal mode. If you get more that ProxyDialerOpenThreshold
	// errors, circuit breaker goes into OPEN mode.
	//
	// When circuit breaker is in OPEN mode, it forbids all request to
	// a given proxy. But after ProxyDialerHalfOpenTimeout it gives a
	// second chance and opens an access for a SINGLE request. If this
	// request success, then circuit breaker closes, otherwise opens
	// again.
	//
	// When circuit breaker is closed, it clears an error states each
	// ProxyDialerResetFailuresTimeout.
	ProxyDialerOpenThreshold = 5

	// ProxyDialerHalfOpenTimeout defines a halfopen timeout for circuit
	// breaker.
	ProxyDialerHalfOpenTimeout = time.Minute

	// ProxyDialerResetFailuresTimeout defines a timeout for resetting a
	// failure.
	ProxyDialerResetFailuresTimeout = 10 * time.Second

	// DefaultDOHHostname defines a default IP address for DOH host.
	// Since mtg is simple, please pass IP address here. We do not
	// have bootstrap servers here embedded.
	DefaultDOHHostname = "9.9.9.9"

	// DNSTimeout defines a timeout for DNS queries.
	DNSTimeout = 5 * time.Second

	// tcpLingerTimeout defines a number of seconds to wait for sending
	// unacknowledged data.
	tcpLingerTimeout = 1
)

var (
	// ErrCircuitBreakerOpened is returned when proxy is being accessed
	// but circuit breaker is opened.
	ErrCircuitBreakerOpened = errors.New("circuit breaker is opened")

	// ErrCannotDialWithAllProxies is returned when load balancing
	// client is trying to access proxies but all of them are failed.
	ErrCannotDialWithAllProxies = errors.New("cannot dial with all proxies")
)

// Dialer defines an interface which is required to bootstrap a network
// instance from.
type Dialer interface {
	Dial(network, address string) (net.Conn, error)
	DialContext(ctx context.Context, network, address string) (net.Conn, error)
}