package smtp

// http://www.rfc-editor.org/rfc/rfc5321.txt

import (
	"errors"
	"log"
	"regexp"
	"strings"

	"github.com/ian-kent/Go-MailHog/mailhog/data"
)

// Protocol is a state machine representing an SMTP session
type Protocol struct {
	state    State
	message  *data.SMTPMessage
	hostname string

	MessageIDHandler       func() (string, error)
	LogHandler             func(message string, args ...interface{})
	MessageReceivedHandler func(*data.Message) (string, error)
}

// Command is a struct representing an SMTP command (verb + arguments)
type Command struct {
	verb string
	args string
}

// Reply is a struct representing an SMTP reply (status code + lines)
type Reply struct {
	status int
	lines  []string
}

// ReplyOk creates a 250 Ok reply
func ReplyOk() *Reply { return &Reply{250, []string{"Ok"}} }

// ReplyBye creates a 221 Bye reply
func ReplyBye() *Reply { return &Reply{221, []string{"Bye"}} }

// ReplyUnrecognisedCommand creates a 500 Unrecognised command reply
func ReplyUnrecognisedCommand() *Reply { return &Reply{500, []string{"Unrecognised command"}} }

// ReplySenderOk creates a 250 Sender ok reply
func ReplySenderOk(sender string) *Reply { return &Reply{250, []string{"Sender " + sender + " ok"}} }

// ReplyRecipientOk creates a 250 Sender ok reply
func ReplyRecipientOk(recipient string) *Reply {
	return &Reply{250, []string{"Recipient " + recipient + " ok"}}
}

// ReplyError creates a 500 error reply
func ReplyError(err error) *Reply { return &Reply{550, []string{err.Error()}} }

// State represents the state of an SMTP conversation
type State int

// SMTP message conversation states
const (
	INVALID   = State(-1)
	ESTABLISH = State(iota)
	AUTH
	AUTHLOGIN
	MAIL
	RCPT
	DATA
	DONE
)

// StateMap provides string representations of SMTP conversation states
var StateMap = map[State]string{
	INVALID:   "INVALID",
	ESTABLISH: "ESTABLISH",
	AUTH:      "AUTH",
	AUTHLOGIN: "AUTHLOGIN",
	MAIL:      "MAIL",
	RCPT:      "RCPT",
	DATA:      "DATA",
	DONE:      "DONE",
}

// NewProtocol returns a new SMTP state machine in INVALID state
// handler is called when a message is received and should return a message ID
func NewProtocol() *Protocol {
	return &Protocol{
		state:   INVALID,
		message: &data.SMTPMessage{},
	}
}

func (proto *Protocol) logf(message string, args ...interface{}) {
	message = strings.Join([]string{"[PROTO: %s]", message}, " ")
	args = append([]interface{}{StateMap[proto.state]}, args...)

	if proto.LogHandler != nil {
		proto.LogHandler(message, args...)
	} else {
		log.Printf(message, args...)
	}
}

// Start begins an SMTP conversation with a 220 reply
func (proto *Protocol) Start(hostname string) *Reply {
	proto.state = ESTABLISH
	proto.hostname = hostname
	return &Reply{
		status: 220,
		lines:  []string{hostname + " ESMTP Go-MailHog"},
	}
}

// Parse parses a line string and returns any remaining line string
// and a reply, if a command was found. Parse does nothing until a
// new line is found.
// - TODO move this to a buffer inside proto?
func (proto *Protocol) Parse(line string) (string, *Reply) {
	var reply *Reply

	for strings.Contains(line, "\n") {
		parts := strings.SplitN(line, "\n", 2)

		if len(parts) == 2 {
			line = parts[1]
		} else {
			line = ""
		}

		if proto.state == DATA {
			return line, proto.ProcessData(parts[0])
		}

		return line, proto.ProcessCommand(parts[0])
	}

	return line, reply
}

// ProcessData handles content received (with newlines stripped) while
// in the SMTP DATA state
func (proto *Protocol) ProcessData(line string) (reply *Reply) {

	proto.message.Data += line + "\n"

	if strings.HasSuffix(proto.message.Data, "\r\n.\r\n") {
		proto.message.Data = strings.Replace(proto.message.Data, "\r\n..", "\r\n.", -1)

		proto.logf("Got EOF, storing message and switching to MAIL state")
		proto.message.Data = strings.TrimSuffix(proto.message.Data, "\r\n.\r\n")
		proto.state = MAIL

		msg := proto.message.Parse(proto.hostname)

		if proto.MessageReceivedHandler != nil {
			id, err := proto.MessageReceivedHandler(msg)
			if err != nil {
				proto.logf("Error storing message: %s", err)
				reply = &Reply{452, []string{"Unable to store message"}}
			} else {
				reply = &Reply{250, []string{"Ok: queued as " + id}}
			}
		} else {
			reply = &Reply{452, []string{"No storage backend"}}
		}
	}

	return
}

// ProcessCommand processes a line of text as a command
// It expects the line string to be a properly formed SMTP verb and arguments
func (proto *Protocol) ProcessCommand(line string) (reply *Reply) {
	line = strings.Trim(line, "\r\n")
	proto.logf("Processing line: %s", line)

	words := strings.Split(line, " ")
	command := strings.ToUpper(words[0])
	args := strings.Join(words[1:len(words)], " ")
	proto.logf("In state %d, got command '%s', args '%s'", proto.state, command, args)

	cmd := &Command{command, args}
	return proto.Command(cmd)
}

// Command applies an SMTP verb and arguments to the state machine
func (proto *Protocol) Command(command *Command) (reply *Reply) {
	switch {
	case "RSET" == command.verb:
		proto.logf("Got RSET command, switching to MAIL state")
		proto.state = MAIL
		proto.message = &data.SMTPMessage{}
		return ReplyOk()
	case "NOOP" == command.verb:
		proto.logf("Got NOOP verb, staying in %s state", StateMap[proto.state])
		return ReplyOk()
	case "QUIT" == command.verb:
		proto.logf("Got QUIT verb, staying in %s state", StateMap[proto.state])
		return ReplyBye()
	case ESTABLISH == proto.state:
		switch command.verb {
		case "HELO":
			return proto.HELO(command.args)
		case "EHLO":
			return proto.EHLO(command.args)
		default:
			proto.logf("Got unknown command for ESTABLISH state: '%s'", command.verb)
			return ReplyUnrecognisedCommand()
		}
	case AUTH == proto.state:
		proto.logf("Got authentication response: '%s', switching to MAIL state", command.args)
		proto.state = MAIL
		return &Reply{235, []string{"Authentication successful"}}
	case AUTHLOGIN == proto.state:
		proto.logf("Got LOGIN authentication response: '%s', switching to AUTH state", command.args)
		proto.state = AUTH
		return &Reply{334, []string{"UGFzc3dvcmQ6"}}
	case MAIL == proto.state:
		switch command.verb {
		case "AUTH":
			proto.logf("Got AUTH command, staying in MAIL state")
			switch {
			case strings.HasPrefix(command.args, "PLAIN "):
				proto.logf("Got PLAIN authentication: %s", strings.TrimPrefix(command.args, "PLAIN "))
				return &Reply{235, []string{"Authentication successful"}}
			case "LOGIN" == command.args:
				proto.logf("Got LOGIN authentication, switching to AUTH state")
				proto.state = AUTHLOGIN
				return &Reply{334, []string{"VXNlcm5hbWU6"}}
			case "PLAIN" == command.args:
				proto.logf("Got PLAIN authentication (no args), switching to AUTH2 state")
				proto.state = AUTH
				return &Reply{334, []string{}}
			case "CRAM-MD5" == command.args:
				proto.logf("Got CRAM-MD5 authentication, switching to AUTH state")
				proto.state = AUTH
				return &Reply{334, []string{"PDQxOTI5NDIzNDEuMTI4Mjg0NzJAc291cmNlZm91ci5hbmRyZXcuY211LmVkdT4="}}
			case strings.HasPrefix(command.args, "EXTERNAL "):
				proto.logf("Got EXTERNAL authentication: %s", strings.TrimPrefix(command.args, "EXTERNAL "))
				return &Reply{235, []string{"Authentication successful"}}
			default:
				return &Reply{504, []string{"Unsupported authentication mechanism"}}
			}
		case "MAIL":
			proto.logf("Got MAIL command, switching to RCPT state")
			from, err := ParseMAIL(command.args)
			if err != nil {
				return ReplyError(err)
			}
			proto.message.From = from
			proto.state = RCPT
			return ReplySenderOk(from)
		case "HELO":
			return proto.HELO(command.args)
		case "EHLO":
			return proto.EHLO(command.args)
		default:
			proto.logf("Got unknown command for MAIL state: '%s'", command)
			return ReplyUnrecognisedCommand()
		}
	case RCPT == proto.state:
		switch command.verb {
		case "RCPT":
			proto.logf("Got RCPT command")
			to, err := ParseRCPT(command.args)
			if err != nil {
				return ReplyError(err)
			}
			proto.message.To = append(proto.message.To, to)
			proto.state = RCPT
			return ReplyRecipientOk(to)
		case "DATA":
			proto.logf("Got DATA command, switching to DATA state")
			proto.state = DATA
			return &Reply{354, []string{"End data with <CR><LF>.<CR><LF>"}}
		default:
			proto.logf("Got unknown command for RCPT state: '%s'", command)
			return ReplyUnrecognisedCommand()
		}
	default:
		return ReplyUnrecognisedCommand()
	}
}

// HELO creates a reply to a HELO command
func (proto *Protocol) HELO(args string) (reply *Reply) {
	proto.logf("Got HELO command, switching to MAIL state")
	proto.state = MAIL
	proto.message.Helo = args
	return &Reply{250, []string{"Hello " + args}}
}

// EHLO creates a reply to a EHLO command
func (proto *Protocol) EHLO(args string) (reply *Reply) {
	proto.logf("Got EHLO command, switching to MAIL state")
	proto.state = MAIL
	proto.message.Helo = args
	return &Reply{250, []string{"Hello " + args, "PIPELINING", "AUTH EXTERNAL CRAM-MD5 LOGIN PLAIN"}}
}

// ParseMAIL returns the forward-path from a MAIL command argument
func ParseMAIL(mail string) (string, error) {
	r := regexp.MustCompile("(?i:From):<([^>]+)>")
	match := r.FindStringSubmatch(mail)
	if len(match) != 2 {
		return "", errors.New("Invalid sender")
	}
	return match[1], nil
}

// ParseRCPT returns the return-path from a RCPT command argument
func ParseRCPT(rcpt string) (string, error) {
	r := regexp.MustCompile("(?i:To):<([^>]+)>")
	match := r.FindStringSubmatch(rcpt)
	if len(match) != 2 {
		return "", errors.New("Invalid recipient")
	}
	return match[1], nil
}