package imap

import (
	"bufio"
	"crypto/tls"
	"fmt"
	"log"
	"net/mail"
	"strconv"
	"strings"
	"time"
)

const (
	tag      = "<<unreadMail>> "
	other    = "* "
	maxRead  = 11000000
	timeout  = 3000 // ms
	taggedOK = tag + "OK"
	otherOK  = other + "OK"
	search   = other + "SEARCH"
)

type Error string

func (err Error) Error() string {
	return string(err)
}

type connection struct {
	*tls.Conn
	user        string
	passwd      string
	unreadMails []*mail.Message
	err         []error
}

type stateFunc func(*connection) stateFunc

func expect(response string, tok string) (string, bool) {
	lines := strings.Split(response, "\n")
	for _, l := range lines {
		line := strings.TrimSpace(l)
		if strings.HasPrefix(line, tok) {
			return strings.TrimSpace(strings.TrimPrefix(line, tok)), true
		}
	}
	return "", false
}

func parseMail(response string) (*mail.Message, error) {
	reader := bufio.NewReader(strings.NewReader(response))
	for {
		line, _ := reader.ReadBytes('\n')
		if !strings.HasPrefix(string(line), other) {
			break
		}
	}
	return mail.ReadMessage(reader)
}

/*-------------- states ----------*/

func connected(c *connection) stateFunc {
	_ = c.read("server hello")
	if c.err != nil {
		return nil
	} else {
		return login
	}
}

func login(c *connection) stateFunc {
	c.write(cmd(fmt.Sprintf("login %s %s", c.user, c.passwd)))
	if c.err != nil {
		return logout
	}
	_ = c.read("login response")
	if c.err != nil {
		return logout
	} else {
		return selectBox
	}
}

func selectBox(c *connection) stateFunc {
	c.write(cmd("select Inbox"))
	if c.err != nil {
		return logout
	}
	_ = c.read("select response")
	if c.err != nil {
		return logout
	} else {
		return searchUnseen
	}
}

func searchUnseen(c *connection) stateFunc {
	c.write(cmd("uid search unseen"))
	if c.err != nil {
		return logout
	}
	s := c.read("search response")
	if c.err != nil {
		return logout
	} else {
		if r, ok := expect(s, search); ok {
			uids := UIDsFromResponse(r)
			return func(c *connection) stateFunc { return fetch(c, uids) }
		} else {
			return logout
		}
	}
}

func fetch(c *connection, ids []int) stateFunc {
	nErrs := 0
	for _, id := range ids {
		c.write(cmd(fmt.Sprintf("uid fetch %d body[]", id)))
		if len(c.err) == nErrs {
			s := c.read("uid fetch of " + strconv.Itoa(id))
			if len(c.err) == nErrs {
				s = strings.TrimSuffix(strings.TrimSpace(s), ")")
				mail, err := parseMail(s)
				if err == nil {
					c.unreadMails = append(c.unreadMails, mail)
				} else {
					c.err = append(c.err, err)
				}
			}
		}
		nErrs = len(c.err)
	}
	return logout
}

func logout(c *connection) stateFunc {
	c.write(cmd("logout"))
	c.read("logout response")
	c.Close()
	return nil
}

/*---------- End of states ----------*/

func UIDsFromResponse(response string) []int {
	ids := make([]int, 0)
	if response == "" {
		return ids
	}
	list := strings.Split(response, " ")
	for _, id := range list {
		val, err := strconv.Atoi(id)
		if err != nil {
			log.Println("Could not convert uid '" + id + "'")
		} else {
			ids = append(ids, val)
		}
	}
	return ids
}

func cmd(s string) string {
	return tag + s + "\r\n"
}

func (c *connection) write(s string) {
	c.Conn.SetWriteDeadline(time.Now().Add(timeout * time.Millisecond))
	bytes := []byte(s)
	n, err := c.Conn.Write(bytes)
	var desc string
	if strings.Contains(s, "login") {
		// would not want to print login credentials in case of error
		desc = "login"
	} else {
		desc = s
	}
	if n != len(bytes) || err != nil {
		c.err = append(c.err, Error("Could not write "+desc))
	}
}

func (c *connection) read(desc string) string {
	buf := make([]byte, 0)
	reader := bufio.NewReader(c.Conn)
	var err error
	var conclusion string
	if desc == "server hello" {
		conclusion = other
	} else {
		conclusion = tag
	}
	for {
		var line []byte
		c.SetReadDeadline(time.Now().Add(timeout * time.Millisecond))
		line, tmpErr := reader.ReadBytes('\n')
		if tmpErr == nil && len(buf)+len(line) <= maxRead {
			if l := string(line); strings.HasPrefix(l, conclusion) {
				if strings.HasPrefix(l, conclusion+"OK") {
					break
				} else {
					buf = nil
					err = Error("Did not receive OK in " + desc + ", got: " + l)
					break
				}
			}
			buf = append(buf, line...)
		} else {
			errMsg := "Error while reading " + desc
			if tmpErr != nil {
				errMsg += "\n" + tmpErr.Error()
			}
			err = Error(errMsg)
			break
		}
	}
	s := string(buf)
	if err != nil {
		c.err = append(c.err, err)
		return ""
	} else {
		return s
	}
}

func Fetch(c *tls.Conn, user string, passwd string) ([]*mail.Message, []error) {
	state := connected
	conn := &connection{Conn: c, user: user, passwd: passwd}
	for state != nil {
		state = state(conn)
	}
	return conn.unreadMails, conn.err
}