package trysmtp

import (
	"crypto/tls"
	"fmt"
	"net"
	"net/smtp"
	"strings"
)

// SMTPAddrs priority list
var SMTPAddrs = []string{":25", ":587", ":465"}

// Connect to SMTP server and call MAIL and RCPT commands
// NOTE: check if client is not nil, because it can return non-fatal errors with initialized client
func Connect(from, to string) (*smtp.Client, error) {
	localname := strings.SplitN(from, "@", 2)[1]
	hostname := strings.SplitN(to, "@", 2)[1]
	client, cerr := initClient(localname, hostname)
	if client == nil {
		return nil, cerr
	}

	err := client.Mail(from)
	if err != nil {
		client.Close()
		return nil, err
	}

	err = client.Rcpt(to)
	if err != nil {
		client.Close()
		return nil, err
	}

	return client, cerr
}

func unwrapErrors(errs []error) error {
	if len(errs) == 0 {
		return nil
	}

	var text strings.Builder
	last := len(errs) - 1
	for i := 0; i < len(errs); i++ {
		text.WriteString(errs[i].Error())
		if i < last {
			text.WriteString("; ")
		}
	}

	return fmt.Errorf(text.String())
}

func initClient(localname, hostname string) (*smtp.Client, error) {
	mxs, err := net.LookupMX(hostname)
	if err != nil {
		return nil, err
	}

	cerrs := []error{}
	var client *smtp.Client
	for _, mx := range mxs {
		if mx.Host == "." {
			continue // no records case
		}
		for _, addr := range SMTPAddrs {
			client, err = trySMTP(localname, strings.TrimSuffix(mx.Host, "."), addr)
			if err != nil {
				cerrs = append(cerrs, err)
			}
			if client != nil {
				return client, unwrapErrors(cerrs)
			}
		}
	}

	// If there are no MX records, according to https://datatracker.ietf.org/doc/html/rfc5321#section-5.1,
	// we're supposed to try talking directly to the host.
	for _, addr := range SMTPAddrs {
		client, err = trySMTP(localname, hostname, addr)
		if err != nil {
			cerrs = append(cerrs, err)
		}
		if client != nil {
			return client, unwrapErrors(cerrs)
		}
	}

	return nil, unwrapErrors(cerrs)
}

func trySMTP(localname, mxhost, addr string) (*smtp.Client, error) {
	target := mxhost + addr
	conn, err := smtp.Dial(target)
	if err != nil {
		return nil, fmt.Errorf("%s: %w", target, err)
	}
	err = conn.Hello(localname)
	if err != nil {
		return nil, fmt.Errorf("%s: %w", target, err)
	}
	if ok, _ := conn.Extension("STARTTLS"); ok {
		config := &tls.Config{ServerName: mxhost}
		conn.StartTLS(config) //nolint:errcheck // if it doesn't work - we can't do anything anyway
	}

	return conn, nil
}