package checker

import (
	"bufio"
	"fmt"
	"net"
	"strings"
)

// EHLOHostname is the hostname sent in the SMTP EHLO command during STARTTLS
// negotiation. Override it at startup (e.g. via -ldflags or programmatically)
// to match the identity of the host running the checker.
var EHLOHostname = "checker.localhost"

func init() {
	registerStartTLS("smtp", starttlsSMTP)
	registerStartTLS("submission", starttlsSMTP)
}

// starttlsSMTP implements ESMTP EHLO + STARTTLS (RFC 3207).
func starttlsSMTP(conn net.Conn, sni string) error {
	rw := bufio.NewReadWriter(bufio.NewReader(conn), bufio.NewWriter(conn))

	if err := readSMTPGreeting(rw.Reader); err != nil {
		return fmt.Errorf("read greeting: %w", err)
	}

	if _, err := fmt.Fprintf(rw, "EHLO %s\r\n", EHLOHostname); err != nil {
		return fmt.Errorf("write ehlo: %w", err)
	}
	if err := rw.Flush(); err != nil {
		return fmt.Errorf("flush ehlo: %w", err)
	}
	lines, err := readSMTPResponse(rw.Reader)
	if err != nil {
		return fmt.Errorf("read ehlo: %w", err)
	}
	if !hasSTARTTLSExt(lines) {
		return fmt.Errorf("%w: EHLO did not advertise STARTTLS", errStartTLSNotOffered)
	}

	if _, err := rw.WriteString("STARTTLS\r\n"); err != nil {
		return fmt.Errorf("write starttls: %w", err)
	}
	if err := rw.Flush(); err != nil {
		return fmt.Errorf("flush starttls: %w", err)
	}
	resp, err := readSMTPResponse(rw.Reader)
	if err != nil {
		return fmt.Errorf("read starttls: %w", err)
	}
	if len(resp) == 0 || !strings.HasPrefix(resp[0], "220") {
		return fmt.Errorf("server refused STARTTLS: %s", strings.Join(resp, " / "))
	}
	return nil
}

func readSMTPGreeting(r *bufio.Reader) error {
	_, err := readSMTPResponse(r)
	return err
}

// readSMTPResponse reads one multi-line SMTP response (lines with "NNN-" are
// continuation, "NNN " terminates).
func readSMTPResponse(r *bufio.Reader) ([]string, error) {
	var out []string
	for {
		line, err := readLineLimited(r)
		if err != nil {
			return out, err
		}
		line = strings.TrimRight(line, "\r\n")
		out = append(out, line)
		if len(line) < 4 || line[3] == ' ' {
			return out, nil
		}
	}
}

func hasSTARTTLSExt(lines []string) bool {
	for _, l := range lines {
		if len(l) < 4 {
			continue
		}
		rest := strings.ToUpper(strings.TrimSpace(l[4:]))
		if rest == "STARTTLS" || strings.HasPrefix(rest, "STARTTLS ") {
			return true
		}
	}
	return false
}