package mailer

import (
	"bytes"
	"crypto/rand"
	"encoding/hex"
	"errors"
	"fmt"
	"mime"
	"mime/quotedprintable"
	"net/mail"
	"strings"
	"time"
)

const (
	senderDisplayName = "RenewCMS"
	messageIDBytes    = 16
	lineBreak         = "\r\n"
)

type message struct {
	from    mail.Address
	to      mail.Address
	subject string
	body    string
	sentAt  time.Time
}

func newMessage(sender string, receiver string, subject string, body string, sentAt time.Time) (message, error) {
	from, err := mail.ParseAddress(sender)
	if err != nil {
		return message{}, fmt.Errorf("invalid sender address: %w", err)
	}
	to, err := mail.ParseAddress(receiver)
	if err != nil {
		return message{}, fmt.Errorf("invalid receiver address: %w", err)
	}
	if strings.ContainsAny(subject, "\r\n") {
		return message{}, errors.New("subject contains a line break")
	}
	return message{
		from:    mail.Address{Name: senderDisplayName, Address: from.Address},
		to:      *to,
		subject: subject,
		body:    body,
		sentAt:  sentAt,
	}, nil
}

func (m message) messageID() (string, error) {
	_, domain, _ := strings.Cut(m.from.Address, "@")
	random := make([]byte, messageIDBytes)
	if _, err := rand.Read(random); err != nil {
		return "", err
	}
	return "<" + hex.EncodeToString(random) + "@" + domain + ">", nil
}

func (m message) bytes() ([]byte, error) {
	id, err := m.messageID()
	if err != nil {
		return nil, err
	}

	var out bytes.Buffer
	headers := []struct{ name, value string }{
		{"From", m.from.String()},
		{"To", m.to.String()},
		{"Subject", mime.QEncoding.Encode("utf-8", m.subject)},
		{"Date", m.sentAt.Format(time.RFC1123Z)},
		{"Message-ID", id},
		{"MIME-Version", "1.0"},
		{"Content-Type", "text/html; charset=UTF-8"},
		{"Content-Transfer-Encoding", "quoted-printable"},
	}
	for _, h := range headers {
		out.WriteString(h.name + ": " + h.value + lineBreak)
	}
	out.WriteString(lineBreak)

	encoder := quotedprintable.NewWriter(&out)
	if _, err := encoder.Write([]byte(m.body)); err != nil {
		return nil, err
	}
	if err := encoder.Close(); err != nil {
		return nil, err
	}
	return out.Bytes(), nil
}