/*
SGBlog -- Git-backed CGI/UCSPI blogging/phlogging/gemlogging engine
Copyright (C) 2020-2023 Sergey Matveev <stargrave@stargrave.org>

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as
published by the Free Software Foundation, version 3 of the License.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU Affero General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

package main

import (
	"bytes"
	"encoding/base64"
	"errors"
	"fmt"
	"io"
	"mime"
	"mime/multipart"
	"mime/quotedprintable"
	"net/mail"
	"strings"
)

const (
	CT  = "Content-Type"
	CTE = "Content-Transfer-Encoding"
	TP  = "text/plain"
)

func processTP(ct, cte string, body io.Reader) (io.Reader, error) {
	_, params, err := mime.ParseMediaType(ct)
	if err != nil {
		return nil, err
	}
	if c := strings.ToLower(params["charset"]); !(c == "" ||
		c == "utf-8" ||
		c == "iso-8859-1" ||
		c == "us-ascii") {
		return nil, errors.New("only utf-8/iso-8859-1/us-ascii charsets supported")
	}
	switch cte {
	case "quoted-printable":
		return quotedprintable.NewReader(body), nil
	case "base64":
		return base64.NewDecoder(base64.StdEncoding, body), nil
	}
	return body, nil
}

func parseEmail(msg *mail.Message) (subj string, body io.Reader, err error) {
	subj = msg.Header.Get("Subject")
	if subj == "" {
		err = errors.New("no Subject")
		return
	}
	subj, err = new(mime.WordDecoder).DecodeHeader(subj)
	if err != nil {
		return
	}
	ct := msg.Header.Get(CT)
	if ct == "" {
		ct = "text/plain"
	}
	if strings.HasPrefix(ct, TP) {
		body, err = processTP(ct, msg.Header.Get(CTE), msg.Body)
		return
	}
	ct, params, err := mime.ParseMediaType(ct)
	if err != nil {
		err = fmt.Errorf("can not ParseMediaType: %w", err)
		return
	}
	if ct != "multipart/signed" {
		err = errors.New("only text/plain and multipart/signed+text/plain Content-Type supported")
		return
	}
	boundary := params["boundary"]
	if len(boundary) == 0 {
		err = errors.New("no boundary string")
		return
	}
	data, err := io.ReadAll(msg.Body)
	if err != nil {
		return
	}
	boundaryIdx := bytes.Index(data, []byte("--"+boundary))
	if boundaryIdx == -1 {
		err = errors.New("no boundary found")
		return
	}
	mpr := multipart.NewReader(bytes.NewReader(data[boundaryIdx:]), boundary)
	var part *multipart.Part
	for {
		part, err = mpr.NextPart()
		if err != nil {
			if err == io.EOF {
				break
			}
			return
		}
		ct = part.Header.Get(CT)
		if strings.HasPrefix(ct, TP) {
			body, err = processTP(ct, part.Header.Get(CTE), part)
			return
		}
		if strings.HasPrefix(ct, "multipart/mixed") {
			_, params, err = mime.ParseMediaType(ct)
			if err != nil {
				err = fmt.Errorf("can not ParseMediaType: %w", err)
				return
			}
			boundary = params["boundary"]
			if len(boundary) == 0 {
				err = errors.New("no boundary string")
				return
			}
			mpr := multipart.NewReader(part, boundary)
			for {
				part, err = mpr.NextPart()
				if err != nil {
					if err == io.EOF {
						break
					}
					return
				}
				ct = part.Header.Get(CT)
				if strings.HasPrefix(ct, TP) {
					body, err = processTP(ct, part.Header.Get(CTE), part)
					return
				}
			}
		}
	}
	err = errors.New("no text/plain part found")
	return
}