// Copyright (c) 2020 Proton Technologies AG
//
// This file is part of ProtonMail Bridge.
//
// ProtonMail Bridge is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// ProtonMail Bridge 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 General Public License
// along with ProtonMail Bridge.  If not, see <https://www.gnu.org/licenses/>.

package transfer

import (
	"bufio"
	"bytes"
	"io"
	"mime"
	"os"
	"strings"
)

type stringSet map[string]bool

const xGmailLabelsHeader = "X-Gmail-Labels"

// filteredOutGmailLabels is set of labels which we don't want to show to users
// as they might be auto-generated by Gmail and unwanted.
var filteredOutGmailLabels = []string{ //nolint[gochecknoglobals]
	"Unread",
	"Opened",
	"IMAP_Junk",
	"IMAP_NonJunk",
	"IMAP_NotJunk",
	"IMAP_$NotJunk",
}

func getGmailLabelsFromMboxFile(filePath string) (stringSet, error) {
	f, err := os.Open(filePath) //nolint[gosec]
	if err != nil {
		return nil, err
	}
	return getGmailLabelsFromMboxReader(f)
}

func getGmailLabelsFromMboxReader(f io.Reader) (stringSet, error) {
	allLabels := stringSet{}

	// Scanner is not used as it does not support long lines and some mbox
	// files contain very long lines even though that should not be happening.
	r := bufio.NewReader(f)
	for {
		b, isPrefix, err := r.ReadLine()
		if err == io.EOF {
			break
		}
		if err != nil {
			return nil, err
		}
		for isPrefix {
			_, isPrefix, err = r.ReadLine()
			if err != nil {
				break
			}
		}
		if bytes.HasPrefix(b, []byte(xGmailLabelsHeader)) {
			for label := range getGmailLabelsFromValue(string(b)) {
				allLabels[label] = true
			}
		}
	}

	return allLabels, nil
}

func getGmailLabelsFromMessage(body []byte) (stringSet, error) {
	header, err := getMessageHeader(body)
	if err != nil {
		return nil, err
	}
	labels := header.Get(xGmailLabelsHeader)
	return getGmailLabelsFromValue(labels), nil
}

func getGmailLabelsFromValue(value string) stringSet {
	value = strings.TrimPrefix(value, xGmailLabelsHeader+":")
	if decoded, err := new(mime.WordDecoder).DecodeHeader(value); err != nil {
		log.WithError(err).Error("Failed to decode header")
	} else {
		value = decoded
	}

	labels := stringSet{}
	for _, label := range strings.Split(value, ",") {
		label = strings.TrimSpace(label)
		if label == "" {
			continue
		}
		skip := false
		for _, filteredOutLabel := range filteredOutGmailLabels {
			if label == filteredOutLabel {
				skip = true
				break
			}
		}
		if skip {
			continue
		}
		labels[label] = true
	}
	return labels
}