// godlighty -- highly-customizable HTTP, HTTP/2, HTTPS server
// Copyright (C) 2021-2024 Sergey Matveev <stargrave@stargrave.org>
//
// This program 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, 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 General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.

package godlighty

import (
	"encoding/base64"
	"encoding/binary"
	"os"
)

func mtimeETag(fd *os.File) (string, error) {
	fi, err := fd.Stat()
	if err != nil {
		return "", err
	}
	mtime := fi.ModTime()
	buf := make([]byte, 8*3)
	binary.BigEndian.PutUint64(buf[:8], uint64(fi.Size()))
	binary.BigEndian.PutUint64(buf[8:16], uint64(mtime.Unix()))
	binary.BigEndian.PutUint64(buf[16:], uint64(mtime.Nanosecond()))
	return `"` + base64.RawURLEncoding.EncodeToString(buf) + `"`, nil
}