package go_ntp

import (
	"fmt"
	"time"

	"github.com/beevik/ntp"
)

// GetDefaultServers returns the addresses of the four servers of the PTB (https://www.ptb.de/cms/)
func GetDefaultServers() []string {
	return []string{
		"ptbtime1.ptb.de",
		"ptbtime2.ptb.de",
		"ptbtime3.ptb.de",
		"ptbtime4.ptb.de",
	}
}

// GetCurrentTime returns either the current time of the first accessible NTP server
// or the current local time and an error
func GetCurrentTime(ntpServers ...string) (syncedTime time.Time, err error) {
	if len(ntpServers) == 0 {
		err = fmt.Errorf("no time servers defined")
		return
	}
	for _, ntpServer := range ntpServers {
		if syncedTime, err = ntp.Time(ntpServer); err == nil {
			break
		}
	}
	return
}

// GetLocalOffset returns either the difference between the current local time and the
// current time of the first accessible NTP server or an empty duration and an error
func GetLocalOffset(ntpServers ...string) (offset time.Duration, err error) {
	var syncedTime time.Time
	if syncedTime, err = GetCurrentTime(ntpServers...); err == nil {
		offset = time.Until(syncedTime)
	}
	return
}