serial.go

package main

import (
	"strconv"
	"time"
)

const Year5 = time.Duration(24*time.Hour*365) * 5
const Year15 = time.Duration(24*time.Hour*365) * 15

func isEpoch(i int64) bool {
	now := time.Now()
	t := time.Unix(i, 0)
	if now.Sub(t) > Year15 {
		return false
	}
	if t.Sub(now) > Year5 {
		return false
	}
	return true
}

func Increase(s []byte) []byte {
	i, err := strconv.ParseInt(string(s), 10, 64)
	if err != nil {
		return s
	}
	if isEpoch(i) { // return current epoch
		e := time.Now().Unix()
		return []byte(strconv.FormatInt(e, 10))
	}
	// otherwise just increase? TODO(miek): smarter later
	i++
	return []byte(strconv.FormatInt(i, 10))
}

// SerialToHuman will detect if a number is epoch, or a coded date, ie:
//
// 1712989081 is epoch, because, when converted is less than 15 years ago, and not more than
// 5 years in the future.
//
// If not epoch, we assume a "date" format: 2024041300. Every sequence number 00, 01, is
// assumed to be an hour.
//
// Both are converted to a more human readable string.
func SerialToHuman(s []byte) string {
	i, err := strconv.ParseInt(string(s), 10, 64)
	if err != nil {
		return "  " + dateToHuman(s)
	}
	if !isEpoch(i) {
		return "  " + dateToHuman(s)
	}
	return "  " + time.Unix(i, 0).UTC().Format(time.RFC1123)
}

func dateToHuman(s []byte) string {
	if len(s) != 10 { // e.g. 2024041300
		return ""
	}
	year, _ := strconv.ParseInt(string(s[:4]), 10, 64)
	mon, _ := strconv.ParseInt(string(s[4:6]), 10, 64)
	day, _ := strconv.ParseInt(string(s[6:8]), 10, 64)
	sequence, _ := strconv.ParseInt(string(s[8:10]), 10, 64)
	// sequence is considered the percentage the day has aged.
	// calculate total minutes and round to hour and remaining minutes
	minutes := 1440 / 100 * sequence
	hour := minutes / 60
	minutes -= hour * 60

	t := time.Date(int(year), time.Month(mon), int(day), int(hour), int(minutes), 0, 0, time.UTC)
	return t.Format(time.RFC1123)
}