Demonstrates Golang formatting and parsing of ISO 8601 weeks

iso8601_week.go

// This snippet demonstrates Golang formatting and parsing of ISO 8601 weeks.

import (
	"errors"
	"fmt"
	"regexp"
	"strconv"
	"time"
)

const (
	MinWeek = 1
	MinYear = 1
	MaxYear = 9999
)

// ErrYearRange is returned when a week is not within our permitted range.
var ErrYearRange = fmt.Errorf("year is out of range (valid range: %d–%d inclusive)", MinYear, MaxYear)

// ErrWeekRange is returned when a week is not within our permitted range.
var ErrWeekRange = fmt.Errorf("week is out of range (valid range: %d–number of iso weeks in the given year inclusive)", MinWeek)

// FormatWeek returns an ISO 8601 week string.
func FormatWeek(date time.Time, shortForm bool) string {
	year, week := date.ISOWeek()
	if shortForm {
		return fmt.Sprintf("%d-W%02d", year, week)
	}

	//date.Weekday() returns a Sunday-started week, with Sunday=0.
	// have to adjust to ISO Monday-started week, with Monday=1.
	dow := ((7 + date.Weekday() - 1) % 7) + 1

	return fmt.Sprintf("%d-W%02d-%d", year, week, dow)
}

func calcP(y int) int {
	return y + (y / 4) - (y / 100) + (y / 400)
}

// ISOYearWeeks returns the number of ISO weeks contained
// in a given Gregorian calendar year.
func ISOYearWeeks(gregYear int) int {
	if (calcP(gregYear)%7 == 4) ||
		(calcP(gregYear-1)%7 == 3) {
		return 53
	}
	return 52
}

// ParseWeek parses an ISO 8601 string representing an ISO week,
// and returns the resultant golang time.Time instance.
// Note: if the ISO week is of the short form (doesn't include day of week),
// this function will return a time.Time instance with day of week of Monday.
func ParseWeek(isoWeek string) (time.Time, error) {
	re := regexp.MustCompile(`^(\d{4})-W([0-5]\d)(?:-([1-7]))?$`)
	matches := re.FindStringSubmatch(isoWeek)
	if matches == nil {
		return time.Time{}, errors.New("isoWeek string is of incorrect format")
	}

	year, err := strconv.Atoi(matches[1])
	if err != nil {
		return time.Time{}, err
	}
	if year < MinYear || year > MaxYear {
		return time.Time{}, ErrYearRange
	}

	week, err := strconv.Atoi(matches[2])
	if week < MinWeek || week > ISOYearWeeks(year) {
		return time.Time{}, ErrWeekRange
	}
	if err != nil {
		return time.Time{}, err
	}

	week--
	daysToAdd := week * 7

	if matches[3] != "" {
		day, err := strconv.Atoi(matches[3])
		if err != nil {
			return time.Time{}, err
		}

		daysToAdd += day - 1
	}

	daysToAdd -= Weekday(year, 1, 4)

	return time.Date(year, time.January, 4+daysToAdd, 0, 0, 0, 0, time.UTC), nil
}

// Weekday returns day of week with Monday=0...Sunday=6.
// Utilizes Zeller's Congruence.  see: https://en.wikipedia.org/wiki/Zeller%27s_congruence
func Weekday(year, month, day int) int {
	if month == 1 {
		month = 13
		year--
	} else if month == 2 {
		month = 14
		year--
	}

	dow := (day + (13 * (month + 1) / 5) + year +
		(year / 4) - (year / 100) +
		(year / 400)) % 7

	return (7 + (dow - 2)) % 7
}