使用Go按周日到周六为一周，获取周数

因为有个业务需要算周别，且是按照周天到周六的算法，和常见的周一到周天，有些区别，所以ISOWeek函数就不好使了，找了下发现Go没有现成的，也没人实现过，所以就有了这篇文章。

推荐代码：

func weekStartEnd(year, week int) (time.Time, time.Time, error) {
	if week < 1 {
		return time.Time{}, time.Time{}, fmt.Errorf("week number must be greater than 0")
	}

	// 计算年份的1月1日
	firstDayOfYear := time.Date(year, time.January, 1, 0, 0, 0, 0, time.UTC)

	// 计算1月1日是周几 (0: 周日, 1: 周一, ..., 6: 周六)
	dayOfWeek := int(firstDayOfYear.Weekday())

	// 计算第一周的起始日期
	var daysToFirstWeek int
	if dayOfWeek == 0 {
		daysToFirstWeek = 1
	} else {
		daysToFirstWeek = 8 - dayOfWeek
	}
	firstWeekStart := firstDayOfYear.AddDate(0, 0, daysToFirstWeek-1)

	// 计算指定周的起始和结束日期
	weekStart := firstWeekStart.AddDate(0, 0, 7*(week-1))
	weekEnd := weekStart.AddDate(0, 0, 6)

	return weekStart, weekEnd, nil
}

func TestTime(t *testing.T) {
	year := 2022
	week := 2

	start, end, err := weekStartEnd(year, week)
	if err != nil {
		fmt.Println("Error:", err)
		return
	}

	fmt.Printf("Year %d, week %d starts on %s and ends on %s\n", year, week, start.Format("2006-01-02"), end.Format("2006-01-02"))
}

以前的实现，直接上代码吧：

type timex struct {
	time.Time
}

func newDate(year int, month time.Month, day int, hour int, min int, sec int, nsec int, loc *time.Location) timex {
	return timex{time.Date(year, month, day, hour, min, sec, nsec, loc)}
}

// ISO Week 周一到周天
func (tx timex) whichWeek() int {
	//tx := time.Now()

	// https://weeknumber.com/?q=02%2F01%2F2022
	
	// Thursday in current week decides the year.
	tm1 := time.Date(tx.Year(), tx.Month(), tx.Day()+3-(int(tx.Weekday())+6)%7, 0, 0, 0, 0, time.UTC)
	// January 4 is always in week 1.
	week1 := time.Date(tm1.Year(), 1, 4, 0, 0, 0, 0, time.UTC)
	// Adjust to Thursday in week 1 and count number of weeks from date to week1.
	abs := 1 + math.Round((float64((tm1.Unix()-week1.Unix())/86400)-float64(3)+float64((int(week1.Weekday())+6)%7))/7)
	return int(abs)
}

// 周天到周六
func (tx timex) whichWeek2() int {

	tm1 := time.Date(tx.Year(), tx.Month(), tx.Day()+4-int(tx.Weekday()), 0, 0, 0, 0, time.UTC)
	yearStart := time.Date(tm1.Year(), 1, 1, 0, 0, 0, 0, time.UTC)
	sd := math.Ceil((float64((tm1.Unix()-yearStart.Unix())/86400) + 1) / 7)

	return int(sd)
}

// 计算周开始的起始时间
func (tx timex) WeekStart(year, week int) time.Time {
	if year == 0 || week == 0 {
		year = tx.Year()
		week = int(tx.Weekday())
	}

	// Start from the middle of the year:
	t2 := newDate(year, 7, 1, 0, 0, 0, 0, time.UTC)

	// Roll back to Monday:
	if wd := t2.Weekday(); wd != time.Sunday {
		t2.Time = t2.AddDate(0, 0, -int(wd))
	}

	// Difference in weeks:
	w := t2.whichWeek2()
	t2.Time = t2.AddDate(0, 0, (week-w)*7)

	if wd := t2.Weekday(); wd != time.Sunday {
		t2.Time = t2.AddDate(0, 0, -6)
	}

	return t2.Time
}

// 计算ISO周别的起始时间
func WeekStart(year, week int) time.Time {
	// Start from the middle of the year:
	t := time.Date(year, 7, 1, 0, 0, 0, 0, time.UTC)

	// Roll back to Monday:
	if wd := t.Weekday(); wd == time.Sunday {
		t = t.AddDate(0, 0, -6)
	} else {
		t = t.AddDate(0, 0, -int(wd)+1)
	}

	// Difference in weeks:
	_, w := t.ISOWeek()
	t = t.AddDate(0, 0, (week-w)*7)

	if wd := t.Weekday(); wd != time.Sunday {
		t = t.AddDate(0, 0, -6)
	} else {
		t = t.AddDate(0, 0, -int(wd)+1)
	}

	return t
}

参考链接：

https://stackoverflow.com/questions/52300644/date-range-by-week-number-golang

https://stackoverflow.com/questions/45910292/get-week-number-with-week-starting-from-sunday