GitHub Copilot で遊ぶ

// WeekdayJp is a type that represents the days of the week in the Japanese context.
// It is based on the time.Weekday type from the standard library.
type WeekdayJp time.Weekday

const (
    Sunday    WeekdayJp = WeekdayJp(time.Sunday) + iota // 日曜日
    Monday                                              // 月曜日
    Tuesday                                             // 火曜日
    Wednesday                                           // 水曜日
    Thursday                                            // 木曜日
    Friday                                              // 金曜日
    Saturday                                            // 土曜日
)

var weekdayNames = [7]string{"日曜日", "月曜日", "火曜日", "水曜日", "木曜日", "金曜日", "土曜日"}
var weekdayShortNames = [7]string{"日", "月", "火", "水", "木", "金", "土"}

// String returns the English name of the Japanese weekday (WeekdayJp)
// by converting it to the standard time.Weekday type and calling its String method.
func (w WeekdayJp) String() string {
    return time.Weekday(w).String()
}

// StringJp returns the Japanese name of the WeekdayJp if it is between Sunday and Saturday.
// If the WeekdayJp is out of this range, it returns the standard time.Weekday string representation.
func (w WeekdayJp) StringJp() string {
    if w < Sunday || w > Saturday {
        return time.Weekday(w).String()
    }
    return weekdayNames[w]
}

// ShortStringJp returns the short Japanese name of the WeekdayJp.
// If the WeekdayJp is not within the valid range (Sunday to Saturday),
// it returns the default string representation of the time.Weekday.
func (w WeekdayJp) ShortStringJp() string {
    if w < Sunday || w > Saturday {
        return time.Weekday(w).String()
    }
    return weekdayShortNames[w]
}