【Swift】日付から干支と12星座を識別するプログラムの作り方と計算方法！

2016 ÷ 12 = 168 ... 0
2017 ÷ 12 = 168 ... 1
2022 ÷ 12 = 168 ... 6

func zodiac(_ date:Date) -> String {
  let df = DateFormatter()
  df.dateFormat = "yyyy/MM/dd"
  df.locale = Locale(identifier: "ja_JP")
  let nowYear = df.string(from: date).prefix(4)
  guard Int(nowYear) != nil else {
      // 文字列の場合
      return "..."
  }
  let num = Int(nowYear)! % 12
    switch num {
        case 4:
            return "ねずみ"
        case 5:
            return "うし"
        case 6:
            return "とら"
        case 7:
            return "うさぎ"
        case 8:
            return "たつ"
        case 9:
            return "へび"
        case 10:
            return "うま"
        case 11:
            return "ひつじ"
        case 0:
            return "さる"
        case 1:
            return "とり"
        case 2:
            return "いぬ"
        case 3:
            return "いのしし"
        default:
            return "..."
    }
}

func signOfZodiac(_ date:Date) -> String {
    let df = DateFormatter()
    df.dateFormat = "yyyy/MM/dd"
    df.locale = Locale(identifier: "ja_JP")
  

    let thisYear = df.string(from: date).prefix(4)
    let nowYear = "\(thisYear)/" // "2023/" ←スラッシュまで抜き出す
    let lateYear = "\(Int(thisYear)! + 1)/" // "2024/" 翌年形式
    
    switch date {
    case df.date(from: String(nowYear + "3/21"))!...df.date(from: String(nowYear + "4/20"))!:
        return "おひつじ座"
    case df.date(from: String(nowYear + "4/20"))!...df.date(from: String(nowYear + "5/21"))!:
        return "おうし座"
    case df.date(from: String(nowYear + "5/21"))!...df.date(from: String(nowYear + "6/22"))!:
        return "ふたご座"
    case df.date(from: String(nowYear + "6/22"))!...df.date(from: String(nowYear + "7/23"))!:
        return "かに座"
    case df.date(from: String(nowYear + "7/23"))!...df.date(from: String(nowYear + "8/23"))!:
        return "しし座"
    case df.date(from: String(nowYear + "8/23"))!...df.date(from: String(nowYear + "9/23"))!:
        return "おとめ座"
    case df.date(from: String(nowYear + "9/23"))!...df.date(from: String(nowYear + "10/24"))!:
        return "てんびん座"
    case df.date(from: String(nowYear + "10/24"))!...df.date(from: String(nowYear + "11/23"))!:
        return "さそり座"
    case df.date(from: String(nowYear + "11/23"))!...df.date(from: String(nowYear + "12/22"))!:
        return "いて座"
    case df.date(from: String(nowYear + "12/22"))!...df.date(from: String(lateYear + "1/1"))!:
        return "やぎ座"
    case df.date(from: String(nowYear + "1/1"))!...df.date(from: String(nowYear + "1/20"))!:
        return "やぎ座"
    case df.date(from: String(nowYear + "1/20"))!...df.date(from: String(nowYear + "2/19"))!:
        return "みずがめ座"
    case df.date(from: String(nowYear + "2/19"))!...df.date(from: String(nowYear + "3/21"))!:
        return "うお座"
    default:
        return "..."
    }
}

Thread 1: Fatal error: Can't form Range with upperBound < lowerBound