print all years in specified era, era number = 1 is Taika, era number = 235 is Heisei

erayears.swift

import Foundation

let japanese = Calendar(identifier: .japanese)
let gregorian = Calendar(identifier: .gregorian)

let locale = Locale(identifier: "ja")
let format = DateFormatter()
format.calendar = japanese
format.dateFormat = "Gy年"
format.locale = locale

///////////////////////////////////////////////
// swiftc -o erayears erayears.swift
// ./erayears (235)
// ./erayears eraNumber
// print all years in specified era, era number = 1 is Taika, era number = 235 is Heisei
///////////////////////////////////////////////


func startDate(forEra era:Int) -> Date {
    let flags = Set<Calendar.Component>([Calendar.Component.era, Calendar.Component.year, Calendar.Component.month, Calendar.Component.day])
    let japanese = Calendar(identifier: Calendar.Identifier.japanese)
    var year = 1, month = 1, day = 1
    var dateComponents = DateComponents(calendar: japanese,
                                    era: era,
                                    year: year,
                                    month: month,
                                    day: day)
    var date = japanese.date(from: dateComponents)!
    dateComponents = japanese.dateComponents(flags, from: date)
    if ( dateComponents.era! != era ) {
        while dateComponents.era! < era {
            month += 1
            dateComponents = DateComponents(calendar: japanese,
                                                era: era,
                                                year: year,
                                                month: month,
                                                day: day)
            date = japanese.date(from: dateComponents)!
            dateComponents = japanese.dateComponents(flags, from: date)
        }
        month -= 1
        dateComponents = DateComponents(calendar: japanese,
                                        era: era,
                                        year: year,
                                        month: month,
                                        day: day)
        date = japanese.date(from: dateComponents)!
        dateComponents = japanese.dateComponents(flags, from: date)
        while dateComponents.era! < era {
            day += 1
            dateComponents = DateComponents(calendar: japanese,
                                            era: era,
                                            year: year,
                                            month: month,
                                            day: day)
            date = japanese.date(from: dateComponents)!
            dateComponents = japanese.dateComponents(flags, from: date)
        }
    }
    return date
}

func printEraYears(era:Int) {
    format.dateStyle = .long
    format.timeStyle = .none
    let date = startDate(forEra: era)
    let gYear = gregorian.component(.year, from: date)
    print(format.string(from: date)+"(\(gYear)年)")
    var year = 2
    repeat {
        let comp = DateComponents(calendar: japanese, era: era, year: year, month: 1, day: 1)
        if let date = japanese.date(from: comp) {
            let flags = Set<Calendar.Component>([.era])
            let dateComps = japanese.dateComponents(flags, from: date)
            if dateComps.era != era {
                return
            }
            let gYear = gregorian.component(.year, from: date)
            print(format.string(from: date)+"(\(gYear)年)")
        } else {
            return
        }
        year += 1
    } while year <= 100
}

///////////////////////////////////////////////

    var era = 235
    
    let argv = ProcessInfo.processInfo.arguments

//    print("count = \(argv.count)")

    if argv.count > 1 {
        let value = Int(argv[1])!
        if value < 235 && value > 0 {
            era = value
        }
    }

    printEraYears(era: era)