print first year of the era include specified year

jera.swift

import Foundation

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

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

///////////////////////////////////////////////
// swiftc -o jera jera.swift
// ./jera (2001 -length 10)
// ./jera startYear (-length 10)
// ./jera startYear stopYear
// print first year of the era include specified year
///////////////////////////////////////////////

func era(year:Int) -> Int? {
    let comp = DateComponents(calendar: gregorian, era: 1, year: year, month: 1, day: 1)
    guard let date = comp.date else {
        return nil
    }
    let jComp = japanease.dateComponents([.era, .year], from: date)
    return jComp.era
}

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 printEras(startYear:Int, stopYear:Int) {
    guard let start = era(year: startYear) else {
        return
    }
    guard let stop = era(year: stopYear) else {
        return
    }
    format.dateStyle = .long
    format.timeStyle = .none
    for era in start...stop {
        let date = startDate(forEra: era)
        let string = format.string(from: date)
        let gYear = gregorian.component(.year, from: date)
        print("\(gYear)年(\(era))"+string)
    }
}

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

    var startYear = 2001
    var length = 10
    
    let argv = ProcessInfo.processInfo.arguments

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

    if argv.count > 1 {
        let value = Int(argv[1])!
        if value < 2100 && value > 645 {
            startYear = value
        }
    }

    if argv.count > 2 {
        if argv[2] == "-length" {
            if argv.count > 3 {
                if let value = Int(argv[3]) {
                    if value > 0 {
                        length = value
                    }
                }
            }
        } else {
	        let value = Int(argv[2])!
		if value < 2100 && value > 645 {
	            let stopYear = value
	            if stopYear > startYear {
	                length = stopYear - startYear
		    }
		}
        }
    }

    printEras(startYear: startYear, stopYear: startYear+length)