exchange.swift

//: Playground - noun: a place where people can play

import Cocoa
import CoreFoundation
//import PlaygroundSupport
//PlaygroundPage.current.needsIndefiniteExecution = true


class CurrencyConverter {
    
    private let requester = FixerRequester()
    private var rateDict = [String: Double]()
    
    func getRateAsync(fromType: String, toType: String, date: Date? = nil, callBack: @escaping (Double?) -> ()) {
        requester.forData(date: date, base: fromType, symbols: [toType]) {
            data in
            guard let dict = data as? [String: Any], let rates = dict["rates"] as? [String: Double] else {
                return
            }
            guard let toRate = rates[toType.uppercased()] else {
                callBack(nil)
                return
            }
            callBack(toRate)
        }
    }
    
    func getRate(fromType: String, toType: String, date: Date? = nil) -> Double? {
        let semaphore = DispatchSemaphore(value: 0)
        var r: Double? = nil
        getRateAsync(fromType: fromType, toType: toType, date: date) {
            rate in
            r = rate
            semaphore.signal()
        }
        semaphore.wait(timeout: DispatchTime.distantFuture)
        return r
    }
    
    private func _convert(fromType: String, toType: String, amount: Double, date: Date? = nil) -> Double? {
        if let rate = getRate(fromType: fromType, toType: toType, date: date) {
            return amount * rate
        } else {
            return nil
        }
    }
    
    func convert(fromType: String, toType: String, amount: Double, date: Date? = nil) -> Double? {
        // memorize
        let dateFormatter = DateFormatter()
        dateFormatter.dateFormat = "yyyy-MM-dd"
        if let rate = rateDict[[fromType, toType, dateFormatter.string(from: date ?? Date())].joined(separator: "-")] {
            return rate
        } else {
            if let rate = _convert(fromType: fromType, toType: toType, amount: amount, date: date) {
                rateDict[[fromType, toType, dateFormatter.string(from: date ?? Date())].joined(separator: "-")] = amount * rate
                return amount * rate
            } else {
                return nil
            }
        }
    }
}

class FixerRequester {
    
    private var baseURLCompoents = URLComponents()
    
    init() {
        baseURLCompoents.scheme = "https"
        baseURLCompoents.host = "api.fixer.io"
    }
    
    func forData(date: Date? = nil, base: String? = nil, symbols: [String]? = nil, completion: @escaping (Any) -> ()) {
        
        var queryItems = [URLQueryItem]()
        
        
        if let d = date {
            let dateFormatter = DateFormatter()
            dateFormatter.dateFormat = "yyyy-MM-dd"
            baseURLCompoents.path = "/\(dateFormatter.string(from: d))"
        } else {
            baseURLCompoents.path = "/latest"
        }
        
        if let b = base {
            queryItems.append(URLQueryItem(name: "base", value: b))
        }
        
        
        if let s = symbols {
            queryItems.append(URLQueryItem(name: "symbols", value: s.joined(separator: ",")))
        }
        
        
        
        baseURLCompoents.queryItems =  queryItems != [] ? queryItems : nil
        let task = URLSession.shared.dataTask(with: baseURLCompoents.url!) {
            data, response, error in
            let json = try! JSONSerialization.jsonObject(with: data!, options: [])
            completion(json)
        }
        task.resume()
        
    }
}

// usage

// latest rate
var c = CurrencyConverter()
print(
    c.convert(fromType: "USD", toType: "CNY", amount: 10)!
)

// rate by date
let formatter = DateFormatter()
formatter.dateFormat = "yyyy MM dd"
let date = formatter.date(from: "2010 2 13")

print(
    c.convert(fromType: "USD", toType: "CNY", amount: 10, date: date)!
)