Initial commit

.gitignore

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+.DS_Store
+/.build
+/Packages
+/*.xcodeproj
+xcuserdata/
+DerivedData/
+.swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata

.swiftpm/xcode/package.xcworkspace/xcshareddata/IDEWorkspaceChecks.plist

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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>IDEDidComputeMac32BitWarning</key>
+	<true/>
+</dict>
+</plist>

.swiftpm/xcode/xcshareddata/xcschemes/currency-converter.xcscheme

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+<?xml version="1.0" encoding="UTF-8"?>
+<Scheme
+   LastUpgradeVersion = "1250"
+   version = "1.3">
+   <BuildAction
+      parallelizeBuildables = "YES"
+      buildImplicitDependencies = "YES">
+      <BuildActionEntries>
+         <BuildActionEntry
+            buildForTesting = "YES"
+            buildForRunning = "YES"
+            buildForProfiling = "YES"
+            buildForArchiving = "YES"
+            buildForAnalyzing = "YES">
+            <BuildableReference
+               BuildableIdentifier = "primary"
+               BlueprintIdentifier = "currency-converter"
+               BuildableName = "currency-converter"
+               BlueprintName = "currency-converter"
+               ReferencedContainer = "container:">
+            </BuildableReference>
+         </BuildActionEntry>
+         <BuildActionEntry
+            buildForTesting = "YES"
+            buildForRunning = "YES"
+            buildForProfiling = "NO"
+            buildForArchiving = "NO"
+            buildForAnalyzing = "YES">
+            <BuildableReference
+               BuildableIdentifier = "primary"
+               BlueprintIdentifier = "currency-converterTests"
+               BuildableName = "currency-converterTests"
+               BlueprintName = "currency-converterTests"
+               ReferencedContainer = "container:">
+            </BuildableReference>
+         </BuildActionEntry>
+      </BuildActionEntries>
+   </BuildAction>
+   <TestAction
+      buildConfiguration = "Debug"
+      selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
+      selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
+      shouldUseLaunchSchemeArgsEnv = "YES">
+      <Testables>
+         <TestableReference
+            skipped = "NO">
+            <BuildableReference
+               BuildableIdentifier = "primary"
+               BlueprintIdentifier = "currency-converterTests"
+               BuildableName = "currency-converterTests"
+               BlueprintName = "currency-converterTests"
+               ReferencedContainer = "container:">
+            </BuildableReference>
+         </TestableReference>
+      </Testables>
+   </TestAction>
+   <LaunchAction
+      buildConfiguration = "Debug"
+      selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
+      selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
+      launchStyle = "0"
+      useCustomWorkingDirectory = "NO"
+      ignoresPersistentStateOnLaunch = "NO"
+      debugDocumentVersioning = "YES"
+      debugServiceExtension = "internal"
+      allowLocationSimulation = "YES">
+   </LaunchAction>
+   <ProfileAction
+      buildConfiguration = "Release"
+      shouldUseLaunchSchemeArgsEnv = "YES"
+      savedToolIdentifier = ""
+      useCustomWorkingDirectory = "NO"
+      debugDocumentVersioning = "YES">
+      <MacroExpansion>
+         <BuildableReference
+            BuildableIdentifier = "primary"
+            BlueprintIdentifier = "currency-converter"
+            BuildableName = "currency-converter"
+            BlueprintName = "currency-converter"
+            ReferencedContainer = "container:">
+         </BuildableReference>
+      </MacroExpansion>
+   </ProfileAction>
+   <AnalyzeAction
+      buildConfiguration = "Debug">
+   </AnalyzeAction>
+   <ArchiveAction
+      buildConfiguration = "Release"
+      revealArchiveInOrganizer = "YES">
+   </ArchiveAction>
+</Scheme>

LICENSE

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+MIT License
+
+Copyright (c) 2021 Hiroki Nagasawa
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Package.swift

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+// swift-tools-version:5.3
+
+import PackageDescription
+
+let package = Package(
+    name: "CurrencyConverter",
+    platforms: [.iOS(.v13), .macOS(.v10_15)],
+    products: [
+        .library(
+            name: "CurrencyConverter",
+            targets: ["CurrencyConverter"]),
+    ],
+    targets: [
+        .target(
+            name: "CurrencyConverter",
+            dependencies: []),
+        .testTarget(
+            name: "CurrencyConverterTests",
+            dependencies: ["CurrencyConverter"]),
+    ]
+)

README.md

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+# currency-converter
+
+A simple currency converter library using the XML data from [European Central Bank](https://www.ecb.europa.eu/home/html/index.en.html).

Sources/CurrencyConverter/CurrencyConverter.swift

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+import Foundation
+
+/// The converter to fetch foreign exchange reference rates from European Central Bank (ECB).
+/// Source: https://www.ecb.europa.eu/stats/policy_and_exchange_rates/euro_reference_exchange_rates/html/index.en.html
+public class CurrencyConverter {
+    private let parser: ReferenceRatesXMLParser
+
+    public init() {
+        parser = ReferenceRatesXMLParser()
+    }
+
+    public init(data: Data) {
+        parser = ReferenceRatesXMLParser(data: data)
+    }
+
+    /// Fetch the latest reference rates from the source.
+    public func fetch(completion: @escaping (Result<ReferenceRates, XMLParserError>) -> Void) {
+        parser.callbacks.parseSucceeded = { referenceRates in
+            completion(.success(referenceRates))
+        }
+        parser.callbacks.parseErrorOccurred = { error in
+            completion(.failure(error))
+        }
+        parser.parse()
+    }
+}

Sources/CurrencyConverter/CurrencyRate.swift

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+import Foundation
+
+/// Represents the currency rate that has the currency code and rate.
+public struct CurrencyRate: Codable, Hashable {
+    /// The currency code, such as "USD", "EUR", and "JPY".
+    public let currencyCode: String
+    /// The currency rate in the currency code.
+    public let rate: Double
+
+    public init(currencyCode: String, rate: Double) {
+        self.currencyCode = currencyCode
+        self.rate = rate
+    }
+}

Sources/CurrencyConverter/ReferenceRates.swift

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+import Foundation
+
+/// Represents the reference rates that has the date code and rates.
+public struct ReferenceRates: Codable, Hashable {
+    private static let defaultCurrencyCode = "EUR"
+
+    /// The date string, e.g. 2021-05-13.
+    public let date: String
+    var rates: [CurrencyRate]
+
+    public init(date: String, rates: [CurrencyRate]) {
+        self.date = date
+        self.rates = rates
+    }
+
+    /// Returns array of currency rates from the given parameters.
+    ///
+    /// - Parameters:
+    ///     - amount: The amount of the currency. The default is 1.
+    ///     - baseCurrencyCode: The base currency code for the currency rates. The default is "EUR".
+    public func rates(amount: Double = 1, baseCurrencyCode: String = "EUR") -> [CurrencyRate] {
+        guard Locale.isoCurrencyCodes.contains(baseCurrencyCode) else {
+            return []
+        }
+
+        if baseCurrencyCode == Self.defaultCurrencyCode {
+            return rates
+        } else {
+            var newRates = rates.filter { $0.currencyCode != baseCurrencyCode }
+
+            if let rate = rate(fromCurrencyCode: baseCurrencyCode, toCurrencyCode: Self.defaultCurrencyCode) {
+                newRates.append(.init(currencyCode: Self.defaultCurrencyCode, rate: rate))
+            }
+
+            newRates = newRates.compactMap {
+                if let rate = rate(fromCurrencyCode: baseCurrencyCode, toCurrencyCode: $0.currencyCode) {
+                    return .init(currencyCode: $0.currencyCode, rate: amount * rate)
+                }
+                return nil
+            }
+
+            return newRates
+        }
+    }
+
+    /// Returns the rate from the given parameters.
+    ///
+    /// - Parameters:
+    ///     - amount: The amount of the currency. The default is 1.
+    ///     - fromCurrencyCode: The currency code that you'd like to converted from.
+    ///     - toCurrencyCode: The currency code that you'd like to converted to.
+    public func rate(amount: Double = 1, fromCurrencyCode: String, toCurrencyCode: String) -> Double? {
+        guard fromCurrencyCode != toCurrencyCode else {
+            return amount
+        }
+
+        let isoCurrencyCodes = Locale.isoCurrencyCodes
+
+        guard isoCurrencyCodes.contains(fromCurrencyCode) && isoCurrencyCodes.contains(toCurrencyCode) else {
+            return nil
+        }
+
+        if let rate = rates.first(where: { $0.currencyCode == toCurrencyCode })?.rate, fromCurrencyCode == Self.defaultCurrencyCode {
+            return amount * rate
+        }
+
+        if let rate = rates.first(where: { $0.currencyCode == fromCurrencyCode })?.rate, toCurrencyCode == Self.defaultCurrencyCode {
+            return amount * 1 / rate
+        }
+
+        if  let fromRate = rates.first(where: { $0.currencyCode == fromCurrencyCode })?.rate,
+            let toRate = rates.first(where: { $0.currencyCode == toCurrencyCode })?.rate {
+            let actualRate = toRate / fromRate
+            return amount * actualRate
+        }
+
+        return nil
+    }
+}

Sources/CurrencyConverter/ReferenceRatesXMLParser.swift

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+import Foundation
+
+class ReferenceRatesXMLParser: NSObject, XMLParserDelegate {
+    private static let defaultXMLURL = URL(string: "https://www.ecb.europa.eu/stats/eurofxref/eurofxref-daily.xml")!
+
+    private let parser: XMLParser?
+
+    private var resultDate: String?
+    private var resultRates: [CurrencyRate] = []
+    var callbacks = Callbacks()
+
+    struct Callbacks {
+        var parseSucceeded: ((ReferenceRates) -> Void)?
+        var parseErrorOccurred: ((XMLParserError) -> Void)?
+    }
+
+    enum XMLParserKeys: String {
+        case time
+        case currency
+        case rate
+    }
+
+    init(contentsOf url: URL = defaultXMLURL) {
+        parser = XMLParser(contentsOf: url)
+        super.init()
+        parser?.delegate = self
+    }
+
+    init(data: Data) {
+        parser = XMLParser(data: data)
+        super.init()
+        parser?.delegate = self
+    }
+
+    func parse() {
+        parser?.parse()
+    }
+
+    // MARK: - XMLParserDelegate
+
+    func parser(_ parser: XMLParser, didStartElement elementName: String, namespaceURI: String?, qualifiedName qName: String?, attributes attributeDict: [String: String] = [:]) {
+        guard elementName == "Cube", !attributeDict.isEmpty else {
+            // Skip throwing an error as there will be element names to ignore,
+            // such as 'gesmes:Envelope', 'gesmes:subject', and so on.
+            return
+        }
+
+        var currencyRate: (currencyCode: String?, rate: Double?)
+
+        for attribute in attributeDict {
+            switch attribute.key {
+            case XMLParserKeys.time.rawValue:
+                let datePattern = #"^\d{4}[-]\d{2}[-]\d{2}$"# // 2021-05-07
+                if attribute.value.range(of: datePattern, options: .regularExpression) != nil {
+                    resultDate = attribute.value
+                } else {
+                    callbacks.parseErrorOccurred?(.custom("Unexpected time value: \(attribute.value)"))
+                    return
+                }
+            case XMLParserKeys.rate.rawValue:
+                if let rate = Double(attribute.value) {
+                    currencyRate.rate = rate
+                } else {
+                    callbacks.parseErrorOccurred?(.custom("Unexpected rate value: \(attribute.value)"))
+                    return
+                }
+            case XMLParserKeys.currency.rawValue:
+                if Locale.isoCurrencyCodes.contains(attribute.value) {
+                    currencyRate.currencyCode = attribute.value
+                } else {
+                    callbacks.parseErrorOccurred?(.custom("Unexpected currency value: \(attribute.value)"))
+                    return
+                }
+            default:
+                break
+            }
+        }
+
+        if let currencyCode = currencyRate.currencyCode, let rate = currencyRate.rate {
+            resultRates.append(.init(currencyCode: currencyCode, rate: rate))
+        }
+    }
+
+    func parserDidEndDocument(_ parser: XMLParser) {
+        if let resultDate = resultDate {
+            callbacks.parseSucceeded?(.init(date: resultDate, rates: resultRates))
+        } else {
+            callbacks.parseErrorOccurred?(.custom("Parse failed"))
+        }
+    }
+
+    func parser(_ parser: XMLParser, parseErrorOccurred parseError: Error) {
+        callbacks.parseErrorOccurred?(.general(parseError))
+    }
+}

Sources/CurrencyConverter/XMLParserError.swift

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+import Foundation
+
+public enum XMLParserError: Error {
+    case general(Error)
+    case custom(String)
+}