TestJSONEncoder.swift

//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//

import Swift
import Foundation
import XCTest

// MARK: - Test Suite

class TestJSONEncoder : XCTestCase {
  // MARK: - Encoding Top-Level Empty Types
  func testEncodingTopLevelEmptyStruct() {
    let empty = EmptyStruct()
    _testRoundTrip(of: empty, expectedJSON: _jsonEmptyDictionary)
  }

  func testEncodingTopLevelEmptyClass() {
    let empty = EmptyClass()
    _testRoundTrip(of: empty, expectedJSON: _jsonEmptyDictionary)
  }

  // MARK: - Encoding Top-Level Single-Value Types
  func testEncodingTopLevelSingleValueEnum() {
    _testRoundTrip(of: Switch.off)
    _testRoundTrip(of: Switch.on)
  }

  func testEncodingTopLevelSingleValueStruct() {
    _testRoundTrip(of: Timestamp(3141592653))
  }

  func testEncodingTopLevelSingleValueClass() {
    _testRoundTrip(of: Counter())
  }

  // MARK: - Encoding Top-Level Structured Types
  func testEncodingTopLevelStructuredStruct() {
    // Address is a struct type with multiple fields.
    let address = Address.testValue
    _testRoundTrip(of: address)
  }

  func testEncodingTopLevelStructuredClass() {
    // Person is a class with multiple fields.
    let expectedJSON = "{\"name\":\"Johnny Appleseed\",\"email\":\"appleseed@apple.com\"}".data(using: .utf8)!
    let person = Person.testValue
    _testRoundTrip(of: person, expectedJSON: expectedJSON)
  }

  func testEncodingTopLevelStructuredSingleStruct() {
    // Numbers is a struct which encodes as an array through a single value container.
    let numbers = Numbers.testValue
    _testRoundTrip(of: numbers)
  }

  func testEncodingTopLevelStructuredSingleClass() {
    // Mapping is a class which encodes as a dictionary through a single value container.
    let mapping = Mapping.testValue
    _testRoundTrip(of: mapping)
  }

  func testEncodingTopLevelDeepStructuredType() {
    // Company is a type with fields which are Codable themselves.
    let company = Company.testValue
    _testRoundTrip(of: company)
  }

  func testEncodingClassWhichSharesEncoderWithSuper() {
    // Employee is a type which shares its encoder & decoder with its superclass, Person.
    let employee = Employee.testValue
    _testRoundTrip(of: employee)
  }

  func testEncodingTopLevelNullableType() {
    // EnhancedBool is a type which encodes either as a Bool or as nil.
    _testRoundTrip(of: EnhancedBool.true, expectedJSON: "true".data(using: .utf8)!)
    _testRoundTrip(of: EnhancedBool.false, expectedJSON: "false".data(using: .utf8)!)
    _testRoundTrip(of: EnhancedBool.fileNotFound, expectedJSON: "null".data(using: .utf8)!)
  }
  
  func testEncodingMultipleNestedContainersWithTheSameTopLevelKey() {
    struct Model : Codable, Equatable {
      let first: String
      let second: String
      
      init(from coder: Decoder) throws {
        let container = try coder.container(keyedBy: TopLevelCodingKeys.self)
        
        let firstNestedContainer = try container.nestedContainer(keyedBy: FirstNestedCodingKeys.self, forKey: .top)
        self.first = try firstNestedContainer.decode(String.self, forKey: .first)
        
        let secondNestedContainer = try container.nestedContainer(keyedBy: SecondNestedCodingKeys.self, forKey: .top)
        self.second = try secondNestedContainer.decode(String.self, forKey: .second)
      }
      
      func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: TopLevelCodingKeys.self)
        
        var firstNestedContainer = container.nestedContainer(keyedBy: FirstNestedCodingKeys.self, forKey: .top)
        try firstNestedContainer.encode(self.first, forKey: .first)
        
        var secondNestedContainer = container.nestedContainer(keyedBy: SecondNestedCodingKeys.self, forKey: .top)
        try secondNestedContainer.encode(self.second, forKey: .second)
      }
      
      init(first: String, second: String) {
        self.first = first
        self.second = second
      }
      
      static var testValue: Model {
        return Model(first: "Johnny Appleseed",
                     second: "appleseed@apple.com")
      }
      
      enum TopLevelCodingKeys : String, CodingKey {
        case top
      }
      
      enum FirstNestedCodingKeys : String, CodingKey {
        case first
      }
      enum SecondNestedCodingKeys : String, CodingKey {
        case second
      }
    }
    
    let model = Model.testValue
    if #available(macOS 10.13, iOS 11.0, watchOS 4.0, tvOS 11.0, *) {
      let expectedJSON = "{\"top\":{\"first\":\"Johnny Appleseed\",\"second\":\"appleseed@apple.com\"}}".data(using: .utf8)!
      _testRoundTrip(of: model, expectedJSON: expectedJSON, outputFormatting: [.sortedKeys])
    } else {
      _testRoundTrip(of: model)
    }
  }

  #if false // FIXME: XCTest doesn't support crash tests yet rdar://20195010&22387653
  func testEncodingConflictedTypeNestedContainersWithTheSameTopLevelKey() {
    struct Model : Encodable, Equatable {
      let first: String

      func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: TopLevelCodingKeys.self)
        
        var firstNestedContainer = container.nestedContainer(keyedBy: FirstNestedCodingKeys.self, forKey: .top)
        try firstNestedContainer.encode(self.first, forKey: .first)
        
        // The following line would fail as it attempts to re-encode into already encoded container is invalid. This will always fail
        var secondNestedContainer = container.nestedUnkeyedContainer(forKey: .top)
        try secondNestedContainer.encode("second")
      }
      
      init(first: String) {
        self.first = first
      }
      
      static var testValue: Model {
        return Model(first: "Johnny Appleseed")
      }
      
      enum TopLevelCodingKeys : String, CodingKey {
        case top
      }
      enum FirstNestedCodingKeys : String, CodingKey {
        case first
      }
    }
    
    let model = Model.testValue
    // This following test would fail as it attempts to re-encode into already encoded container is invalid. This will always fail
    expectCrashLater()
    if #available(macOS 10.13, iOS 11.0, watchOS 4.0, tvOS 11.0, *) {
      _testEncodeFailure(of: model)
    } else {
      _testEncodeFailure(of: model)
    }
  }
  #endif
  
  // MARK: - Output Formatting Tests
  func testEncodingOutputFormattingDefault() {
    let expectedJSON = "{\"name\":\"Johnny Appleseed\",\"email\":\"appleseed@apple.com\"}".data(using: .utf8)!
    let person = Person.testValue
    _testRoundTrip(of: person, expectedJSON: expectedJSON)
  }

  func testEncodingOutputFormattingPrettyPrinted() {
    let expectedJSON = "{\n  \"name\" : \"Johnny Appleseed\",\n  \"email\" : \"appleseed@apple.com\"\n}".data(using: .utf8)!
    let person = Person.testValue
    _testRoundTrip(of: person, expectedJSON: expectedJSON, outputFormatting: [.prettyPrinted])
  }

  func testEncodingOutputFormattingSortedKeys() {
    if #available(macOS 10.13, iOS 11.0, watchOS 4.0, tvOS 11.0, *) {
      let expectedJSON = "{\"email\":\"appleseed@apple.com\",\"name\":\"Johnny Appleseed\"}".data(using: .utf8)!
      let person = Person.testValue
      _testRoundTrip(of: person, expectedJSON: expectedJSON, outputFormatting: [.sortedKeys])
    }
  }

  func testEncodingOutputFormattingPrettyPrintedSortedKeys() {
    if #available(macOS 10.13, iOS 11.0, watchOS 4.0, tvOS 11.0, *) {
      let expectedJSON = "{\n  \"email\" : \"appleseed@apple.com\",\n  \"name\" : \"Johnny Appleseed\"\n}".data(using: .utf8)!
      let person = Person.testValue
      _testRoundTrip(of: person, expectedJSON: expectedJSON, outputFormatting: [.prettyPrinted, .sortedKeys])
    }
  }

  // MARK: - Date Strategy Tests

  // Disabled for now till we resolve rdar://52618414
  func x_testEncodingDate() {

    func formattedLength(of value: Double) -> Int {
      let empty = UnsafeMutablePointer<Int8>.allocate(capacity: 0)
      defer { empty.deallocate() }
      let length = snprintf(ptr: empty, 0, "%0.*g", DBL_DECIMAL_DIG, value)
      return Int(length)
    }

    // Duplicated to handle a special case
    func localTestRoundTrip<T: Codable & Equatable>(of value: T) {
      var payload: Data! = nil
      do {
        let encoder = JSONEncoder()
        payload = try encoder.encode(value)
      } catch {
        XCTFail("Failed to encode \(T.self) to JSON: \(error)")
      }

      do {
        let decoder = JSONDecoder()
        let decoded = try decoder.decode(T.self, from: payload)

        /// `snprintf`'s `%g`, which `JSONSerialization` uses internally for double values, does not respect 
        /// our precision requests in every case. This bug effects Darwin, FreeBSD, and Linux currently
        /// causing this test (which uses the current time) to fail occasionally.
        if formattedLength(of: (decoded as! Date).timeIntervalSinceReferenceDate) > DBL_DECIMAL_DIG + 2 {
          let adjustedTimeIntervalSinceReferenceDate: (Date) -> Double = { date in
              let adjustment = pow(10, Double(DBL_DECIMAL_DIG))
              return Double(floor(adjustment * date.timeIntervalSinceReferenceDate).rounded() / adjustment)
          }
          
          let decodedAprox = adjustedTimeIntervalSinceReferenceDate(decoded as! Date)
          let valueAprox = adjustedTimeIntervalSinceReferenceDate(value as! Date)
          XCTAssertEqual(decodedAprox, valueAprox, "\(T.self) did not round-trip to an equal value after DBL_DECIMAL_DIG adjustment \(decodedAprox) != \(valueAprox).")
          return
        }

        XCTAssertEqual(decoded, value, "\(T.self) did not round-trip to an equal value. \((decoded as! Date).timeIntervalSinceReferenceDate) != \((value as! Date).timeIntervalSinceReferenceDate)")
      } catch {
        XCTFail("Failed to decode \(T.self) from JSON: \(error)")
      }
    }

    // Test the above `snprintf` edge case evaluation with a known triggering case
    let knownBadDate = Date(timeIntervalSinceReferenceDate: 0.0021413276231263384)
    localTestRoundTrip(of: knownBadDate)

    localTestRoundTrip(of: Date())

    // Optional dates should encode the same way.
    localTestRoundTrip(of: Optional(Date()))
  }

  func testEncodingDateSecondsSince1970() {
    // Cannot encode an arbitrary number of seconds since we've lost precision since 1970.
    let seconds = 1000.0
    let expectedJSON = "1000".data(using: .utf8)!

    _testRoundTrip(of: Date(timeIntervalSince1970: seconds),
                   expectedJSON: expectedJSON,
                   dateEncodingStrategy: .secondsSince1970,
                   dateDecodingStrategy: .secondsSince1970)

    // Optional dates should encode the same way.
    _testRoundTrip(of: Optional(Date(timeIntervalSince1970: seconds)),
                   expectedJSON: expectedJSON,
                   dateEncodingStrategy: .secondsSince1970,
                   dateDecodingStrategy: .secondsSince1970)
  }

  func testEncodingDateMillisecondsSince1970() {
    // Cannot encode an arbitrary number of seconds since we've lost precision since 1970.
    let seconds = 1000.0
    let expectedJSON = "1000000".data(using: .utf8)!

    _testRoundTrip(of: Date(timeIntervalSince1970: seconds),
                   expectedJSON: expectedJSON,
                   dateEncodingStrategy: .millisecondsSince1970,
                   dateDecodingStrategy: .millisecondsSince1970)

    // Optional dates should encode the same way.
    _testRoundTrip(of: Optional(Date(timeIntervalSince1970: seconds)),
                   expectedJSON: expectedJSON,
                   dateEncodingStrategy: .millisecondsSince1970,
                   dateDecodingStrategy: .millisecondsSince1970)
  }

  func testEncodingDateISO8601() {
    if #available(macOS 10.12, iOS 10.0, watchOS 3.0, tvOS 10.0, *) {
      let formatter = ISO8601DateFormatter()
      formatter.formatOptions = .withInternetDateTime

      let timestamp = Date(timeIntervalSince1970: 1000)
      let expectedJSON = "\"\(formatter.string(from: timestamp))\"".data(using: .utf8)!

      _testRoundTrip(of: timestamp,
                     expectedJSON: expectedJSON,
                     dateEncodingStrategy: .iso8601,
                     dateDecodingStrategy: .iso8601)


      // Optional dates should encode the same way.
      _testRoundTrip(of: Optional(timestamp),
                     expectedJSON: expectedJSON,
                     dateEncodingStrategy: .iso8601,
                     dateDecodingStrategy: .iso8601)
    }
  }

  func testEncodingDateFormatted() {
    let formatter = DateFormatter()
    formatter.dateStyle = .full
    formatter.timeStyle = .full

    let timestamp = Date(timeIntervalSince1970: 1000)
    let expectedJSON = "\"\(formatter.string(from: timestamp))\"".data(using: .utf8)!

    _testRoundTrip(of: timestamp,
                   expectedJSON: expectedJSON,
                   dateEncodingStrategy: .formatted(formatter),
                   dateDecodingStrategy: .formatted(formatter))

    // Optional dates should encode the same way.
    _testRoundTrip(of: Optional(timestamp),
                   expectedJSON: expectedJSON,
                   dateEncodingStrategy: .formatted(formatter),
                   dateDecodingStrategy: .formatted(formatter))
  }

  func testEncodingDateCustom() {
    let timestamp = Date()

    // We'll encode a number instead of a date.
    let encode = { (_ data: Date, _ encoder: Encoder) throws -> Void in
      var container = encoder.singleValueContainer()
      try container.encode(42)
    }
    let decode = { (_: Decoder) throws -> Date in return timestamp }

    let expectedJSON = "42".data(using: .utf8)!
    _testRoundTrip(of: timestamp,
                   expectedJSON: expectedJSON,
                   dateEncodingStrategy: .custom(encode),
                   dateDecodingStrategy: .custom(decode))

    // Optional dates should encode the same way.
    _testRoundTrip(of: Optional(timestamp),
                   expectedJSON: expectedJSON,
                   dateEncodingStrategy: .custom(encode),
                   dateDecodingStrategy: .custom(decode))
  }

  func testEncodingDateCustomEmpty() {
    let timestamp = Date()

    // Encoding nothing should encode an empty keyed container ({}).
    let encode = { (_: Date, _: Encoder) throws -> Void in }
    let decode = { (_: Decoder) throws -> Date in return timestamp }

    let expectedJSON = "{}".data(using: .utf8)!
    _testRoundTrip(of: timestamp,
                   expectedJSON: expectedJSON,
                   dateEncodingStrategy: .custom(encode),
                   dateDecodingStrategy: .custom(decode))

    // Optional dates should encode the same way.
    _testRoundTrip(of: Optional(timestamp),
                   expectedJSON: expectedJSON,
                   dateEncodingStrategy: .custom(encode),
                   dateDecodingStrategy: .custom(decode))
  }

  // MARK: - Data Strategy Tests
  func testEncodingData() {
    let data = Data(bytes: [0xDE, 0xAD, 0xBE, 0xEF])

    let expectedJSON = "[222,173,190,239]".data(using: .utf8)!
    _testRoundTrip(of: data,
                   expectedJSON: expectedJSON,
                   dataEncodingStrategy: .deferredToData,
                   dataDecodingStrategy: .deferredToData)

    // Optional data should encode the same way.
    _testRoundTrip(of: Optional(data),
                   expectedJSON: expectedJSON,
                   dataEncodingStrategy: .deferredToData,
                   dataDecodingStrategy: .deferredToData)
  }

  func testEncodingDataBase64() {
    let data = Data(bytes: [0xDE, 0xAD, 0xBE, 0xEF])

    let expectedJSON = "\"3q2+7w==\"".data(using: .utf8)!
    _testRoundTrip(of: data, expectedJSON: expectedJSON)

    // Optional data should encode the same way.
    _testRoundTrip(of: Optional(data), expectedJSON: expectedJSON)
  }

  func testEncodingDataCustom() {
    // We'll encode a number instead of data.
    let encode = { (_ data: Data, _ encoder: Encoder) throws -> Void in
      var container = encoder.singleValueContainer()
      try container.encode(42)
    }
    let decode = { (_: Decoder) throws -> Data in return Data() }

    let expectedJSON = "42".data(using: .utf8)!
    _testRoundTrip(of: Data(),
                   expectedJSON: expectedJSON,
                   dataEncodingStrategy: .custom(encode),
                   dataDecodingStrategy: .custom(decode))

    // Optional data should encode the same way.
    _testRoundTrip(of: Optional(Data()),
                   expectedJSON: expectedJSON,
                   dataEncodingStrategy: .custom(encode),
                   dataDecodingStrategy: .custom(decode))
  }

  func testEncodingDataCustomEmpty() {
    // Encoding nothing should encode an empty keyed container ({}).
    let encode = { (_: Data, _: Encoder) throws -> Void in }
    let decode = { (_: Decoder) throws -> Data in return Data() }

    let expectedJSON = "{}".data(using: .utf8)!
    _testRoundTrip(of: Data(),
                   expectedJSON: expectedJSON,
                   dataEncodingStrategy: .custom(encode),
                   dataDecodingStrategy: .custom(decode))

    // Optional Data should encode the same way.
    _testRoundTrip(of: Optional(Data()),
                   expectedJSON: expectedJSON,
                   dataEncodingStrategy: .custom(encode),
                   dataDecodingStrategy: .custom(decode))
  }

  // MARK: - Non-Conforming Floating Point Strategy Tests
  func testEncodingNonConformingFloats() {
    _testEncodeFailure(of: Float.infinity)
    _testEncodeFailure(of: Float.infinity)
    _testEncodeFailure(of: -Float.infinity)
    _testEncodeFailure(of: Float.nan)

    _testEncodeFailure(of: Double.infinity)
    _testEncodeFailure(of: -Double.infinity)
    _testEncodeFailure(of: Double.nan)

    // Optional Floats/Doubles should encode the same way.
    _testEncodeFailure(of: Float.infinity)
    _testEncodeFailure(of: -Float.infinity)
    _testEncodeFailure(of: Float.nan)

    _testEncodeFailure(of: Double.infinity)
    _testEncodeFailure(of: -Double.infinity)
    _testEncodeFailure(of: Double.nan)
  }

  func testEncodingNonConformingFloatStrings() {
    let encodingStrategy: JSONEncoder.NonConformingFloatEncodingStrategy = .convertToString(positiveInfinity: "INF", negativeInfinity: "-INF", nan: "NaN")
    let decodingStrategy: JSONDecoder.NonConformingFloatDecodingStrategy = .convertFromString(positiveInfinity: "INF", negativeInfinity: "-INF", nan: "NaN")

    _testRoundTrip(of: Float.infinity,
                   expectedJSON: "\"INF\"".data(using: .utf8)!,
                   nonConformingFloatEncodingStrategy: encodingStrategy,
                   nonConformingFloatDecodingStrategy: decodingStrategy)
    _testRoundTrip(of: -Float.infinity,
                   expectedJSON: "\"-INF\"".data(using: .utf8)!,
                   nonConformingFloatEncodingStrategy: encodingStrategy,
                   nonConformingFloatDecodingStrategy: decodingStrategy)

    // Since Float.nan != Float.nan, we have to use a placeholder that'll encode NaN but actually round-trip.
    _testRoundTrip(of: FloatNaNPlaceholder(),
                   expectedJSON: "\"NaN\"".data(using: .utf8)!,
                   nonConformingFloatEncodingStrategy: encodingStrategy,
                   nonConformingFloatDecodingStrategy: decodingStrategy)

    _testRoundTrip(of: Double.infinity,
                   expectedJSON: "\"INF\"".data(using: .utf8)!,
                   nonConformingFloatEncodingStrategy: encodingStrategy,
                   nonConformingFloatDecodingStrategy: decodingStrategy)
    _testRoundTrip(of: -Double.infinity,
                   expectedJSON: "\"-INF\"".data(using: .utf8)!,
                   nonConformingFloatEncodingStrategy: encodingStrategy,
                   nonConformingFloatDecodingStrategy: decodingStrategy)

    // Since Double.nan != Double.nan, we have to use a placeholder that'll encode NaN but actually round-trip.
    _testRoundTrip(of: DoubleNaNPlaceholder(),
                   expectedJSON: "\"NaN\"".data(using: .utf8)!,
                   nonConformingFloatEncodingStrategy: encodingStrategy,
                   nonConformingFloatDecodingStrategy: decodingStrategy)

    // Optional Floats and Doubles should encode the same way.
    _testRoundTrip(of: Optional(Float.infinity),
                   expectedJSON: "\"INF\"".data(using: .utf8)!,
                   nonConformingFloatEncodingStrategy: encodingStrategy,
                   nonConformingFloatDecodingStrategy: decodingStrategy)
    _testRoundTrip(of: Optional(-Float.infinity),
                   expectedJSON: "\"-INF\"".data(using: .utf8)!,
                   nonConformingFloatEncodingStrategy: encodingStrategy,
                   nonConformingFloatDecodingStrategy: decodingStrategy)
    _testRoundTrip(of: Optional(Double.infinity),
                   expectedJSON: "\"INF\"".data(using: .utf8)!,
                   nonConformingFloatEncodingStrategy: encodingStrategy,
                   nonConformingFloatDecodingStrategy: decodingStrategy)
    _testRoundTrip(of: Optional(-Double.infinity),
                   expectedJSON: "\"-INF\"".data(using: .utf8)!,
                   nonConformingFloatEncodingStrategy: encodingStrategy,
                   nonConformingFloatDecodingStrategy: decodingStrategy)
  }

  // MARK: - Key Strategy Tests
  private struct EncodeMe : Encodable {
    var keyName: String
    func encode(to coder: Encoder) throws {
      var c = coder.container(keyedBy: _TestKey.self)
      try c.encode("test", forKey: _TestKey(stringValue: keyName)!)
    }
  }

  func testEncodingKeyStrategySnake() {
    let toSnakeCaseTests = [
      ("simpleOneTwo", "simple_one_two"),
      ("myURL", "my_url"),
      ("singleCharacterAtEndX", "single_character_at_end_x"),
      ("thisIsAnXMLProperty", "this_is_an_xml_property"),
      ("single", "single"), // no underscore
      ("", ""), // don't die on empty string
      ("a", "a"), // single character
      ("aA", "a_a"), // two characters
      ("version4Thing", "version4_thing"), // numerics
      ("partCAPS", "part_caps"), // only insert underscore before first all caps
      ("partCAPSLowerAGAIN", "part_caps_lower_again"), // switch back and forth caps.
      ("manyWordsInThisThing", "many_words_in_this_thing"), // simple lowercase + underscore + more
      ("asdfĆqer", "asdf_ćqer"),
      ("already_snake_case", "already_snake_case"),
      ("dataPoint22", "data_point22"),
      ("dataPoint22Word", "data_point22_word"),
      ("_oneTwoThree", "_one_two_three"),
      ("oneTwoThree_", "one_two_three_"),
      ("__oneTwoThree", "__one_two_three"),
      ("oneTwoThree__", "one_two_three__"),
      ("_oneTwoThree_", "_one_two_three_"),
      ("__oneTwoThree", "__one_two_three"),
      ("__oneTwoThree__", "__one_two_three__"),
      ("_test", "_test"),
      ("_test_", "_test_"),
      ("__test", "__test"),
      ("test__", "test__"),
      ("m͉̟̹y̦̳G͍͚͎̳r̤͉̤͕ͅea̲͕t͇̥̼͖U͇̝̠R͙̻̥͓̣L̥̖͎͓̪̫ͅR̩͖̩eq͈͓u̞e̱s̙t̤̺ͅ", "m͉̟̹y̦̳_g͍͚͎̳r̤͉̤͕ͅea̲͕t͇̥̼͖_u͇̝̠r͙̻̥͓̣l̥̖͎͓̪̫ͅ_r̩͖̩eq͈͓u̞e̱s̙t̤̺ͅ"), // because Itai wanted to test this
      ("🐧🐟", "🐧🐟") // fishy emoji example?
    ]

    for test in toSnakeCaseTests {
      let expected = "{\"\(test.1)\":\"test\"}"
      let encoded = EncodeMe(keyName: test.0)
      
      let encoder = JSONEncoder()
      encoder.keyEncodingStrategy = .convertToSnakeCase
      let resultData = try! encoder.encode(encoded)
      let resultString = String(bytes: resultData, encoding: .utf8)
      
      XCTAssertEqual(expected, resultString)
    }
  }
  
  func testEncodingKeyStrategyCustom() {
    let expected = "{\"QQQhello\":\"test\"}"
    let encoded = EncodeMe(keyName: "hello")
    
    let encoder = JSONEncoder()
    let customKeyConversion = { (_ path : [CodingKey]) -> CodingKey in
      let key = _TestKey(stringValue: "QQQ" + path.last!.stringValue)!
      return key
    }
    encoder.keyEncodingStrategy = .custom(customKeyConversion)
    let resultData = try! encoder.encode(encoded)
    let resultString = String(bytes: resultData, encoding: .utf8)
    
    XCTAssertEqual(expected, resultString)
  }

  func testEncodingDictionaryStringKeyConversionUntouched() {
    let expected = "{\"leaveMeAlone\":\"test\"}"
    let toEncode: [String: String] = ["leaveMeAlone": "test"]

    let encoder = JSONEncoder()
    encoder.keyEncodingStrategy = .convertToSnakeCase
    let resultData = try! encoder.encode(toEncode)
    let resultString = String(bytes: resultData, encoding: .utf8)

    XCTAssertEqual(expected, resultString)
  }

  private struct EncodeFailure : Encodable {
    var someValue: Double
  }

  private struct EncodeFailureNested : Encodable {
    var nestedValue: EncodeFailure
  }

  func testEncodingDictionaryFailureKeyPath() {
    let toEncode: [String: EncodeFailure] = ["key": EncodeFailure(someValue: Double.nan)]

    let encoder = JSONEncoder()
    encoder.keyEncodingStrategy = .convertToSnakeCase
    do {
      _ = try encoder.encode(toEncode)
    } catch EncodingError.invalidValue(_, let context) {
      XCTAssertEqual(2, context.codingPath.count)
      XCTAssertEqual("key", context.codingPath[0].stringValue)
      XCTAssertEqual("someValue", context.codingPath[1].stringValue)
    } catch {
      XCTFail("Unexpected error: \(String(describing: error))")
    }
  }

  func testEncodingDictionaryFailureKeyPathNested() {
    let toEncode: [String: [String: EncodeFailureNested]] = ["key": ["sub_key": EncodeFailureNested(nestedValue: EncodeFailure(someValue: Double.nan))]]

    let encoder = JSONEncoder()
    encoder.keyEncodingStrategy = .convertToSnakeCase
    do {
      _ = try encoder.encode(toEncode)
    } catch EncodingError.invalidValue(_, let context) {
      XCTAssertEqual(4, context.codingPath.count)
      XCTAssertEqual("key", context.codingPath[0].stringValue)
      XCTAssertEqual("sub_key", context.codingPath[1].stringValue)
      XCTAssertEqual("nestedValue", context.codingPath[2].stringValue)
      XCTAssertEqual("someValue", context.codingPath[3].stringValue)
    } catch {
      XCTFail("Unexpected error: \(String(describing: error))")
    }
  }

  private struct EncodeNested : Encodable {
    let nestedValue: EncodeMe
  }
  
  private struct EncodeNestedNested : Encodable {
    let outerValue: EncodeNested
  }
  
  func testEncodingKeyStrategyPath() {
    // Make sure a more complex path shows up the way we want
    // Make sure the path reflects keys in the Swift, not the resulting ones in the JSON
    let expected = "{\"QQQouterValue\":{\"QQQnestedValue\":{\"QQQhelloWorld\":\"test\"}}}"
    let encoded = EncodeNestedNested(outerValue: EncodeNested(nestedValue: EncodeMe(keyName: "helloWorld")))
    
    let encoder = JSONEncoder()
    var callCount = 0
    
    let customKeyConversion = { (_ path : [CodingKey]) -> CodingKey in
      // This should be called three times:
      // 1. to convert 'outerValue' to something
      // 2. to convert 'nestedValue' to something
      // 3. to convert 'helloWorld' to something
      callCount = callCount + 1
      
      if path.count == 0 {
        XCTFail("The path should always have at least one entry")
      } else if path.count == 1 {
        XCTAssertEqual(["outerValue"], path.map { $0.stringValue })
      } else if path.count == 2 {
        XCTAssertEqual(["outerValue", "nestedValue"], path.map { $0.stringValue })
      } else if path.count == 3 {
        XCTAssertEqual(["outerValue", "nestedValue", "helloWorld"], path.map { $0.stringValue })
      } else {
        XCTFail("The path mysteriously had more entries")
      }
      
      let key = _TestKey(stringValue: "QQQ" + path.last!.stringValue)!
      return key
    }
    encoder.keyEncodingStrategy = .custom(customKeyConversion)
    let resultData = try! encoder.encode(encoded)
    let resultString = String(bytes: resultData, encoding: .utf8)
    
    XCTAssertEqual(expected, resultString)
    XCTAssertEqual(3, callCount)
  }
  
  private struct DecodeMe : Decodable {
    let found: Bool
    init(from coder: Decoder) throws {
      let c = try coder.container(keyedBy: _TestKey.self)
      // Get the key that we expect to be passed in (camel case)
      let camelCaseKey = try c.decode(String.self, forKey: _TestKey(stringValue: "camelCaseKey")!)
        
      // Use the camel case key to decode from the JSON. The decoder should convert it to snake case to find it.
      found = try c.decode(Bool.self, forKey: _TestKey(stringValue: camelCaseKey)!)
    }
  }

  func testDecodingKeyStrategyCamel() {
    let fromSnakeCaseTests = [
      ("", ""), // don't die on empty string
      ("a", "a"), // single character
      ("ALLCAPS", "ALLCAPS"), // If no underscores, we leave the word as-is
      ("ALL_CAPS", "allCaps"), // Conversion from screaming snake case
      ("single", "single"), // do not capitalize anything with no underscore
      ("snake_case", "snakeCase"), // capitalize a character
      ("one_two_three", "oneTwoThree"), // more than one word
      ("one_2_three", "one2Three"), // numerics
      ("one2_three", "one2Three"), // numerics, part 2
      ("snake_Ćase", "snakeĆase"), // do not further modify a capitalized diacritic
      ("snake_ćase", "snakeĆase"), // capitalize a diacritic
      ("alreadyCamelCase", "alreadyCamelCase"), // do not modify already camel case
      ("__this_and_that", "__thisAndThat"),
      ("_this_and_that", "_thisAndThat"),
      ("this__and__that", "thisAndThat"),
      ("this_and_that__", "thisAndThat__"),
      ("this_aNd_that", "thisAndThat"),
      ("_one_two_three", "_oneTwoThree"),
      ("one_two_three_", "oneTwoThree_"),
      ("__one_two_three", "__oneTwoThree"),
      ("one_two_three__", "oneTwoThree__"),
      ("_one_two_three_", "_oneTwoThree_"),
      ("__one_two_three", "__oneTwoThree"),
      ("__one_two_three__", "__oneTwoThree__"),
      ("_test", "_test"),
      ("_test_", "_test_"),
      ("__test", "__test"),
      ("test__", "test__"),
      ("_", "_"),
      ("__", "__"),
      ("___", "___"),
      ("m͉̟̹y̦̳G͍͚͎̳r̤͉̤͕ͅea̲͕t͇̥̼͖U͇̝̠R͙̻̥͓̣L̥̖͎͓̪̫ͅR̩͖̩eq͈͓u̞e̱s̙t̤̺ͅ", "m͉̟̹y̦̳G͍͚͎̳r̤͉̤͕ͅea̲͕t͇̥̼͖U͇̝̠R͙̻̥͓̣L̥̖͎͓̪̫ͅR̩͖̩eq͈͓u̞e̱s̙t̤̺ͅ"), // because Itai wanted to test this
      ("🐧_🐟", "🐧🐟") // fishy emoji example?
    ]
    
    for test in fromSnakeCaseTests {
      // This JSON contains the camel case key that the test object should decode with, then it uses the snake case key (test.0) as the actual key for the boolean value.
      let input = "{\"camelCaseKey\":\"\(test.1)\",\"\(test.0)\":true}".data(using: .utf8)!
      
      let decoder = JSONDecoder()
      decoder.keyDecodingStrategy = .convertFromSnakeCase
      
      let result = try! decoder.decode(DecodeMe.self, from: input)
      
      XCTAssertTrue(result.found)
    }
  }
  
  private struct DecodeMe2 : Decodable { var hello: String }
  
  func testDecodingKeyStrategyCustom() {
    let input = "{\"----hello\":\"test\"}".data(using: .utf8)!
    let decoder = JSONDecoder()
    let customKeyConversion = { (_ path: [CodingKey]) -> CodingKey in
      // This converter removes the first 4 characters from the start of all string keys, if it has more than 4 characters
      let string = path.last!.stringValue
      guard string.count > 4 else { return path.last! }
      let newString = String(string.dropFirst(4))
      return _TestKey(stringValue: newString)!
    }
    decoder.keyDecodingStrategy = .custom(customKeyConversion)
    let result = try! decoder.decode(DecodeMe2.self, from: input)
    
    XCTAssertEqual("test", result.hello)
  }

  func testDecodingDictionaryStringKeyConversionUntouched() {
    let input = "{\"leave_me_alone\":\"test\"}".data(using: .utf8)!
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase
    let result = try! decoder.decode([String: String].self, from: input)

    XCTAssertEqual(["leave_me_alone": "test"], result)
  }

  func testDecodingDictionaryFailureKeyPath() {
    let input = "{\"leave_me_alone\":\"test\"}".data(using: .utf8)!
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase
    do {
      _ = try decoder.decode([String: Int].self, from: input)
    } catch DecodingError.typeMismatch(_, let context) {
      XCTAssertEqual(1, context.codingPath.count)
      XCTAssertEqual("leave_me_alone", context.codingPath[0].stringValue)
    } catch {
      XCTFail("Unexpected error: \(String(describing: error))")
    }
  }

  private struct DecodeFailure : Decodable {
    var intValue: Int
  }

  private struct DecodeFailureNested : Decodable {
    var nestedValue: DecodeFailure
  }

  func testDecodingDictionaryFailureKeyPathNested() {
    let input = "{\"top_level\": {\"sub_level\": {\"nested_value\": {\"int_value\": \"not_an_int\"}}}}".data(using: .utf8)!
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase
    do {
      _ = try decoder.decode([String: [String : DecodeFailureNested]].self, from: input)
    } catch DecodingError.typeMismatch(_, let context) {
      XCTAssertEqual(4, context.codingPath.count)
      XCTAssertEqual("top_level", context.codingPath[0].stringValue)
      XCTAssertEqual("sub_level", context.codingPath[1].stringValue)
      XCTAssertEqual("nestedValue", context.codingPath[2].stringValue)
      XCTAssertEqual("intValue", context.codingPath[3].stringValue)
    } catch {
      XCTFail("Unexpected error: \(String(describing: error))")
    }
  }
  
  private struct DecodeMe3 : Codable {
      var thisIsCamelCase : String
  }
    
  func testEncodingKeyStrategySnakeGenerated() {
    // Test that this works with a struct that has automatically generated keys
    let input = "{\"this_is_camel_case\":\"test\"}".data(using: .utf8)!
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase
    let result = try! decoder.decode(DecodeMe3.self, from: input)
    
    XCTAssertEqual("test", result.thisIsCamelCase)
  }
    
  func testDecodingKeyStrategyCamelGenerated() {
    let encoded = DecodeMe3(thisIsCamelCase: "test")
    let encoder = JSONEncoder()
    encoder.keyEncodingStrategy = .convertToSnakeCase
    let resultData = try! encoder.encode(encoded)
    let resultString = String(bytes: resultData, encoding: .utf8)
    XCTAssertEqual("{\"this_is_camel_case\":\"test\"}", resultString)
  }
    
  func testKeyStrategySnakeGeneratedAndCustom() {
    // Test that this works with a struct that has automatically generated keys
    struct DecodeMe4 : Codable {
        var thisIsCamelCase : String
        var thisIsCamelCaseToo : String
        private enum CodingKeys : String, CodingKey {
            case thisIsCamelCase = "fooBar"
            case thisIsCamelCaseToo
        }
    }

    // Decoding
    let input = "{\"foo_bar\":\"test\",\"this_is_camel_case_too\":\"test2\"}".data(using: .utf8)!
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase
    let decodingResult = try! decoder.decode(DecodeMe4.self, from: input)
    
    XCTAssertEqual("test", decodingResult.thisIsCamelCase)
    XCTAssertEqual("test2", decodingResult.thisIsCamelCaseToo)
    
    // Encoding
    let encoded = DecodeMe4(thisIsCamelCase: "test", thisIsCamelCaseToo: "test2")
    let encoder = JSONEncoder()
    encoder.keyEncodingStrategy = .convertToSnakeCase
    let encodingResultData = try! encoder.encode(encoded)
    let encodingResultString = String(bytes: encodingResultData, encoding: .utf8)
    XCTAssertEqual("{\"foo_bar\":\"test\",\"this_is_camel_case_too\":\"test2\"}", encodingResultString)
  }

  func testKeyStrategyDuplicateKeys() {
    // This test is mostly to make sure we don't assert on duplicate keys
    struct DecodeMe5 : Codable {
        var oneTwo : String
        var numberOfKeys : Int
        
        enum CodingKeys : String, CodingKey {
          case oneTwo
          case oneTwoThree
        }
        
        init() {
            oneTwo = "test"
            numberOfKeys = 0
        }
        
        init(from decoder: Decoder) throws {
          let container = try decoder.container(keyedBy: CodingKeys.self)
          oneTwo = try container.decode(String.self, forKey: .oneTwo)
          numberOfKeys = container.allKeys.count
        }
        
        func encode(to encoder: Encoder) throws {
          var container = encoder.container(keyedBy: CodingKeys.self)
          try container.encode(oneTwo, forKey: .oneTwo)
          try container.encode("test2", forKey: .oneTwoThree)
        }
    }

    let customKeyConversion = { (_ path: [CodingKey]) -> CodingKey in
      // All keys are the same!
      return _TestKey(stringValue: "oneTwo")!
    }
    
    // Decoding
    // This input has a dictionary with two keys, but only one will end up in the container
    let input = "{\"unused key 1\":\"test1\",\"unused key 2\":\"test2\"}".data(using: .utf8)!
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .custom(customKeyConversion)

    let decodingResult = try! decoder.decode(DecodeMe5.self, from: input)
    // There will be only one result for oneTwo (the second one in the json)
    XCTAssertEqual(1, decodingResult.numberOfKeys)
    
    // Encoding
    let encoded = DecodeMe5()
    let encoder = JSONEncoder()
    encoder.keyEncodingStrategy = .custom(customKeyConversion)
    let decodingResultData = try! encoder.encode(encoded)
    let decodingResultString = String(bytes: decodingResultData, encoding: .utf8)
    
    // There will be only one value in the result (the second one encoded)
    XCTAssertEqual("{\"oneTwo\":\"test2\"}", decodingResultString)
  }

  // MARK: - Encoder Features
  func testNestedContainerCodingPaths() {
    let encoder = JSONEncoder()
    do {
      let _ = try encoder.encode(NestedContainersTestType())
    } catch let error as NSError {
      XCTFail("Caught error during encoding nested container types: \(error)")
    }
  }

  func testSuperEncoderCodingPaths() {
    let encoder = JSONEncoder()
    do {
      let _ = try encoder.encode(NestedContainersTestType(testSuperEncoder: true))
    } catch let error as NSError {
      XCTFail("Caught error during encoding nested container types: \(error)")
    }
  }

  func testInterceptDecimal() {
    let expectedJSON = "10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000".data(using: .utf8)!

    // Want to make sure we write out a JSON number, not the keyed encoding here.
    // 1e127 is too big to fit natively in a Double, too, so want to make sure it's encoded as a Decimal.
    let decimal = Decimal(sign: .plus, exponent: 127, significand: Decimal(1))
    _testRoundTrip(of: decimal, expectedJSON: expectedJSON)

    // Optional Decimals should encode the same way.
    _testRoundTrip(of: Optional(decimal), expectedJSON: expectedJSON)
  }

  func testInterceptURL() {
    // Want to make sure JSONEncoder writes out single-value URLs, not the keyed encoding.
    let expectedJSON = "\"http:\\/\\/swift.org\"".data(using: .utf8)!
    let url = URL(string: "http://swift.org")!
    _testRoundTrip(of: url, expectedJSON: expectedJSON)

    // Optional URLs should encode the same way.
    _testRoundTrip(of: Optional(url), expectedJSON: expectedJSON)
  }

  func testInterceptURLWithoutEscapingOption() {
    if #available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *) {
      // Want to make sure JSONEncoder writes out single-value URLs, not the keyed encoding.
      let expectedJSON = "\"http://swift.org\"".data(using: .utf8)!
      let url = URL(string: "http://swift.org")!
      _testRoundTrip(of: url, expectedJSON: expectedJSON, outputFormatting: [.withoutEscapingSlashes])

      // Optional URLs should encode the same way.
      _testRoundTrip(of: Optional(url), expectedJSON: expectedJSON, outputFormatting: [.withoutEscapingSlashes])
    }
  }
    
  // MARK: - Type coercion
  func testTypeCoercion() {
    _testRoundTripTypeCoercionFailure(of: [false, true], as: [Int].self)
    _testRoundTripTypeCoercionFailure(of: [false, true], as: [Int8].self)
    _testRoundTripTypeCoercionFailure(of: [false, true], as: [Int16].self)
    _testRoundTripTypeCoercionFailure(of: [false, true], as: [Int32].self)
    _testRoundTripTypeCoercionFailure(of: [false, true], as: [Int64].self)
    _testRoundTripTypeCoercionFailure(of: [false, true], as: [UInt].self)
    _testRoundTripTypeCoercionFailure(of: [false, true], as: [UInt8].self)
    _testRoundTripTypeCoercionFailure(of: [false, true], as: [UInt16].self)
    _testRoundTripTypeCoercionFailure(of: [false, true], as: [UInt32].self)
    _testRoundTripTypeCoercionFailure(of: [false, true], as: [UInt64].self)
    _testRoundTripTypeCoercionFailure(of: [false, true], as: [Float].self)
    _testRoundTripTypeCoercionFailure(of: [false, true], as: [Double].self)
    _testRoundTripTypeCoercionFailure(of: [0, 1] as [Int], as: [Bool].self)
    _testRoundTripTypeCoercionFailure(of: [0, 1] as [Int8], as: [Bool].self)
    _testRoundTripTypeCoercionFailure(of: [0, 1] as [Int16], as: [Bool].self)
    _testRoundTripTypeCoercionFailure(of: [0, 1] as [Int32], as: [Bool].self)
    _testRoundTripTypeCoercionFailure(of: [0, 1] as [Int64], as: [Bool].self)
    _testRoundTripTypeCoercionFailure(of: [0, 1] as [UInt], as: [Bool].self)
    _testRoundTripTypeCoercionFailure(of: [0, 1] as [UInt8], as: [Bool].self)
    _testRoundTripTypeCoercionFailure(of: [0, 1] as [UInt16], as: [Bool].self)
    _testRoundTripTypeCoercionFailure(of: [0, 1] as [UInt32], as: [Bool].self)
    _testRoundTripTypeCoercionFailure(of: [0, 1] as [UInt64], as: [Bool].self)
    _testRoundTripTypeCoercionFailure(of: [0.0, 1.0] as [Float], as: [Bool].self)
    _testRoundTripTypeCoercionFailure(of: [0.0, 1.0] as [Double], as: [Bool].self)
  }

  func testDecodingConcreteTypeParameter() {
      let encoder = JSONEncoder()
      guard let json = try? encoder.encode(Employee.testValue) else {
          XCTFail("Unable to encode Employee.")
          return
      }

      let decoder = JSONDecoder()
      guard let decoded = try? decoder.decode(Employee.self as Person.Type, from: json) else {
          XCTFail("Failed to decode Employee as Person from JSON.")
          return
      }

      expectEqual(type(of: decoded), Employee.self, "Expected decoded value to be of type Employee; got \(type(of: decoded)) instead.")
  }

  // MARK: - Encoder State
  // SR-6078
  func testEncoderStateThrowOnEncode() {
    struct ReferencingEncoderWrapper<T : Encodable> : Encodable {
      let value: T
      init(_ value: T) { self.value = value }

      func encode(to encoder: Encoder) throws {
        // This approximates a subclass calling into its superclass, where the superclass encodes a value that might throw.
        // The key here is that getting the superEncoder creates a referencing encoder.
        var container = encoder.unkeyedContainer()
        let superEncoder = container.superEncoder()

        // Pushing a nested container on leaves the referencing encoder with multiple containers.
        var nestedContainer = superEncoder.unkeyedContainer()
        try nestedContainer.encode(value)
      }
    }

    // The structure that would be encoded here looks like
    //
    //   [[[Float.infinity]]]
    //
    // The wrapper asks for an unkeyed container ([^]), gets a super encoder, and creates a nested container into that ([[^]]).
    // We then encode an array into that ([[[^]]]), which happens to be a value that causes us to throw an error.
    //
    // The issue at hand reproduces when you have a referencing encoder (superEncoder() creates one) that has a container on the stack (unkeyedContainer() adds one) that encodes a value going through box_() (Array does that) that encodes something which throws (Float.infinity does that).
    // When reproducing, this will cause a test failure via fatalError().
    _ = try? JSONEncoder().encode(ReferencingEncoderWrapper([Float.infinity]))
  }

  func testEncoderStateThrowOnEncodeCustomDate() {
    // This test is identical to testEncoderStateThrowOnEncode, except throwing via a custom Date closure.
    struct ReferencingEncoderWrapper<T : Encodable> : Encodable {
      let value: T
      init(_ value: T) { self.value = value }
      func encode(to encoder: Encoder) throws {
        var container = encoder.unkeyedContainer()
        let superEncoder = container.superEncoder()
        var nestedContainer = superEncoder.unkeyedContainer()
        try nestedContainer.encode(value)
      }
    }

    // The closure needs to push a container before throwing an error to trigger.
    let encoder = JSONEncoder()
    encoder.dateEncodingStrategy = .custom({ _, encoder in
      let _ = encoder.unkeyedContainer()
      enum CustomError : Error { case foo }
      throw CustomError.foo
    })

    _ = try? encoder.encode(ReferencingEncoderWrapper(Date()))
  }

  func testEncoderStateThrowOnEncodeCustomData() {
    // This test is identical to testEncoderStateThrowOnEncode, except throwing via a custom Data closure.
    struct ReferencingEncoderWrapper<T : Encodable> : Encodable {
      let value: T
      init(_ value: T) { self.value = value }
      func encode(to encoder: Encoder) throws {
        var container = encoder.unkeyedContainer()
        let superEncoder = container.superEncoder()
        var nestedContainer = superEncoder.unkeyedContainer()
        try nestedContainer.encode(value)
      }
    }

    // The closure needs to push a container before throwing an error to trigger.
    let encoder = JSONEncoder()
    encoder.dataEncodingStrategy = .custom({ _, encoder in
      let _ = encoder.unkeyedContainer()
      enum CustomError : Error { case foo }
      throw CustomError.foo
    })

    _ = try? encoder.encode(ReferencingEncoderWrapper(Data()))
  }

  // MARK: - Decoder State
  // SR-6048
  func testDecoderStateThrowOnDecode() {
    // The container stack here starts as [[1,2,3]]. Attempting to decode as [String] matches the outer layer (Array), and begins decoding the array.
    // Once Array decoding begins, 1 is pushed onto the container stack ([[1,2,3], 1]), and 1 is attempted to be decoded as String. This throws a .typeMismatch, but the container is not popped off the stack.
    // When attempting to decode [Int], the container stack is still ([[1,2,3], 1]), and 1 fails to decode as [Int].
    let json = "[1,2,3]".data(using: .utf8)!
    let _ = try! JSONDecoder().decode(EitherDecodable<[String], [Int]>.self, from: json)
  }

  func testDecoderStateThrowOnDecodeCustomDate() {
    // This test is identical to testDecoderStateThrowOnDecode, except we're going to fail because our closure throws an error, not because we hit a type mismatch.
    let decoder = JSONDecoder()
    decoder.dateDecodingStrategy = .custom({ decoder in
      enum CustomError : Error { case foo }
      throw CustomError.foo
    })

    let json = "1".data(using: .utf8)!
    let _ = try! decoder.decode(EitherDecodable<Date, Int>.self, from: json)
  }

  func testDecoderStateThrowOnDecodeCustomData() {
    // This test is identical to testDecoderStateThrowOnDecode, except we're going to fail because our closure throws an error, not because we hit a type mismatch.
    let decoder = JSONDecoder()
    decoder.dataDecodingStrategy = .custom({ decoder in
      enum CustomError : Error { case foo }
      throw CustomError.foo
    })

    let json = "1".data(using: .utf8)!
    let _ = try! decoder.decode(EitherDecodable<Data, Int>.self, from: json)
  }

  // MARK: - Helper Functions
  private var _jsonEmptyDictionary: Data {
    return "{}".data(using: .utf8)!
  }

  private func _testEncodeFailure<T : Encodable>(of value: T) {
    do {
      let _ = try JSONEncoder().encode(value)
      XCTFail("Encode of top-level \(T.self) was expected to fail.")
    } catch {}
  }

  private func _testRoundTrip<T>(of value: T,
                                 expectedJSON json: Data? = nil,
                                 outputFormatting: JSONEncoder.OutputFormatting = [],
                                 dateEncodingStrategy: JSONEncoder.DateEncodingStrategy = .deferredToDate,
                                 dateDecodingStrategy: JSONDecoder.DateDecodingStrategy = .deferredToDate,
                                 dataEncodingStrategy: JSONEncoder.DataEncodingStrategy = .base64,
                                 dataDecodingStrategy: JSONDecoder.DataDecodingStrategy = .base64,
                                 keyEncodingStrategy: JSONEncoder.KeyEncodingStrategy = .useDefaultKeys,
                                 keyDecodingStrategy: JSONDecoder.KeyDecodingStrategy = .useDefaultKeys,
                                 nonConformingFloatEncodingStrategy: JSONEncoder.NonConformingFloatEncodingStrategy = .throw,
                                 nonConformingFloatDecodingStrategy: JSONDecoder.NonConformingFloatDecodingStrategy = .throw) where T : Codable, T : Equatable {
    var payload: Data! = nil
    do {
      let encoder = JSONEncoder()
      encoder.outputFormatting = outputFormatting
      encoder.dateEncodingStrategy = dateEncodingStrategy
      encoder.dataEncodingStrategy = dataEncodingStrategy
      encoder.nonConformingFloatEncodingStrategy = nonConformingFloatEncodingStrategy
      encoder.keyEncodingStrategy = keyEncodingStrategy
      payload = try encoder.encode(value)
    } catch {
      XCTFail("Failed to encode \(T.self) to JSON: \(error)")
    }

    if let expectedJSON = json {
        XCTAssertEqual(expectedJSON, payload, "Produced JSON not identical to expected JSON.")
    }

    do {
      let decoder = JSONDecoder()
      decoder.dateDecodingStrategy = dateDecodingStrategy
      decoder.dataDecodingStrategy = dataDecodingStrategy
      decoder.nonConformingFloatDecodingStrategy = nonConformingFloatDecodingStrategy
      decoder.keyDecodingStrategy = keyDecodingStrategy
      let decoded = try decoder.decode(T.self, from: payload)
      XCTAssertEqual(decoded, value, "\(T.self) did not round-trip to an equal value.")
    } catch {
      XCTFail("Failed to decode \(T.self) from JSON: \(error)")
    }
  }

    private func _testRoundTripTypeCoercionFailure<T,U>(of value: T, as type: U.Type) where T : Codable, U : Codable {
        do {
            let data = try JSONEncoder().encode(value)
            let _ = try JSONDecoder().decode(U.self, from: data)
            XCTFail("Coercion from \(T.self) to \(U.self) was expected to fail.")
        } catch {}
    }
}

// MARK: - Helper Global Functions
func expectEqualPaths(_ lhs: [CodingKey], _ rhs: [CodingKey], _ prefix: String) {
  if lhs.count != rhs.count {
    XCTFail("\(prefix) [CodingKey].count mismatch: \(lhs.count) != \(rhs.count)")
    return
  }

  for (key1, key2) in zip(lhs, rhs) {
    switch (key1.intValue, key2.intValue) {
    case (.none, .none): break
    case (.some(let i1), .none):
      XCTFail("\(prefix) CodingKey.intValue mismatch: \(type(of: key1))(\(i1)) != nil")
      return
    case (.none, .some(let i2)):
      XCTFail("\(prefix) CodingKey.intValue mismatch: nil != \(type(of: key2))(\(i2))")
      return
    case (.some(let i1), .some(let i2)):
        guard i1 == i2 else {
            XCTFail("\(prefix) CodingKey.intValue mismatch: \(type(of: key1))(\(i1)) != \(type(of: key2))(\(i2))")
            return
        }
    }

    XCTAssertEqual(key1.stringValue, key2.stringValue, "\(prefix) CodingKey.stringValue mismatch: \(type(of: key1))('\(key1.stringValue)') != \(type(of: key2))('\(key2.stringValue)')")
  }
}

// MARK: - Test Types
/* FIXME: Import from %S/Inputs/Coding/SharedTypes.swift somehow. */

// MARK: - Empty Types
fileprivate struct EmptyStruct : Codable, Equatable {
  static func ==(_ lhs: EmptyStruct, _ rhs: EmptyStruct) -> Bool {
    return true
  }
}

fileprivate class EmptyClass : Codable, Equatable {
  static func ==(_ lhs: EmptyClass, _ rhs: EmptyClass) -> Bool {
    return true
  }
}

// MARK: - Single-Value Types
/// A simple on-off switch type that encodes as a single Bool value.
fileprivate enum Switch : Codable {
  case off
  case on

  init(from decoder: Decoder) throws {
    let container = try decoder.singleValueContainer()
    switch try container.decode(Bool.self) {
    case false: self = .off
    case true:  self = .on
    }
  }

  func encode(to encoder: Encoder) throws {
    var container = encoder.singleValueContainer()
    switch self {
    case .off: try container.encode(false)
    case .on:  try container.encode(true)
    }
  }
}

/// A simple timestamp type that encodes as a single Double value.
fileprivate struct Timestamp : Codable, Equatable {
  let value: Double

  init(_ value: Double) {
    self.value = value
  }

  init(from decoder: Decoder) throws {
    let container = try decoder.singleValueContainer()
    value = try container.decode(Double.self)
  }

  func encode(to encoder: Encoder) throws {
    var container = encoder.singleValueContainer()
    try container.encode(self.value)
  }

  static func ==(_ lhs: Timestamp, _ rhs: Timestamp) -> Bool {
    return lhs.value == rhs.value
  }
}

/// A simple referential counter type that encodes as a single Int value.
fileprivate final class Counter : Codable, Equatable {
  var count: Int = 0

  init() {}

  init(from decoder: Decoder) throws {
    let container = try decoder.singleValueContainer()
    count = try container.decode(Int.self)
  }

  func encode(to encoder: Encoder) throws {
    var container = encoder.singleValueContainer()
    try container.encode(self.count)
  }

  static func ==(_ lhs: Counter, _ rhs: Counter) -> Bool {
    return lhs === rhs || lhs.count == rhs.count
  }
}

// MARK: - Structured Types
/// A simple address type that encodes as a dictionary of values.
fileprivate struct Address : Codable, Equatable {
  let street: String
  let city: String
  let state: String
  let zipCode: Int
  let country: String

  init(street: String, city: String, state: String, zipCode: Int, country: String) {
    self.street = street
    self.city = city
    self.state = state
    self.zipCode = zipCode
    self.country = country
  }

  static func ==(_ lhs: Address, _ rhs: Address) -> Bool {
    return lhs.street == rhs.street &&
           lhs.city == rhs.city &&
           lhs.state == rhs.state &&
           lhs.zipCode == rhs.zipCode &&
           lhs.country == rhs.country
  }

  static var testValue: Address {
    return Address(street: "1 Infinite Loop",
                   city: "Cupertino",
                   state: "CA",
                   zipCode: 95014,
                   country: "United States")
  }
}

/// A simple person class that encodes as a dictionary of values.
fileprivate class Person : Codable, Equatable {
  let name: String
  let email: String
  let website: URL?

  init(name: String, email: String, website: URL? = nil) {
    self.name = name
    self.email = email
    self.website = website
  }

  func isEqual(_ other: Person) -> Bool {
    return self.name == other.name &&
           self.email == other.email &&
           self.website == other.website
  }

  static func ==(_ lhs: Person, _ rhs: Person) -> Bool {
    return lhs.isEqual(rhs)
  }

  class var testValue: Person {
    return Person(name: "Johnny Appleseed", email: "appleseed@apple.com")
  }
}

/// A class which shares its encoder and decoder with its superclass.
fileprivate class Employee : Person {
  let id: Int

  init(name: String, email: String, website: URL? = nil, id: Int) {
    self.id = id
    super.init(name: name, email: email, website: website)
  }

  enum CodingKeys : String, CodingKey {
    case id
  }

  required init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    id = try container.decode(Int.self, forKey: .id)
    try super.init(from: decoder)
  }

  override func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    try container.encode(id, forKey: .id)
    try super.encode(to: encoder)
  }

  override func isEqual(_ other: Person) -> Bool {
    if let employee = other as? Employee {
      guard self.id == employee.id else { return false }
    }

    return super.isEqual(other)
  }

  override class var testValue: Employee {
    return Employee(name: "Johnny Appleseed", email: "appleseed@apple.com", id: 42)
  }
}

/// A simple company struct which encodes as a dictionary of nested values.
fileprivate struct Company : Codable, Equatable {
  let address: Address
  var employees: [Employee]

  init(address: Address, employees: [Employee]) {
    self.address = address
    self.employees = employees
  }

  static func ==(_ lhs: Company, _ rhs: Company) -> Bool {
    return lhs.address == rhs.address && lhs.employees == rhs.employees
  }

  static var testValue: Company {
    return Company(address: Address.testValue, employees: [Employee.testValue])
  }
}

/// An enum type which decodes from Bool?.
fileprivate enum EnhancedBool : Codable {
  case `true`
  case `false`
  case fileNotFound

  init(from decoder: Decoder) throws {
    let container = try decoder.singleValueContainer()
    if container.decodeNil() {
      self = .fileNotFound
    } else {
      let value = try container.decode(Bool.self)
      self = value ? .true : .false
    }
  }

  func encode(to encoder: Encoder) throws {
    var container = encoder.singleValueContainer()
    switch self {
    case .true: try container.encode(true)
    case .false: try container.encode(false)
    case .fileNotFound: try container.encodeNil()
    }
  }
}

/// A type which encodes as an array directly through a single value container.
private struct Numbers : Codable, Equatable {
  let values = [4, 8, 15, 16, 23, 42]

  init() {}

  init(from decoder: Decoder) throws {
    let container = try decoder.singleValueContainer()
    let decodedValues = try container.decode([Int].self)
    guard decodedValues == values else {
      throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "The Numbers are wrong!"))
    }
  }

  func encode(to encoder: Encoder) throws {
    var container = encoder.singleValueContainer()
    try container.encode(values)
  }

  static func ==(_ lhs: Numbers, _ rhs: Numbers) -> Bool {
    return lhs.values == rhs.values
  }

  static var testValue: Numbers {
    return Numbers()
  }
}

/// A type which encodes as a dictionary directly through a single value container.
fileprivate final class Mapping : Codable, Equatable {
  let values: [String : URL]

  init(values: [String : URL]) {
    self.values = values
  }

  init(from decoder: Decoder) throws {
    let container = try decoder.singleValueContainer()
    values = try container.decode([String : URL].self)
  }

  func encode(to encoder: Encoder) throws {
    var container = encoder.singleValueContainer()
    try container.encode(values)
  }

  static func ==(_ lhs: Mapping, _ rhs: Mapping) -> Bool {
    return lhs === rhs || lhs.values == rhs.values
  }

  static var testValue: Mapping {
    return Mapping(values: ["Apple": URL(string: "http://apple.com")!,
                            "localhost": URL(string: "http://127.0.0.1")!])
  }
}

private struct NestedContainersTestType : Encodable {
  let testSuperEncoder: Bool

  init(testSuperEncoder: Bool = false) {
    self.testSuperEncoder = testSuperEncoder
  }

  enum TopLevelCodingKeys : Int, CodingKey {
    case a
    case b
    case c
  }

  enum IntermediateCodingKeys : Int, CodingKey {
      case one
      case two
  }

  func encode(to encoder: Encoder) throws {
    if self.testSuperEncoder {
      var topLevelContainer = encoder.container(keyedBy: TopLevelCodingKeys.self)
      expectEqualPaths(encoder.codingPath, [], "Top-level Encoder's codingPath changed.")
      expectEqualPaths(topLevelContainer.codingPath, [], "New first-level keyed container has non-empty codingPath.")

      let superEncoder = topLevelContainer.superEncoder(forKey: .a)
      expectEqualPaths(encoder.codingPath, [], "Top-level Encoder's codingPath changed.")
      expectEqualPaths(topLevelContainer.codingPath, [], "First-level keyed container's codingPath changed.")
      expectEqualPaths(superEncoder.codingPath, [TopLevelCodingKeys.a], "New superEncoder had unexpected codingPath.")
      _testNestedContainers(in: superEncoder, baseCodingPath: [TopLevelCodingKeys.a])
    } else {
      _testNestedContainers(in: encoder, baseCodingPath: [])
    }
  }

  func _testNestedContainers(in encoder: Encoder, baseCodingPath: [CodingKey]) {
    expectEqualPaths(encoder.codingPath, baseCodingPath, "New encoder has non-empty codingPath.")

    // codingPath should not change upon fetching a non-nested container.
    var firstLevelContainer = encoder.container(keyedBy: TopLevelCodingKeys.self)
    expectEqualPaths(encoder.codingPath, baseCodingPath, "Top-level Encoder's codingPath changed.")
    expectEqualPaths(firstLevelContainer.codingPath, baseCodingPath, "New first-level keyed container has non-empty codingPath.")

    // Nested Keyed Container
    do {
      // Nested container for key should have a new key pushed on.
      var secondLevelContainer = firstLevelContainer.nestedContainer(keyedBy: IntermediateCodingKeys.self, forKey: .a)
      expectEqualPaths(encoder.codingPath, baseCodingPath, "Top-level Encoder's codingPath changed.")
      expectEqualPaths(firstLevelContainer.codingPath, baseCodingPath, "First-level keyed container's codingPath changed.")
      expectEqualPaths(secondLevelContainer.codingPath, baseCodingPath + [TopLevelCodingKeys.a], "New second-level keyed container had unexpected codingPath.")

      // Inserting a keyed container should not change existing coding paths.
      let thirdLevelContainerKeyed = secondLevelContainer.nestedContainer(keyedBy: IntermediateCodingKeys.self, forKey: .one)
      expectEqualPaths(encoder.codingPath, baseCodingPath, "Top-level Encoder's codingPath changed.")
      expectEqualPaths(firstLevelContainer.codingPath, baseCodingPath, "First-level keyed container's codingPath changed.")
      expectEqualPaths(secondLevelContainer.codingPath, baseCodingPath + [TopLevelCodingKeys.a], "Second-level keyed container's codingPath changed.")
      expectEqualPaths(thirdLevelContainerKeyed.codingPath, baseCodingPath + [TopLevelCodingKeys.a, IntermediateCodingKeys.one], "New third-level keyed container had unexpected codingPath.")

      // Inserting an unkeyed container should not change existing coding paths.
      let thirdLevelContainerUnkeyed = secondLevelContainer.nestedUnkeyedContainer(forKey: .two)
      expectEqualPaths(encoder.codingPath, baseCodingPath + [], "Top-level Encoder's codingPath changed.")
      expectEqualPaths(firstLevelContainer.codingPath, baseCodingPath + [], "First-level keyed container's codingPath changed.")
      expectEqualPaths(secondLevelContainer.codingPath, baseCodingPath + [TopLevelCodingKeys.a], "Second-level keyed container's codingPath changed.")
      expectEqualPaths(thirdLevelContainerUnkeyed.codingPath, baseCodingPath + [TopLevelCodingKeys.a, IntermediateCodingKeys.two], "New third-level unkeyed container had unexpected codingPath.")
    }

    // Nested Unkeyed Container
    do {
      // Nested container for key should have a new key pushed on.
      var secondLevelContainer = firstLevelContainer.nestedUnkeyedContainer(forKey: .b)
      expectEqualPaths(encoder.codingPath, baseCodingPath, "Top-level Encoder's codingPath changed.")
      expectEqualPaths(firstLevelContainer.codingPath, baseCodingPath, "First-level keyed container's codingPath changed.")
      expectEqualPaths(secondLevelContainer.codingPath, baseCodingPath + [TopLevelCodingKeys.b], "New second-level keyed container had unexpected codingPath.")

      // Appending a keyed container should not change existing coding paths.
      let thirdLevelContainerKeyed = secondLevelContainer.nestedContainer(keyedBy: IntermediateCodingKeys.self)
      expectEqualPaths(encoder.codingPath, baseCodingPath, "Top-level Encoder's codingPath changed.")
      expectEqualPaths(firstLevelContainer.codingPath, baseCodingPath, "First-level keyed container's codingPath changed.")
      expectEqualPaths(secondLevelContainer.codingPath, baseCodingPath + [TopLevelCodingKeys.b], "Second-level unkeyed container's codingPath changed.")
      expectEqualPaths(thirdLevelContainerKeyed.codingPath, baseCodingPath + [TopLevelCodingKeys.b, _TestKey(index: 0)], "New third-level keyed container had unexpected codingPath.")

      // Appending an unkeyed container should not change existing coding paths.
      let thirdLevelContainerUnkeyed = secondLevelContainer.nestedUnkeyedContainer()
      expectEqualPaths(encoder.codingPath, baseCodingPath, "Top-level Encoder's codingPath changed.")
      expectEqualPaths(firstLevelContainer.codingPath, baseCodingPath, "First-level keyed container's codingPath changed.")
      expectEqualPaths(secondLevelContainer.codingPath, baseCodingPath + [TopLevelCodingKeys.b], "Second-level unkeyed container's codingPath changed.")
      expectEqualPaths(thirdLevelContainerUnkeyed.codingPath, baseCodingPath + [TopLevelCodingKeys.b, _TestKey(index: 1)], "New third-level unkeyed container had unexpected codingPath.")
    }
  }
}

// MARK: - Helper Types

/// A key type which can take on any string or integer value.
/// This needs to mirror _JSONKey.
fileprivate struct _TestKey : CodingKey {
  var stringValue: String
  var intValue: Int?

  init?(stringValue: String) {
    self.stringValue = stringValue
    self.intValue = nil
  }

  init?(intValue: Int) {
    self.stringValue = "\(intValue)"
    self.intValue = intValue
  }

  init(index: Int) {
    self.stringValue = "Index \(index)"
    self.intValue = index
  }
}

fileprivate struct FloatNaNPlaceholder : Codable, Equatable {
  init() {}

  func encode(to encoder: Encoder) throws {
    var container = encoder.singleValueContainer()
    try container.encode(Float.nan)
  }

  init(from decoder: Decoder) throws {
    let container = try decoder.singleValueContainer()
    let float = try container.decode(Float.self)
    if !float.isNaN {
      throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Couldn't decode NaN."))
    }
  }

  static func ==(_ lhs: FloatNaNPlaceholder, _ rhs: FloatNaNPlaceholder) -> Bool {
    return true
  }
}

fileprivate struct DoubleNaNPlaceholder : Codable, Equatable {
  init() {}

  func encode(to encoder: Encoder) throws {
    var container = encoder.singleValueContainer()
    try container.encode(Double.nan)
  }

  init(from decoder: Decoder) throws {
    let container = try decoder.singleValueContainer()
    let double = try container.decode(Double.self)
    if !double.isNaN {
      throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Couldn't decode NaN."))
    }
  }

  static func ==(_ lhs: DoubleNaNPlaceholder, _ rhs: DoubleNaNPlaceholder) -> Bool {
    return true
  }
}

fileprivate enum EitherDecodable<T : Decodable, U : Decodable> : Decodable {
  case t(T)
  case u(U)

  init(from decoder: Decoder) throws {
    let container = try decoder.singleValueContainer()
    do {
      self = .t(try container.decode(T.self))
    } catch {
      self = .u(try container.decode(U.self))
    }
  }
}