TestJSONEncoder.swift

// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See http://swift.org/LICENSE.txt for license information
// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//

struct TopLevelObjectWrapper<T: Codable & Equatable>: Codable, Equatable {
    var value: T

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

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

class TestJSONEncoder : XCTestCase {

    // MARK: - Encoding Top-Level fragments
    func test_encodingTopLevelFragments() {

        func _testFragment<T: Codable & Equatable>(value: T, fragment: String) {
            let data: Data
            let payload: String

            do {
                data = try JSONEncoder().encode(value)
                payload = try XCTUnwrap(String.init(decoding: data, as: UTF8.self))
                XCTAssertEqual(fragment, payload)
            } catch {
                XCTFail("Failed to encode \(T.self) to JSON: \(error)")
                return
            }
            do {
                let decodedValue = try JSONDecoder().decode(T.self, from: data)
                XCTAssertEqual(value, decodedValue)
            } catch {
                XCTFail("Failed to decode \(payload) to \(T.self): \(error)")
            }
        }

        _testFragment(value: 2, fragment: "2")
        _testFragment(value: false, fragment: "false")
        _testFragment(value: true, fragment: "true")
        _testFragment(value: Float(1), fragment: "1")
        _testFragment(value: Double(2), fragment: "2")
        _testFragment(value: Decimal(Double(Float.leastNormalMagnitude)), fragment: "0.000000000000000000000000000000000000011754943508222875648")
        _testFragment(value: "test", fragment: "\"test\"")
        let v: Int? = nil
        _testFragment(value: v, fragment: "null")
    }

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

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

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

        _testRoundTrip(of: TopLevelArrayWrapper(Switch.off))
        _testRoundTrip(of: TopLevelArrayWrapper(Switch.on))
    }

    func test_encodingTopLevelSingleValueStruct() {
        _testRoundTrip(of: Timestamp(3141592653))
        _testRoundTrip(of: TopLevelArrayWrapper(Timestamp(3141592653)))
    }

    func test_encodingTopLevelSingleValueClass() {
        _testRoundTrip(of: Counter())
        _testRoundTrip(of: TopLevelArrayWrapper(Counter()))
    }

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

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

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

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

    // MARK: - Output Formatting Tests
    func test_encodingOutputFormattingDefault() {
        let expectedJSON = "{\"name\":\"Johnny Appleseed\",\"email\":\"appleseed@apple.com\"}".data(using: .utf8)!
        let person = Person.testValue
        _testRoundTrip(of: person, expectedJSON: expectedJSON)
    }

    func test_encodingOutputFormattingPrettyPrinted() throws {
        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])

        let encoder = JSONEncoder()
        if #available(macOS 10.13, *) {
            encoder.outputFormatting = [.prettyPrinted, .sortedKeys]
        } else {
            // Fallback on earlier versions
            encoder.outputFormatting = [.prettyPrinted]
        }

        let emptyArray: [Int] = []
        let arrayOutput = try encoder.encode(emptyArray)
        XCTAssertEqual(String.init(decoding: arrayOutput, as: UTF8.self), "[\n\n]")

        let emptyDictionary: [String: Int] = [:]
        let dictionaryOutput = try encoder.encode(emptyDictionary)
        XCTAssertEqual(String.init(decoding: dictionaryOutput, as: UTF8.self), "{\n\n}")

        struct DataType: Encodable {
            let array = [1, 2, 3]
            let dictionary: [String: Int] = [:]
            let emptyAray: [Int] = []
            let secondArray: [Int] = [4, 5, 6]
            let secondDictionary: [String: Int] = [ "one": 1, "two": 2, "three": 3]
            let singleElement: [Int] = [1]
            let subArray: [String: [Int]] = [ "array": [] ]
            let subDictionary: [String: [String: Int]] = [ "dictionary": [:] ]
        }

        let dataOutput = try encoder.encode([DataType(), DataType()])
        XCTAssertEqual(String.init(decoding: dataOutput, as: UTF8.self), """
[
  {
    "array" : [
      1,
      2,
      3
    ],
    "dictionary" : {

    },
    "emptyAray" : [

    ],
    "secondArray" : [
      4,
      5,
      6
    ],
    "secondDictionary" : {
      "one" : 1,
      "three" : 3,
      "two" : 2
    },
    "singleElement" : [
      1
    ],
    "subArray" : {
      "array" : [

      ]
    },
    "subDictionary" : {
      "dictionary" : {

      }
    }
  },
  {
    "array" : [
      1,
      2,
      3
    ],
    "dictionary" : {

    },
    "emptyAray" : [

    ],
    "secondArray" : [
      4,
      5,
      6
    ],
    "secondDictionary" : {
      "one" : 1,
      "three" : 3,
      "two" : 2
    },
    "singleElement" : [
      1
    ],
    "subArray" : {
      "array" : [

      ]
    },
    "subDictionary" : {
      "dictionary" : {

      }
    }
  }
]
""")
    }

    func test_encodingOutputFormattingSortedKeys() throws {
        let expectedJSON = try XCTUnwrap("""
        {"2":"2","7":"7","25":"25","ａｌｉｃｅ":"ａｌｉｃｅ","bob":"bob","Charlie":"Charlie","中国":"中国","日本":"日本","韓国":"韓国"}
        """.data(using: .utf8))
        let testValue = [
            "2": "2",
            "25": "25",
            "7": "7",
            "ａｌｉｃｅ": "ａｌｉｃｅ",
            "bob": "bob",
            "Charlie": "Charlie",
            "日本": "日本",
            "中国": "中国",
            "韓国": "韓国",
        ]
#if os(macOS) || DARWIN_COMPATIBILITY_TESTS
        if #available(macOS 10.13, iOS 11.0, watchOS 4.0, tvOS 11.0, *) {
            let encoder = JSONEncoder()
            encoder.outputFormatting = .sortedKeys
            let payload = try encoder.encode(testValue)
            XCTAssertEqual(expectedJSON, payload)
        }
#else
        let encoder = JSONEncoder()
        encoder.outputFormatting = .sortedKeys
        let payload = try encoder.encode(testValue)
        XCTAssertEqual(expectedJSON, payload)
#endif
    }

    func test_encodingOutputFormattingPrettyPrintedSortedKeys() throws {
        let expectedJSON = try XCTUnwrap("""
        {
          "2" : "2",
          "7" : "7",
          "25" : "25",
          "ａｌｉｃｅ" : "ａｌｉｃｅ",
          "bob" : "bob",
          "Charlie" : "Charlie",
          "中国" : "中国",
          "日本" : "日本",
          "韓国" : "韓国"
        }
        """.data(using: .utf8))
        let testValue = [
            "2": "2",
            "25": "25",
            "7": "7",
            "ａｌｉｃｅ": "ａｌｉｃｅ",
            "bob": "bob",
            "Charlie": "Charlie",
            "日本": "日本",
            "中国": "中国",
            "韓国": "韓国",
        ]
#if os(macOS) || DARWIN_COMPATIBILITY_TESTS
        if #available(macOS 10.13, iOS 11.0, watchOS 4.0, tvOS 11.0, *) {
            let encoder = JSONEncoder()
            encoder.outputFormatting = [.prettyPrinted, .sortedKeys]
            let payload = try encoder.encode(testValue)
            XCTAssertEqual(expectedJSON, payload)
        }
#else
        let encoder = JSONEncoder()
        encoder.outputFormatting = [.prettyPrinted, .sortedKeys]
        let payload = try encoder.encode(testValue)
        XCTAssertEqual(expectedJSON, payload)
#endif
    }

    // MARK: - Date Strategy Tests
    func test_encodingDate() {
        // We can't encode a top-level Date, so it'll be wrapped in an array.
        _testRoundTrip(of: TopLevelArrayWrapper(Date()))
    }

    func test_encodingDateSecondsSince1970() {
        // Cannot encode an arbitrary number of seconds since we've lost precision since 1970.
        let seconds = 1000.0
        let expectedJSON = "[1000]".data(using: .utf8)!

        // We can't encode a top-level Date, so it'll be wrapped in an array.
        _testRoundTrip(of: TopLevelArrayWrapper(Date(timeIntervalSince1970: seconds)),
                       expectedJSON: expectedJSON,
                       dateEncodingStrategy: .secondsSince1970,
                       dateDecodingStrategy: .secondsSince1970)
    }

    func test_encodingDateMillisecondsSince1970() {
        // Cannot encode an arbitrary number of seconds since we've lost precision since 1970.
        let seconds = 1000.0
        let expectedJSON = "[1000000]".data(using: .utf8)!

        // We can't encode a top-level Date, so it'll be wrapped in an array.
        _testRoundTrip(of: TopLevelArrayWrapper(Date(timeIntervalSince1970: seconds)),
                       expectedJSON: expectedJSON,
                       dateEncodingStrategy: .millisecondsSince1970,
                       dateDecodingStrategy: .millisecondsSince1970)
    }

    func test_encodingDateISO8601() {
        let formatter = ISO8601DateFormatter()
        formatter.formatOptions = .withInternetDateTime

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

        // We can't encode a top-level Date, so it'll be wrapped in an array.
        _testRoundTrip(of: TopLevelArrayWrapper(timestamp),
                       expectedJSON: expectedJSON,
                       dateEncodingStrategy: .iso8601,
                       dateDecodingStrategy: .iso8601)

    }

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

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

        // We can't encode a top-level Date, so it'll be wrapped in an array.
        _testRoundTrip(of: TopLevelArrayWrapper(timestamp),
                       expectedJSON: expectedJSON,
                       dateEncodingStrategy: .formatted(formatter),
                       dateDecodingStrategy: .formatted(formatter))
    }

    func test_encodingDateCustom() {
        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 }

        // We can't encode a top-level Date, so it'll be wrapped in an array.
        let expectedJSON = "[42]".data(using: .utf8)!
        _testRoundTrip(of: TopLevelArrayWrapper(timestamp),
                       expectedJSON: expectedJSON,
                       dateEncodingStrategy: .custom(encode),
                       dateDecodingStrategy: .custom(decode))
    }

    func test_encodingDateCustomEmpty() {
        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 }

        // We can't encode a top-level Date, so it'll be wrapped in an array.
        let expectedJSON = "[{}]".data(using: .utf8)!
        _testRoundTrip(of: TopLevelArrayWrapper(timestamp),
                       expectedJSON: expectedJSON,
                       dateEncodingStrategy: .custom(encode),
                       dateDecodingStrategy: .custom(decode))
    }

    // MARK: - Data Strategy Tests
    func test_encodingBase64Data() {
        let data = Data([0xDE, 0xAD, 0xBE, 0xEF])

        // We can't encode a top-level Data, so it'll be wrapped in an array.
        let expectedJSON = "[\"3q2+7w==\"]".data(using: .utf8)!
        _testRoundTrip(of: TopLevelArrayWrapper(data), expectedJSON: expectedJSON)
    }

    func test_encodingCustomData() {
        // 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() }

        // We can't encode a top-level Data, so it'll be wrapped in an array.
        let expectedJSON = "[42]".data(using: .utf8)!
        _testRoundTrip(of: TopLevelArrayWrapper(Data()),
                       expectedJSON: expectedJSON,
                       dataEncodingStrategy: .custom(encode),
                       dataDecodingStrategy: .custom(decode))
    }

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

        // We can't encode a top-level Data, so it'll be wrapped in an array.
        let expectedJSON = "[{}]".data(using: .utf8)!
        _testRoundTrip(of: TopLevelArrayWrapper(Data()),
                       expectedJSON: expectedJSON,
                       dataEncodingStrategy: .custom(encode),
                       dataDecodingStrategy: .custom(decode))
    }

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

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

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


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

        _testRoundTrip(of: TopLevelArrayWrapper(Double.infinity),
                       expectedJSON: "[\"INF\"]".data(using: .utf8)!,
                       nonConformingFloatEncodingStrategy: encodingStrategy,
                       nonConformingFloatDecodingStrategy: decodingStrategy)
        _testRoundTrip(of: TopLevelArrayWrapper(-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: TopLevelArrayWrapper(DoubleNaNPlaceholder()),
                       expectedJSON: "[\"NaN\"]".data(using: .utf8)!,
                       nonConformingFloatEncodingStrategy: encodingStrategy,
                       nonConformingFloatDecodingStrategy: decodingStrategy)
    }

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

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

    func test_notFoundSuperDecoder() {
        struct NotFoundSuperDecoderTestType: Decodable {
            init(from decoder: Decoder) throws {
                let container = try decoder.container(keyedBy: CodingKeys.self)
                _ = try container.superDecoder(forKey: .superDecoder)
            }

            private enum CodingKeys: String, CodingKey {
                case superDecoder = "super"
            }
        }
        let decoder = JSONDecoder()
        do {
            let _ = try decoder.decode(NotFoundSuperDecoderTestType.self, from: Data(#"{}"#.utf8))
        } catch {
            XCTFail("Caught error during decoding empty super decoder: \(error)")
        }
    }
    
    func test_childTypeDecoder() {
        class BaseTestType: Decodable { }
        class ChildTestType: BaseTestType { }

        func dynamicTestType() -> BaseTestType.Type {
            return ChildTestType.self
        }

        let decoder = JSONDecoder()
        do {
            let testType = dynamicTestType()
            let instance = try decoder.decode(testType, from: Data(#"{}"#.utf8))
            XCTAssertTrue(instance is ChildTestType)
        } catch {
            XCTFail("Caught error during decoding empty super decoder: \(error)")
        }
    }

    // MARK: - Test encoding and decoding of built-in Codable types
    func test_codingOfBool() {
        test_codingOf(value: Bool(true), toAndFrom: "true")
        test_codingOf(value: Bool(false), toAndFrom: "false")

        do {
            _ = try JSONDecoder().decode([Bool].self, from: "[1]".data(using: .utf8)!)
            XCTFail("Coercing non-boolean numbers into Bools was expected to fail")
        } catch { }


        // Check that a Bool false or true isn't converted to 0 or 1
        struct Foo: Decodable {
            var intValue: Int?
            var int8Value: Int8?
            var int16Value: Int16?
            var int32Value: Int32?
            var int64Value: Int64?
            var uintValue: UInt?
            var uint8Value: UInt8?
            var uint16Value: UInt16?
            var uint32Value: UInt32?
            var uint64Value: UInt64?
            var floatValue: Float?
            var doubleValue: Double?
            var decimalValue: Decimal?
            let boolValue: Bool
        }

        func testValue(_ valueName: String) {
            do {
                let jsonData = "{ \"\(valueName)\": false }".data(using: .utf8)!
                _ = try JSONDecoder().decode(Foo.self, from: jsonData)
                XCTFail("Decoded 'false' as non Bool for \(valueName)")
            } catch {}
            do {
                let jsonData = "{ \"\(valueName)\": true }".data(using: .utf8)!
                _ = try JSONDecoder().decode(Foo.self, from: jsonData)
                XCTFail("Decoded 'true' as non Bool for \(valueName)")
            } catch {}
        }

        testValue("intValue")
        testValue("int8Value")
        testValue("int16Value")
        testValue("int32Value")
        testValue("int64Value")
        testValue("uintValue")
        testValue("uint8Value")
        testValue("uint16Value")
        testValue("uint32Value")
        testValue("uint64Value")
        testValue("floatValue")
        testValue("doubleValue")
        testValue("decimalValue")
        let falseJsonData = "{ \"boolValue\": false }".data(using: .utf8)!
        if let falseFoo = try? JSONDecoder().decode(Foo.self, from: falseJsonData) {
            XCTAssertFalse(falseFoo.boolValue)
        } else {
            XCTFail("Could not decode 'false' as a Bool")
        }

        let trueJsonData = "{ \"boolValue\": true }".data(using: .utf8)!
        if let trueFoo = try? JSONDecoder().decode(Foo.self, from: trueJsonData) {
            XCTAssertTrue(trueFoo.boolValue)
        } else {
            XCTFail("Could not decode 'true' as a Bool")
        }
    }

    func test_codingOfNil() {
        let x: Int? = nil
        test_codingOf(value: x, toAndFrom: "null")
    }

    func test_codingOfInt8() {
        test_codingOf(value: Int8(-42), toAndFrom: "-42")
    }

    func test_codingOfUInt8() {
        test_codingOf(value: UInt8(42), toAndFrom: "42")
    }

    func test_codingOfInt16() {
        test_codingOf(value: Int16(-30042), toAndFrom: "-30042")
    }

    func test_codingOfUInt16() {
        test_codingOf(value: UInt16(30042), toAndFrom: "30042")
    }

    func test_codingOfInt32() {
        test_codingOf(value: Int32(-2000000042), toAndFrom: "-2000000042")
    }

    func test_codingOfUInt32() {
        test_codingOf(value: UInt32(2000000042), toAndFrom: "2000000042")
    }

    func test_codingOfInt64() {
#if !arch(arm)
        test_codingOf(value: Int64(-9000000000000000042), toAndFrom: "-9000000000000000042")
#endif
    }

    func test_codingOfUInt64() {
#if !arch(arm)
        test_codingOf(value: UInt64(9000000000000000042), toAndFrom: "9000000000000000042")
#endif
    }

    func test_codingOfInt() {
        let intSize = MemoryLayout<Int>.size
        switch intSize {
        case 4: // 32-bit
            test_codingOf(value: Int(-2000000042), toAndFrom: "-2000000042")
        case 8: // 64-bit
#if arch(arm)
            break
#else
            test_codingOf(value: Int(-9000000000000000042), toAndFrom: "-9000000000000000042")
#endif
        default:
            XCTFail("Unexpected UInt size: \(intSize)")
        }
    }

    func test_codingOfUInt() {
        let uintSize = MemoryLayout<UInt>.size
        switch uintSize {
        case 4: // 32-bit
            test_codingOf(value: UInt(2000000042), toAndFrom: "2000000042")
        case 8: // 64-bit
#if arch(arm)
            break
#else
            test_codingOf(value: UInt(9000000000000000042), toAndFrom: "9000000000000000042")
#endif
        default:
            XCTFail("Unexpected UInt size: \(uintSize)")
        }
    }

    func test_codingOfFloat() {
        test_codingOf(value: Float(1.5), toAndFrom: "1.5")

        // Check value too large fails to decode.
        XCTAssertThrowsError(try JSONDecoder().decode(Float.self, from: "1e100".data(using: .utf8)!))
    }

    func test_codingOfDouble() {
        test_codingOf(value: Double(1.5), toAndFrom: "1.5")

        // Check value too large fails to decode.
        XCTAssertThrowsError(try JSONDecoder().decode(Double.self, from: "100e323".data(using: .utf8)!))
    }

    func test_codingOfDecimal() {
        test_codingOf(value: Decimal.pi, toAndFrom: "3.14159265358979323846264338327950288419")

        // Check value too large fails to decode.
        XCTAssertThrowsError(try JSONDecoder().decode(Decimal.self, from: "100e200".data(using: .utf8)!))
    }

    func test_codingOfString() {
        test_codingOf(value: "Hello, world!", toAndFrom: "\"Hello, world!\"")
    }

    func test_codingOfURL() {
        test_codingOf(value: URL(string: "https://swift.org")!, toAndFrom: "\"https://swift.org\"")
    }


    // UInt and Int
    func test_codingOfUIntMinMax() {

        struct MyValue: Encodable {
            let int64Min = Int64.min
            let int64Max = Int64.max
            let uint64Min = UInt64.min
            let uint64Max = UInt64.max
        }

        func compareJSON(_ s1: String, _ s2: String) {
            let ss1 = s1.trimmingCharacters(in: CharacterSet(charactersIn: "{}")).split(separator: Character(",")).sorted()
            let ss2 = s2.trimmingCharacters(in: CharacterSet(charactersIn: "{}")).split(separator: Character(",")).sorted()
            XCTAssertEqual(ss1, ss2)
        }

        do {
            let encoder = JSONEncoder()
            let myValue = MyValue()
            let result = try encoder.encode(myValue)
            let r = String(data: result, encoding: .utf8) ?? "nil"
            compareJSON(r, "{\"uint64Min\":0,\"uint64Max\":18446744073709551615,\"int64Min\":-9223372036854775808,\"int64Max\":9223372036854775807}")
        } catch {
            XCTFail(String(describing: error))
        }
    }

    func test_encodeDecodeNumericTypesBaseline() throws {
        struct NumericTypesStruct: Codable, Equatable {
            let int8Value: Int8
            let uint8Value: UInt8
            let int16Value: Int16
            let uint16Value: UInt16
            let int32Value: Int32
            let uint32Value: UInt32
            let int64Value: Int64
            let intValue: Int
            let uintValue: UInt
            let uint64Value: UInt64
            let floatValue: Float
            let doubleValue: Double
            let decimalValue: Decimal
        }

        let source = NumericTypesStruct(
            int8Value: -12,
            uint8Value: 34,
            int16Value: -5678, 
            uint16Value: 9011,
            int32Value: -12141516,
            uint32Value: 17181920,
            int64Value: -21222324252627,
            intValue: -2829303132,
            uintValue: 33343536,
            uint64Value: 373839404142,
            floatValue: 1.234,
            doubleValue: 5.101520,
            decimalValue: Decimal(10))

        let data = try JSONEncoder().encode(source)
        let destination = try JSONDecoder().decode(NumericTypesStruct.self, from: data)
        XCTAssertEqual(source, destination)

        // Ensure that if a value is expressed as a floating point number, it casts correctly into the underlying type.

        let json = """
        {
            "int8Value": -12.0,
            "uint8Value": 34.0,
            "int16Value": -5678.0, 
            "uint16Value": 9011.0,
            "int32Value": -12141516.0,
            "uint32Value": 17181920.0,
            "int64Value": -21222324252627.0,
            "intValue": -2829303132.0,
            "uintValue": 33343536.0,
            "uint64Value": 373839404142.0,
            "floatValue": 1.234,
            "doubleValue": 5.101520,
            "decimalValue": 10.0
        }
        """

        let destination2 = try JSONDecoder().decode(NumericTypesStruct.self, from: Data(json.utf8))
        XCTAssertEqual(source, destination2)
    }

    func test_numericLimits() {
        struct DataStruct: Codable {
            let int8Value: Int8?
            let uint8Value: UInt8?
            let int16Value: Int16?
            let uint16Value: UInt16?
            let int32Value: Int32?
            let uint32Value: UInt32?
            let int64Value: Int64?
            let intValue: Int?
            let uintValue: UInt?
            let uint64Value: UInt64?
            let floatValue: Float?
            let doubleValue: Double?
            let decimalValue: Decimal?
        }

        func decode(_ type: String, _ value: String) throws {
            var key = type.lowercased()
            key.append("Value")
            _ = try JSONDecoder().decode(DataStruct.self, from: "{ \"\(key)\": \(value) }".data(using: .utf8)!)
        }

        func testGoodValue(_ type: String, _ value: String) {
            do {
                try decode(type, value)
            } catch {
                XCTFail("Unexpected error: \(error) for parsing \(value) to \(type)")
            }
        }

        func testErrorThrown(_ type: String, _ value: String, errorMessage: String) {
            do {
                try decode(type, value)
                XCTFail("Decode of \(value) to \(type) should not succeed")
            } catch DecodingError.dataCorrupted(let context) {
                XCTAssertEqual(context.debugDescription, errorMessage)
            } catch {
                XCTAssertEqual(String(describing: error), errorMessage)
            }
        }


        var goodValues = [
            ("Int8", "0"), ("Int8", "1"), ("Int8", "-1"), ("Int8", "-128"), ("Int8", "127"),
            ("UInt8", "0"), ("UInt8", "1"), ("UInt8", "255"), ("UInt8", "-0"),

            ("Int16", "0"), ("Int16", "1"), ("Int16", "-1"), ("Int16", "-32768"), ("Int16", "32767"),
            ("UInt16", "0"), ("UInt16", "1"), ("UInt16", "65535"), ("UInt16", "34.0"),

            ("Int32", "0"), ("Int32", "1"), ("Int32", "-1"), ("Int32", "-2147483648"), ("Int32", "2147483647"),
            ("UInt32", "0"), ("UInt32", "1"), ("UInt32", "4294967295"),

            ("Int64", "0"), ("Int64", "1"), ("Int64", "-1"), ("Int64", "-9223372036854775808"), ("Int64", "9223372036854775807"),
            ("UInt64", "0"), ("UInt64", "1"), ("UInt64", "18446744073709551615"),

            ("Double", "0"), ("Double", "1"), ("Double", "-1"), ("Double", "2.2250738585072014e-308"), ("Double", "1.7976931348623157e+308"),
            ("Double", "5e-324"), ("Double", "3.141592653589793"),

            ("Decimal", "1.2"), ("Decimal", "3.14159265358979323846264338327950288419"),
            ("Decimal", "3402823669209384634633746074317682114550000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"),
            ("Decimal", "-3402823669209384634633746074317682114550000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"),
        ]

        if Int.max == Int64.max {
            goodValues += [
                ("Int", "0"), ("Int", "1"), ("Int", "-1"), ("Int", "-9223372036854775808"), ("Int", "9223372036854775807"),
                ("UInt", "0"), ("UInt", "1"), ("UInt", "18446744073709551615"),
                ]
        } else {
            goodValues += [
                ("Int", "0"), ("Int", "1"), ("Int", "-1"), ("Int", "-2147483648"), ("Int", "2147483647"),
                ("UInt", "0"), ("UInt", "1"), ("UInt", "4294967295"),
            ]
        }

        let badValues = [
            ("Int8", "-129"), ("Int8", "128"), ("Int8", "1.2"),
            ("UInt8", "-1"), ("UInt8", "256"),

            ("Int16", "-32769"), ("Int16", "32768"),
            ("UInt16", "-1"), ("UInt16", "65536"),

            ("Int32", "-2147483649"), ("Int32", "2147483648"),
            ("UInt32", "-1"), ("UInt32", "4294967296"),

            ("Int64", "9223372036854775808"), ("Int64", "9223372036854775808"), ("Int64", "-100000000000000000000"),
            ("UInt64", "-1"), ("UInt64", "18446744073709600000"), ("Int64", "10000000000000000000000000000000000000"),
        ]

        for value in goodValues {
            testGoodValue(value.0, value.1)
        }

        for (type, value) in badValues {
            testErrorThrown(type, value, errorMessage: "Parsed JSON number <\(value)> does not fit in \(type).")
        }

        // Invalid JSON number formats
        testErrorThrown("Int8", "0000000000000000000000000000001", errorMessage: "The given data was not valid JSON.")
        testErrorThrown("Double", "-.1", errorMessage: "The given data was not valid JSON.")
        testErrorThrown("Int32", "+1", errorMessage: "The given data was not valid JSON.")
        testErrorThrown("Int", ".012", errorMessage: "The given data was not valid JSON.")
    }

    func test_snake_case_encoding() throws {
        struct MyTestData: Codable, Equatable {
            let thisIsAString: String
            let thisIsABool: Bool
            let thisIsAnInt: Int
            let thisIsAnInt8: Int8
            let thisIsAnInt16: Int16
            let thisIsAnInt32: Int32
            let thisIsAnInt64: Int64
            let thisIsAUint: UInt
            let thisIsAUint8: UInt8
            let thisIsAUint16: UInt16
            let thisIsAUint32: UInt32
            let thisIsAUint64: UInt64
            let thisIsAFloat: Float
            let thisIsADouble: Double
            let thisIsADate: Date
            let thisIsAnArray: Array<Int>
            let thisIsADictionary: Dictionary<String, Bool>
        }

        let data = MyTestData(thisIsAString: "Hello",
                              thisIsABool: true,
                              thisIsAnInt: 1,
                              thisIsAnInt8: 2,
                              thisIsAnInt16: 3,
                              thisIsAnInt32: 4,
                              thisIsAnInt64: 5,
                              thisIsAUint: 6,
                              thisIsAUint8: 7,
                              thisIsAUint16: 8,
                              thisIsAUint32: 9,
                              thisIsAUint64: 10,
                              thisIsAFloat: 11,
                              thisIsADouble: 12,
                              thisIsADate: Date.init(timeIntervalSince1970: 0),
                              thisIsAnArray: [1, 2, 3],
                              thisIsADictionary: [ "trueValue": true, "falseValue": false]
        )

        let encoder = JSONEncoder()
        encoder.keyEncodingStrategy = .convertToSnakeCase
        encoder.dateEncodingStrategy = .iso8601
        let encodedData = try encoder.encode(data)
        guard let jsonObject = try JSONSerialization.jsonObject(with: encodedData) as? [String: Any] else {
            XCTFail("Cant decode json object")
            return
        }
        XCTAssertEqual(jsonObject["this_is_a_string"] as? String, "Hello")
        XCTAssertEqual(jsonObject["this_is_a_bool"] as? Bool, true)
        XCTAssertEqual(jsonObject["this_is_an_int"] as? Int, 1)
        XCTAssertEqual(jsonObject["this_is_an_int8"] as? Int8, 2)
        XCTAssertEqual(jsonObject["this_is_an_int16"] as? Int16, 3)
        XCTAssertEqual(jsonObject["this_is_an_int32"] as? Int32, 4)
        XCTAssertEqual(jsonObject["this_is_an_int64"] as? Int64, 5)
        XCTAssertEqual(jsonObject["this_is_a_uint"] as? UInt, 6)
        XCTAssertEqual(jsonObject["this_is_a_uint8"] as? UInt8, 7)
        XCTAssertEqual(jsonObject["this_is_a_uint16"] as? UInt16, 8)
        XCTAssertEqual(jsonObject["this_is_a_uint32"] as? UInt32, 9)
        XCTAssertEqual(jsonObject["this_is_a_uint64"] as? UInt64, 10)
        XCTAssertEqual(jsonObject["this_is_a_float"] as? Float, 11)
        XCTAssertEqual(jsonObject["this_is_a_double"] as? Double, 12)
        XCTAssertEqual(jsonObject["this_is_a_date"] as? String, "1970-01-01T00:00:00Z")
        XCTAssertEqual(jsonObject["this_is_an_array"] as? [Int], [1, 2, 3])
        XCTAssertEqual(jsonObject["this_is_a_dictionary"] as? [String: Bool], ["trueValue": true, "falseValue": false ])

        let decoder = JSONDecoder()
        decoder.keyDecodingStrategy = .convertFromSnakeCase
        decoder.dateDecodingStrategy = .iso8601
        let decodedData = try decoder.decode(MyTestData.self, from: encodedData)
        XCTAssertEqual(data, decodedData)
    }

    func test_dictionary_snake_case_decoding() throws {
        let decoder = JSONDecoder()
        decoder.keyDecodingStrategy = .convertFromSnakeCase
        let snakeCaseJSONData = """
        {
            "snake_case_key": {
                "nested_dictionary": 1
            }
        }
        """.data(using: .utf8)!
        let decodedDictionary = try decoder.decode([String: [String: Int]].self, from: snakeCaseJSONData)
        let expectedDictionary = ["snake_case_key": ["nested_dictionary": 1]]
        XCTAssertEqual(decodedDictionary, expectedDictionary)
    }

    func test_dictionary_snake_case_encoding() throws {
        let encoder = JSONEncoder()
        encoder.keyEncodingStrategy = .convertToSnakeCase
        let camelCaseDictionary = ["camelCaseKey": ["nested_dictionary": 1]]
        let encodedData = try encoder.encode(camelCaseDictionary)
        guard let jsonObject = try JSONSerialization.jsonObject(with: encodedData) as? [String: [String: Int]] else {
            XCTFail("Cant decode json object")
            return
        }
        XCTAssertEqual(jsonObject, camelCaseDictionary)
    }

    func test_OutputFormattingValues() {
        XCTAssertEqual(JSONEncoder.OutputFormatting.prettyPrinted.rawValue, 1)
        if #available(macOS 10.13, *) {
            XCTAssertEqual(JSONEncoder.OutputFormatting.sortedKeys.rawValue, 2)
        }
        XCTAssertEqual(JSONEncoder.OutputFormatting.withoutEscapingSlashes.rawValue, 8)
    }

    func test_SR17581_codingEmptyDictionaryWithNonstringKeyDoesRoundtrip() throws {
        struct Something: Codable {
            struct Key: Codable, Hashable {
                var x: String
            }

            var dict: [Key: String]

            enum CodingKeys: String, CodingKey {
                case dict
            }

            init(from decoder: Decoder) throws {
                let container = try decoder.container(keyedBy: CodingKeys.self)
                self.dict = try container.decode([Key: String].self, forKey: .dict)
            }

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

            init(dict: [Key: String]) {
                self.dict = dict
            }
        }

        let toEncode = Something(dict: [:])
        let data = try JSONEncoder().encode(toEncode)
        let result = try JSONDecoder().decode(Something.self, from: data)
        XCTAssertEqual(result.dict.count, 0)
    }

    // 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,
                                   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
            payload = try encoder.encode(value)
        } catch {
            XCTFail("Failed to encode \(T.self) to JSON: \(error)")
        }
        
        if let expectedJSON = json {
            // We do not compare expectedJSON to payload directly, because they might have values like
            // {"name": "Bob", "age": 22}
            // and
            // {"age": 22, "name": "Bob"}
            // which if compared as Data would not be equal, but the contained JSON values are equal.
            // So we wrap them in a JSON type, which compares data as if it were a json.

            let expectedJSONObject: JSON
            let payloadJSONObject: JSON

            do {
                expectedJSONObject = try JSON(data: expectedJSON)
            } catch {
                XCTFail("Invalid JSON data passed as expectedJSON: \(error)")
                return
            }

            do {
                payloadJSONObject = try JSON(data: payload)
            } catch {
                XCTFail("Produced data is not a valid JSON: \(error)")
                return
            }

            XCTAssertEqual(expectedJSONObject, payloadJSONObject, "Produced JSON not identical to expected JSON.")
        }
        
        do {
            let decoder = JSONDecoder()
            decoder.dateDecodingStrategy = dateDecodingStrategy
            decoder.dataDecodingStrategy = dataDecodingStrategy
            decoder.nonConformingFloatDecodingStrategy = nonConformingFloatDecodingStrategy
            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)")
        }
    }

    func test_codingOf<T: Codable & Equatable>(value: T, toAndFrom stringValue: String) {
        _testRoundTrip(of: TopLevelObjectWrapper(value),
                       expectedJSON: "{\"value\":\(stringValue)}".data(using: .utf8)!)

        _testRoundTrip(of: TopLevelArrayWrapper(value),
                       expectedJSON: "[\(stringValue)]".data(using: .utf8)!)
    }
}

// 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 (k1, k2) in zip(lhs, rhs) {
        switch (k1, k2) {
        case (nil, nil): continue
        case (let _k1?, nil):
            XCTFail("\(prefix) CodingKey mismatch: \(type(of: _k1)) != nil")
            return
        case (nil, let _k2?):
            XCTFail("\(prefix) CodingKey mismatch: nil != \(type(of: _k2))")
            return
        default: break
        }

        let key1 = k1!
        let key2 = k2!

        switch (key1.intValue, key2.intValue) {
        case (nil, nil): break
        case (let i1?, nil):
            XCTFail("\(prefix) CodingKey.intValue mismatch: \(type(of: key1))(\(i1)) != nil")
            return
        case (nil, let i2?):
            XCTFail("\(prefix) CodingKey.intValue mismatch: nil != \(type(of: key2))(\(i2))")
            return
        case (let i1?, 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

    // FIXME: This property is present only in order to test the expected result of Codable synthesis in the compiler.
    // We want to test against expected encoded output (to ensure this generates an encodeIfPresent call), but we need an output format for that.
    // Once we have a VerifyingEncoder for compiler unit tests, we should move this test there.
    let website: URL?

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

    static func ==(_ lhs: Person, _ rhs: Person) -> Bool {
        return lhs.name == rhs.name &&
            lhs.email == rhs.email &&
            lhs.website == rhs.website
    }

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

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

    init(address: Address, employees: [Person]) {
        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: [Person.testValue])
    }
}

// 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
  }
}

/// Wraps a type T so that it can be encoded at the top level of a payload.
fileprivate struct TopLevelArrayWrapper<T> : Codable, Equatable where T : Codable, T : Equatable {
    let value: T

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

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

    init(from decoder: Decoder) throws {
        var container = try decoder.unkeyedContainer()
        value = try container.decode(T.self)
        assert(container.isAtEnd)
    }

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

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
    }
}

/// A type which encodes as an array directly through a single value container.
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")!])
    }
}

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: - Helpers

fileprivate struct JSON: Equatable {
    private var jsonObject: Any

    fileprivate init(data: Data) throws {
        self.jsonObject = try JSONSerialization.jsonObject(with: data, options: [])
    }

    static func ==(lhs: JSON, rhs: JSON) -> Bool {
        switch (lhs.jsonObject, rhs.jsonObject) {
        case let (lhs, rhs) as ([AnyHashable: Any], [AnyHashable: Any]):
            return NSDictionary(dictionary: lhs) == NSDictionary(dictionary: rhs)
        case let (lhs, rhs) as ([Any], [Any]):
            return NSArray(array: lhs) == NSArray(array: rhs)
        default:
            return false
        }
    }
}

// MARK: - Run Tests

extension TestJSONEncoder {
    static var allTests: [(String, (TestJSONEncoder) -> () throws -> Void)] {
        return [
            ("test_encodingTopLevelFragments", test_encodingTopLevelFragments),
            ("test_encodingTopLevelEmptyStruct", test_encodingTopLevelEmptyStruct),
            ("test_encodingTopLevelEmptyClass", test_encodingTopLevelEmptyClass),
            ("test_encodingTopLevelSingleValueEnum", test_encodingTopLevelSingleValueEnum),
            ("test_encodingTopLevelSingleValueStruct", test_encodingTopLevelSingleValueStruct),
            ("test_encodingTopLevelSingleValueClass", test_encodingTopLevelSingleValueClass),
            ("test_encodingTopLevelStructuredStruct", test_encodingTopLevelStructuredStruct),
            ("test_encodingTopLevelStructuredClass", test_encodingTopLevelStructuredClass),
            ("test_encodingTopLevelStructuredSingleStruct", test_encodingTopLevelStructuredSingleStruct),
            ("test_encodingTopLevelStructuredSingleClass", test_encodingTopLevelStructuredSingleClass),
            ("test_encodingTopLevelDeepStructuredType", test_encodingTopLevelDeepStructuredType),
            ("test_encodingOutputFormattingDefault", test_encodingOutputFormattingDefault),
            ("test_encodingOutputFormattingPrettyPrinted", test_encodingOutputFormattingPrettyPrinted),
            ("test_encodingOutputFormattingSortedKeys", test_encodingOutputFormattingSortedKeys),
            ("test_encodingOutputFormattingPrettyPrintedSortedKeys", test_encodingOutputFormattingPrettyPrintedSortedKeys),
            ("test_encodingDate", test_encodingDate),
            ("test_encodingDateSecondsSince1970", test_encodingDateSecondsSince1970),
            ("test_encodingDateMillisecondsSince1970", test_encodingDateMillisecondsSince1970),
            ("test_encodingDateISO8601", test_encodingDateISO8601),
            ("test_encodingDateFormatted", test_encodingDateFormatted),
            ("test_encodingDateCustom", test_encodingDateCustom),
            ("test_encodingDateCustomEmpty", test_encodingDateCustomEmpty),
            ("test_encodingBase64Data", test_encodingBase64Data),
            ("test_encodingCustomData", test_encodingCustomData),
            ("test_encodingCustomDataEmpty", test_encodingCustomDataEmpty),
            ("test_encodingNonConformingFloats", test_encodingNonConformingFloats),
            ("test_encodingNonConformingFloatStrings", test_encodingNonConformingFloatStrings),
            ("test_encodeDecodeNumericTypesBaseline", test_encodeDecodeNumericTypesBaseline),
            ("test_nestedContainerCodingPaths", test_nestedContainerCodingPaths),
            ("test_superEncoderCodingPaths", test_superEncoderCodingPaths),
            ("test_notFoundSuperDecoder", test_notFoundSuperDecoder),
            ("test_childTypeDecoder", test_childTypeDecoder),
            ("test_codingOfBool", test_codingOfBool),
            ("test_codingOfNil", test_codingOfNil),
            ("test_codingOfInt8", test_codingOfInt8),
            ("test_codingOfUInt8", test_codingOfUInt8),
            ("test_codingOfInt16", test_codingOfInt16),
            ("test_codingOfUInt16", test_codingOfUInt16),
            ("test_codingOfInt32", test_codingOfInt32),
            ("test_codingOfUInt32", test_codingOfUInt32),
            ("test_codingOfInt64", test_codingOfInt64),
            ("test_codingOfUInt64", test_codingOfUInt64),
            ("test_codingOfInt", test_codingOfInt),
            ("test_codingOfUInt", test_codingOfUInt),
            ("test_codingOfFloat", test_codingOfFloat),
            ("test_codingOfDouble", test_codingOfDouble),
            ("test_codingOfDecimal", test_codingOfDecimal),
            ("test_codingOfString", test_codingOfString),
            ("test_codingOfURL", test_codingOfURL),
            ("test_codingOfUIntMinMax", test_codingOfUIntMinMax),
            ("test_numericLimits", test_numericLimits),
            ("test_snake_case_encoding", test_snake_case_encoding),
            ("test_dictionary_snake_case_decoding", test_dictionary_snake_case_decoding),
            ("test_dictionary_snake_case_encoding", test_dictionary_snake_case_encoding),
            ("test_OutputFormattingValues", test_OutputFormattingValues),
            ("test_SR17581_codingEmptyDictionaryWithNonstringKeyDoesRoundtrip", test_SR17581_codingEmptyDictionaryWithNonstringKeyDoesRoundtrip),
        ]
    }
}