Codable.swift.gyb

//===----------------------------------------------------------------------===//
//
// 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 https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
%{
codable_types = ['Bool', 'String', 'Double', 'Float',
                 'Int', 'Int8', 'Int16', 'Int32', 'Int64',
                 'UInt', 'UInt8', 'UInt16', 'UInt32', 'UInt64']
}%

//===----------------------------------------------------------------------===//
// Codable
//===----------------------------------------------------------------------===//

/// A type that can encode itself to an external representation.
public protocol Encodable {
  /// Encodes this value into the given encoder.
  ///
  /// If the value fails to encode anything, `encoder` will encode an empty
  /// keyed container in its place.
  ///
  /// This function throws an error if any values are invalid for the given
  /// encoder's format.
  ///
  /// - Parameter encoder: The encoder to write data to.
  func encode(to encoder: Encoder) throws
}

/// A type that can decode itself from an external representation.
public protocol Decodable {
  /// Creates a new instance by decoding from the given decoder.
  ///
  /// This initializer throws an error if reading from the decoder fails, or
  /// if the data read is corrupted or otherwise invalid.
  ///
  /// - Parameter decoder: The decoder to read data from.
  init(from decoder: Decoder) throws
}

/// A type that can convert itself into and out of an external representation.
///
/// `Codable` is a type alias for the `Encodable` and `Decodable` protocols.
/// When you use `Codable` as a type or a generic constraint, it matches
/// any type that conforms to both protocols.
public typealias Codable = Encodable & Decodable

//===----------------------------------------------------------------------===//
// CodingKey
//===----------------------------------------------------------------------===//

/// A type that can be used as a key for encoding and decoding.
public protocol CodingKey :
  CustomStringConvertible, CustomDebugStringConvertible
{
  /// The string to use in a named collection (e.g. a string-keyed dictionary).
  var stringValue: String { get }

  /// Creates a new instance from the given string.
  ///
  /// If the string passed as `stringValue` does not correspond to any instance
  /// of this type, the result is `nil`.
  ///
  /// - parameter stringValue: The string value of the desired key.
  init?(stringValue: String)

  /// The value to use in an integer-indexed collection (e.g. an int-keyed
  /// dictionary).
  var intValue: Int? { get }

  /// Creates a new instance from the specified integer.
  ///
  /// If the value passed as `intValue` does not correspond to any instance of
  /// this type, the result is `nil`.
  ///
  /// - parameter intValue: The integer value of the desired key.
  init?(intValue: Int)
}

extension CodingKey {
  /// A textual representation of this key.
  public var description: String {
    let intValue = self.intValue?.description ?? "nil"
    return "\(type(of: self))(stringValue: \"\(stringValue)\", intValue: \(intValue))"
  }

  /// A textual representation of this key, suitable for debugging.
  public var debugDescription: String {
    return description
  }
}

//===----------------------------------------------------------------------===//
// Encoder & Decoder
//===----------------------------------------------------------------------===//

/// A type that can encode values into a native format for external
/// representation.
public protocol Encoder {
  /// The path of coding keys taken to get to this point in encoding.
  var codingPath: [CodingKey] { get }

  /// Any contextual information set by the user for encoding.
  var userInfo: [CodingUserInfoKey : Any] { get }

  /// Returns an encoding container appropriate for holding multiple values
  /// keyed by the given key type.
  ///
  /// You must use only one kind of top-level encoding container. This method
  /// must not be called after a call to `unkeyedContainer()` or after
  /// encoding a value through a call to `singleValueContainer()`
  ///
  /// - parameter type: The key type to use for the container.
  /// - returns: A new keyed encoding container.
  func container<Key>(keyedBy type: Key.Type) -> KeyedEncodingContainer<Key>

  /// Returns an encoding container appropriate for holding multiple unkeyed
  /// values.
  ///
  /// You must use only one kind of top-level encoding container. This method
  /// must not be called after a call to `container(keyedBy:)` or after
  /// encoding a value through a call to `singleValueContainer()`
  ///
  /// - returns: A new empty unkeyed container.
  func unkeyedContainer() -> UnkeyedEncodingContainer

  /// Returns an encoding container appropriate for holding a single primitive
  /// value.
  ///
  /// You must use only one kind of top-level encoding container. This method
  /// must not be called after a call to `unkeyedContainer()` or
  /// `container(keyedBy:)`, or after encoding a value through a call to
  /// `singleValueContainer()`
  ///
  /// - returns: A new empty single value container.
  func singleValueContainer() -> SingleValueEncodingContainer
}

/// A type that can decode values from a native format into in-memory
/// representations.
public protocol Decoder {
  /// The path of coding keys taken to get to this point in decoding.
  var codingPath: [CodingKey] { get }

  /// Any contextual information set by the user for decoding.
  var userInfo: [CodingUserInfoKey : Any] { get }

  /// Returns the data stored in this decoder as represented in a container
  /// keyed by the given key type.
  ///
  /// - parameter type: The key type to use for the container.
  /// - returns: A keyed decoding container view into this decoder.
  /// - throws: `DecodingError.typeMismatch` if the encountered stored value is
  ///   not a keyed container.
  func container<Key>(keyedBy type: Key.Type) throws -> KeyedDecodingContainer<Key>

  /// Returns the data stored in this decoder as represented in a container
  /// appropriate for holding values with no keys.
  ///
  /// - returns: An unkeyed container view into this decoder.
  /// - throws: `DecodingError.typeMismatch` if the encountered stored value is
  ///   not an unkeyed container.
  func unkeyedContainer() throws -> UnkeyedDecodingContainer

  /// Returns the data stored in this decoder as represented in a container
  /// appropriate for holding a single primitive value.
  ///
  /// - returns: A single value container view into this decoder.
  /// - throws: `DecodingError.typeMismatch` if the encountered stored value is
  ///   not a single value container.
  func singleValueContainer() throws -> SingleValueDecodingContainer
}

//===----------------------------------------------------------------------===//
// Keyed Encoding Containers
//===----------------------------------------------------------------------===//

/// A type that provides a view into an encoder's storage and is used to hold
/// the encoded properties of an encodable type in a keyed manner.
///
/// Encoders should provide types conforming to
/// `KeyedEncodingContainerProtocol` for their format.
public protocol KeyedEncodingContainerProtocol {
  associatedtype Key : CodingKey

  /// The path of coding keys taken to get to this point in encoding.
  var codingPath: [CodingKey] { get }

  /// Encodes a null value for the given key.
  ///
  /// - parameter key: The key to associate the value with.
  /// - throws: `EncodingError.invalidValue` if a null value is invalid in the
  ///   current context for this format.
  mutating func encodeNil(forKey key: Key) throws

% for type in codable_types:
  /// Encodes the given value for the given key.
  ///
  /// - parameter value: The value to encode.
  /// - parameter key: The key to associate the value with.
  /// - throws: `EncodingError.invalidValue` if the given value is invalid in
  ///   the current context for this format.
  mutating func encode(_ value: ${type}, forKey key: Key) throws
% end

  /// Encodes the given value for the given key.
  ///
  /// - parameter value: The value to encode.
  /// - parameter key: The key to associate the value with.
  /// - throws: `EncodingError.invalidValue` if the given value is invalid in
  ///   the current context for this format.
  mutating func encode<T : Encodable>(_ value: T, forKey key: Key) throws

  /// Encodes a reference to the given object only if it is encoded
  /// unconditionally elsewhere in the payload (previously, or in the future).
  ///
  /// For encoders which don't support this feature, the default implementation
  /// encodes the given object unconditionally.
  ///
  /// - parameter object: The object to encode.
  /// - parameter key: The key to associate the object with.
  /// - throws: `EncodingError.invalidValue` if the given value is invalid in
  ///   the current context for this format.
  mutating func encodeConditional<T : AnyObject & Encodable>(
    _ object: T, forKey key: Key) throws

% for type in codable_types:
  /// Encodes the given value for the given key if it is not `nil`.
  ///
  /// - parameter value: The value to encode.
  /// - parameter key: The key to associate the value with.
  /// - throws: `EncodingError.invalidValue` if the given value is invalid in
  ///   the current context for this format.
  mutating func encodeIfPresent(_ value: ${type}?, forKey key: Key) throws
% end

  /// Encodes the given value for the given key if it is not `nil`.
  ///
  /// - parameter value: The value to encode.
  /// - parameter key: The key to associate the value with.
  /// - throws: `EncodingError.invalidValue` if the given value is invalid in
  ///   the current context for this format.
  mutating func encodeIfPresent<T : Encodable>(
    _ value: T?, forKey key: Key) throws

  /// Stores a keyed encoding container for the given key and returns it.
  ///
  /// - parameter keyType: The key type to use for the container.
  /// - parameter key: The key to encode the container for.
  /// - returns: A new keyed encoding container.
  mutating func nestedContainer<NestedKey>(
    keyedBy keyType: NestedKey.Type, forKey key: Key
  ) -> KeyedEncodingContainer<NestedKey>

  /// Stores an unkeyed encoding container for the given key and returns it.
  ///
  /// - parameter key: The key to encode the container for.
  /// - returns: A new unkeyed encoding container.
  mutating func nestedUnkeyedContainer(
    forKey key: Key) -> UnkeyedEncodingContainer

  /// Stores a new nested container for the default `super` key and returns A
  /// new encoder instance for encoding `super` into that container.
  ///
  /// Equivalent to calling `superEncoder(forKey:)` with
  /// `Key(stringValue: "super", intValue: 0)`.
  ///
  /// - returns: A new encoder to pass to `super.encode(to:)`.
  mutating func superEncoder() -> Encoder

  /// Stores a new nested container for the given key and returns A new encoder
  /// instance for encoding `super` into that container.
  ///
  /// - parameter key: The key to encode `super` for.
  /// - returns: A new encoder to pass to `super.encode(to:)`.
  mutating func superEncoder(forKey key: Key) -> Encoder
}

// An implementation of _KeyedEncodingContainerBase and
// _KeyedEncodingContainerBox are given at the bottom of this file.

/// A concrete container that provides a view into an encoder's storage, making
/// the encoded properties of an encodable type accessible by keys.
public struct KeyedEncodingContainer<K : CodingKey> :
  KeyedEncodingContainerProtocol
{
  public typealias Key = K

  /// The container for the concrete encoder. The type is _*Base so that it's
  /// generic on the key type.
  internal var _box: _KeyedEncodingContainerBase<Key>

  /// Creates a new instance with the given container.
  ///
  /// - parameter container: The container to hold.
  public init<Container : KeyedEncodingContainerProtocol>(
    _ container: Container) where Container.Key == Key
  {
    _box = _KeyedEncodingContainerBox(container)
  }

  /// The path of coding keys taken to get to this point in encoding.
  public var codingPath: [CodingKey] {
    return _box.codingPath
  }

  /// Encodes a null value for the given key.
  ///
  /// - parameter key: The key to associate the value with.
  /// - throws: `EncodingError.invalidValue` if a null value is invalid in the
  ///   current context for this format.
  public mutating func encodeNil(forKey key: Key) throws {
    try _box.encodeNil(forKey: key)
  }

% for type in codable_types:
  /// Encodes the given value for the given key.
  ///
  /// - parameter value: The value to encode.
  /// - parameter key: The key to associate the value with.
  /// - throws: `EncodingError.invalidValue` if the given value is invalid in
  ///   the current context for this format.
  public mutating func encode(_ value: ${type}, forKey key: Key) throws {
    try _box.encode(value, forKey: key)
  }
% end

  /// Encodes the given value for the given key.
  ///
  /// - parameter value: The value to encode.
  /// - parameter key: The key to associate the value with.
  /// - throws: `EncodingError.invalidValue` if the given value is invalid in
  ///   the current context for this format.
  public mutating func encode<T : Encodable>(
    _ value: T, forKey key: Key) throws
  {
    try _box.encode(value, forKey: key)
  }

  /// Encodes a reference to the given object only if it is encoded
  /// unconditionally elsewhere in the payload (previously, or in the future).
  ///
  /// For encoders which don't support this feature, the default implementation
  /// encodes the given object unconditionally.
  ///
  /// - parameter object: The object to encode.
  /// - parameter key: The key to associate the object with.
  /// - throws: `EncodingError.invalidValue` if the given value is invalid in
  ///   the current context for this format.
  public mutating func encodeConditional<T : AnyObject & Encodable>(
    _ object: T, forKey key: Key) throws
  {
    try _box.encodeConditional(object, forKey: key)
  }

% for type in codable_types:
  /// Encodes the given value for the given key if it is not `nil`.
  ///
  /// - parameter value: The value to encode.
  /// - parameter key: The key to associate the value with.
  /// - throws: `EncodingError.invalidValue` if the given value is invalid in
  ///   the current context for this format.
  public mutating func encodeIfPresent(
    _ value: ${type}?, forKey key: Key) throws
  {
    try _box.encodeIfPresent(value, forKey: key)
  }
% end

  /// Encodes the given value for the given key if it is not `nil`.
  ///
  /// - parameter value: The value to encode.
  /// - parameter key: The key to associate the value with.
  /// - throws: `EncodingError.invalidValue` if the given value is invalid in
  ///   the current context for this format.
  public mutating func encodeIfPresent<T : Encodable>(
    _ value: T?, forKey key: Key) throws
  {
    try _box.encodeIfPresent(value, forKey: key)
  }

  /// Stores a keyed encoding container for the given key and returns it.
  ///
  /// - parameter keyType: The key type to use for the container.
  /// - parameter key: The key to encode the container for.
  /// - returns: A new keyed encoding container.
  public mutating func nestedContainer<NestedKey>(
    keyedBy keyType: NestedKey.Type, forKey key: Key
  ) -> KeyedEncodingContainer<NestedKey> {
    return _box.nestedContainer(keyedBy: NestedKey.self, forKey: key)
  }

  /// Stores an unkeyed encoding container for the given key and returns it.
  ///
  /// - parameter key: The key to encode the container for.
  /// - returns: A new unkeyed encoding container.
  public mutating func nestedUnkeyedContainer(
    forKey key: Key) -> UnkeyedEncodingContainer
  {
    return _box.nestedUnkeyedContainer(forKey: key)
  }

  /// Stores a new nested container for the default `super` key and returns A
  /// new encoder instance for encoding `super` into that container.
  ///
  /// Equivalent to calling `superEncoder(forKey:)` with
  /// `Key(stringValue: "super", intValue: 0)`.
  ///
  /// - returns: A new encoder to pass to `super.encode(to:)`.
  public mutating func superEncoder() -> Encoder {
    return _box.superEncoder()
  }

  /// Stores a new nested container for the given key and returns A new encoder
  /// instance for encoding `super` into that container.
  ///
  /// - parameter key: The key to encode `super` for.
  /// - returns: A new encoder to pass to `super.encode(to:)`.
  public mutating func superEncoder(forKey key: Key) -> Encoder {
    return _box.superEncoder(forKey: key)
  }
}

/// A type that provides a view into a decoder's storage and is used to hold
/// the encoded properties of a decodable type in a keyed manner.
///
/// Decoders should provide types conforming to `UnkeyedDecodingContainer` for
/// their format.
public protocol KeyedDecodingContainerProtocol {
  associatedtype Key : CodingKey

  /// The path of coding keys taken to get to this point in decoding.
  var codingPath: [CodingKey] { get }

  /// All the keys the `Decoder` has for this container.
  ///
  /// Different keyed containers from the same `Decoder` may return different
  /// keys here; it is possible to encode with multiple key types which are
  /// not convertible to one another. This should report all keys present
  /// which are convertible to the requested type.
  var allKeys: [Key] { get }

  /// Returns a Boolean value indicating whether the decoder contains a value
  /// associated with the given key.
  ///
  /// The value associated with `key` may be a null value as appropriate for
  /// the data format.
  ///
  /// - parameter key: The key to search for.
  /// - returns: Whether the `Decoder` has an entry for the given key.
  func contains(_ key: Key) -> Bool

  /// Decodes a null value for the given key.
  ///
  /// - parameter key: The key that the decoded value is associated with.
  /// - returns: Whether the encountered value was null.
  /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry
  ///   for the given key.
  func decodeNil(forKey key: Key) throws -> Bool

% for type in codable_types:
  /// Decodes a value of the given type for the given key.
  ///
  /// - parameter type: The type of value to decode.
  /// - parameter key: The key that the decoded value is associated with.
  /// - returns: A value of the requested type, if present for the given key
  ///   and convertible to the requested type.
  /// - throws: `DecodingError.typeMismatch` if the encountered encoded value
  ///   is not convertible to the requested type.
  /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry
  ///   for the given key.
  /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for
  ///   the given key.
  func decode(_ type: ${type}.Type, forKey key: Key) throws -> ${type}
% end

  /// Decodes a value of the given type for the given key.
  ///
  /// - parameter type: The type of value to decode.
  /// - parameter key: The key that the decoded value is associated with.
  /// - returns: A value of the requested type, if present for the given key
  ///   and convertible to the requested type.
  /// - throws: `DecodingError.typeMismatch` if the encountered encoded value
  ///   is not convertible to the requested type.
  /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry
  ///   for the given key.
  /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for
  ///   the given key.
  func decode<T : Decodable>(_ type: T.Type, forKey key: Key) throws -> T

% for type in codable_types:
  /// Decodes a value of the given type for the given key, if present.
  ///
  /// This method returns `nil` if the container does not have a value
  /// associated with `key`, or if the value is null. The difference between
  /// these states can be distinguished with a `contains(_:)` call.
  ///
  /// - parameter type: The type of value to decode.
  /// - parameter key: The key that the decoded value is associated with.
  /// - returns: A decoded value of the requested type, or `nil` if the
  ///   `Decoder` does not have an entry associated with the given key, or if
  ///   the value is a null value.
  /// - throws: `DecodingError.typeMismatch` if the encountered encoded value
  ///   is not convertible to the requested type.
  func decodeIfPresent(_ type: ${type}.Type, forKey key: Key) throws -> ${type}?
% end

  /// Decodes a value of the given type for the given key, if present.
  ///
  /// This method returns `nil` if the container does not have a value
  /// associated with `key`, or if the value is null. The difference between
  /// these states can be distinguished with a `contains(_:)` call.
  ///
  /// - parameter type: The type of value to decode.
  /// - parameter key: The key that the decoded value is associated with.
  /// - returns: A decoded value of the requested type, or `nil` if the
  ///   `Decoder` does not have an entry associated with the given key, or if
  ///   the value is a null value.
  /// - throws: `DecodingError.typeMismatch` if the encountered encoded value
  ///   is not convertible to the requested type.
  func decodeIfPresent<T : Decodable>(
    _ type: T.Type, forKey key: Key) throws -> T?

  /// Returns the data stored for the given key as represented in a container
  /// keyed by the given key type.
  ///
  /// - parameter type: The key type to use for the container.
  /// - parameter key: The key that the nested container is associated with.
  /// - returns: A keyed decoding container view into `self`.
  /// - throws: `DecodingError.typeMismatch` if the encountered stored value is
  ///   not a keyed container.
  func nestedContainer<NestedKey>(
    keyedBy type: NestedKey.Type, forKey key: Key
  ) throws -> KeyedDecodingContainer<NestedKey>

  /// Returns the data stored for the given key as represented in an unkeyed
  /// container.
  ///
  /// - parameter key: The key that the nested container is associated with.
  /// - returns: An unkeyed decoding container view into `self`.
  /// - throws: `DecodingError.typeMismatch` if the encountered stored value is
  ///   not an unkeyed container.
  func nestedUnkeyedContainer(
    forKey key: Key) throws -> UnkeyedDecodingContainer

  /// Returns a `Decoder` instance for decoding `super` from the container
  /// associated with the default `super` key.
  ///
  /// Equivalent to calling `superDecoder(forKey:)` with
  /// `Key(stringValue: "super", intValue: 0)`.
  ///
  /// - returns: A new `Decoder` to pass to `super.init(from:)`.
  /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry
  ///   for the default `super` key.
  /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for
  ///   the default `super` key.
  func superDecoder() throws -> Decoder

  /// Returns a `Decoder` instance for decoding `super` from the container
  /// associated with the given key.
  ///
  /// - parameter key: The key to decode `super` for.
  /// - returns: A new `Decoder` to pass to `super.init(from:)`.
  /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry
  ///   for the given key.
  /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for
  ///   the given key.
  func superDecoder(forKey key: Key) throws -> Decoder
}

// An implementation of _KeyedDecodingContainerBase and
// _KeyedDecodingContainerBox are given at the bottom of this file.

/// A concrete container that provides a view into a decoder's storage, making
/// the encoded properties of a decodable type accessible by keys.
public struct KeyedDecodingContainer<K : CodingKey> :
  KeyedDecodingContainerProtocol
{
  public typealias Key = K

  /// The container for the concrete decoder. The type is _*Base so that it's
  /// generic on the key type.
  internal var _box: _KeyedDecodingContainerBase<Key>

  /// Creates a new instance with the given container.
  ///
  /// - parameter container: The container to hold.
  public init<Container : KeyedDecodingContainerProtocol>(
    _ container: Container) where Container.Key == Key
  {
    _box = _KeyedDecodingContainerBox(container)
  }

  /// The path of coding keys taken to get to this point in decoding.
  public var codingPath: [CodingKey] {
    return _box.codingPath
  }

  /// All the keys the decoder has for this container.
  ///
  /// Different keyed containers from the same decoder may return different
  /// keys here, because it is possible to encode with multiple key types
  /// which are not convertible to one another. This should report all keys
  /// present which are convertible to the requested type.
  public var allKeys: [Key] {
    return _box.allKeys
  }

  /// Returns a Boolean value indicating whether the decoder contains a value
  /// associated with the given key.
  ///
  /// The value associated with the given key may be a null value as
  /// appropriate for the data format.
  ///
  /// - parameter key: The key to search for.
  /// - returns: Whether the `Decoder` has an entry for the given key.
  public func contains(_ key: Key) -> Bool {
    return _box.contains(key)
  }

  /// Decodes a null value for the given key.
  ///
  /// - parameter key: The key that the decoded value is associated with.
  /// - returns: Whether the encountered value was null.
  /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry
  ///   for the given key.
  public func decodeNil(forKey key: Key) throws -> Bool {
    return try _box.decodeNil(forKey: key)
  }

% for type in codable_types:
  /// Decodes a value of the given type for the given key.
  ///
  /// - parameter type: The type of value to decode.
  /// - parameter key: The key that the decoded value is associated with.
  /// - returns: A value of the requested type, if present for the given key
  ///   and convertible to the requested type.
  /// - throws: `DecodingError.typeMismatch` if the encountered encoded value
  ///   is not convertible to the requested type.
  /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry
  ///   for the given key.
  /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for
  ///   the given key.
  public func decode(_ type: ${type}.Type, forKey key: Key) throws -> ${type} {
    return try _box.decode(${type}.self, forKey: key)
  }
% end

  /// Decodes a value of the given type for the given key.
  ///
  /// - parameter type: The type of value to decode.
  /// - parameter key: The key that the decoded value is associated with.
  /// - returns: A value of the requested type, if present for the given key
  ///   and convertible to the requested type.
  /// - throws: `DecodingError.typeMismatch` if the encountered encoded value
  ///   is not convertible to the requested type.
  /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry
  ///   for the given key.
  /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for
  ///   the given key.
  public func decode<T : Decodable>(_ type: T.Type, forKey key: Key) throws -> T {
    return try _box.decode(T.self, forKey: key)
  }

% for type in codable_types:
  /// Decodes a value of the given type for the given key, if present.
  ///
  /// This method returns `nil` if the container does not have a value
  /// associated with `key`, or if the value is null. The difference between
  /// these states can be distinguished with a `contains(_:)` call.
  ///
  /// - parameter type: The type of value to decode.
  /// - parameter key: The key that the decoded value is associated with.
  /// - returns: A decoded value of the requested type, or `nil` if the
  ///   `Decoder` does not have an entry associated with the given key, or if
  ///   the value is a null value.
  /// - throws: `DecodingError.typeMismatch` if the encountered encoded value
  ///   is not convertible to the requested type.
  public func decodeIfPresent(
    _ type: ${type}.Type, forKey key: Key) throws -> ${type}?
  {
    return try _box.decodeIfPresent(${type}.self, forKey: key)
  }
% end

  /// Decodes a value of the given type for the given key, if present.
  ///
  /// This method returns `nil` if the container does not have a value
  /// associated with `key`, or if the value is null. The difference between
  /// these states can be distinguished with a `contains(_:)` call.
  ///
  /// - parameter type: The type of value to decode.
  /// - parameter key: The key that the decoded value is associated with.
  /// - returns: A decoded value of the requested type, or `nil` if the
  ///   `Decoder` does not have an entry associated with the given key, or if
  ///   the value is a null value.
  /// - throws: `DecodingError.typeMismatch` if the encountered encoded value
  ///   is not convertible to the requested type.
  public func decodeIfPresent<T : Decodable>(
    _ type: T.Type, forKey key: Key) throws -> T?
  {
    return try _box.decodeIfPresent(T.self, forKey: key)
  }

  /// Returns the data stored for the given key as represented in a container
  /// keyed by the given key type.
  ///
  /// - parameter type: The key type to use for the container.
  /// - parameter key: The key that the nested container is associated with.
  /// - returns: A keyed decoding container view into `self`.
  /// - throws: `DecodingError.typeMismatch` if the encountered stored value is
  ///   not a keyed container.
  public func nestedContainer<NestedKey>(
    keyedBy type: NestedKey.Type, forKey key: Key
  ) throws -> KeyedDecodingContainer<NestedKey> {
    return try _box.nestedContainer(keyedBy: NestedKey.self, forKey: key)
  }

  /// Returns the data stored for the given key as represented in an unkeyed
  /// container.
  ///
  /// - parameter key: The key that the nested container is associated with.
  /// - returns: An unkeyed decoding container view into `self`.
  /// - throws: `DecodingError.typeMismatch` if the encountered stored value is
  ///   not an unkeyed container.
  public func nestedUnkeyedContainer(
    forKey key: Key) throws -> UnkeyedDecodingContainer
  {
    return try _box.nestedUnkeyedContainer(forKey: key)
  }

  /// Returns a `Decoder` instance for decoding `super` from the container
  /// associated with the default `super` key.
  ///
  /// Equivalent to calling `superDecoder(forKey:)` with
  /// `Key(stringValue: "super", intValue: 0)`.
  ///
  /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry
  ///   for the default `super` key.
  /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for
  ///   the default `super` key.
  public func superDecoder() throws -> Decoder {
    return try _box.superDecoder()
  }

  /// Returns a `Decoder` instance for decoding `super` from the container
  /// associated with the given key.
  ///
  /// - parameter key: The key to decode `super` for.
  /// - returns: A new `Decoder` to pass to `super.init(from:)`.
  /// - throws: `DecodingError.keyNotFound` if `self` does not have an entry
  ///   for the given key.
  /// - throws: `DecodingError.valueNotFound` if `self` has a null entry for
  ///   the given key.
  public func superDecoder(forKey key: Key) throws -> Decoder {
    return try _box.superDecoder(forKey: key)
  }
}

//===----------------------------------------------------------------------===//
// Unkeyed Encoding Containers
//===----------------------------------------------------------------------===//

/// A type that provides a view into an encoder's storage and is used to hold
/// the encoded properties of an encodable type sequentially, without keys.
///
/// Encoders should provide types conforming to `UnkeyedEncodingContainer` for
/// their format.
public protocol UnkeyedEncodingContainer {
  /// The path of coding keys taken to get to this point in encoding.
  var codingPath: [CodingKey] { get }

  /// The number of elements encoded into the container.
  var count: Int { get }

  /// Encodes a null value.
  ///
  /// - throws: `EncodingError.invalidValue` if a null value is invalid in the
  ///   current context for this format.
  mutating func encodeNil() throws

% for type in codable_types:
  /// Encodes the given value.
  ///
  /// - parameter value: The value to encode.
  /// - throws: `EncodingError.invalidValue` if the given value is invalid in
  ///   the current context for this format.
  mutating func encode(_ value: ${type}) throws
% end

  /// Encodes the given value.
  ///
  /// - parameter value: The value to encode.
  /// - throws: `EncodingError.invalidValue` if the given value is invalid in
  ///   the current context for this format.
  mutating func encode<T : Encodable>(_ value: T) throws

  /// Encodes a reference to the given object only if it is encoded
  /// unconditionally elsewhere in the payload (previously, or in the future).
  ///
  /// For encoders which don't support this feature, the default implementation
  /// encodes the given object unconditionally.
  ///
  /// For formats which don't support this feature, the default implementation
  /// encodes the given object unconditionally.
  ///
  /// - parameter object: The object to encode.
  /// - throws: `EncodingError.invalidValue` if the given value is invalid in
  ///   the current context for this format.
  mutating func encodeConditional<T : AnyObject & Encodable>(_ object: T) throws

% for type in codable_types:
  /// Encodes the elements of the given sequence.
  ///
  /// - parameter sequence: The sequences whose contents to encode.
  /// - throws: An error if any of the contained values throws an error.
  mutating func encode<T : Sequence>(
    contentsOf sequence: T) throws where T.Element == ${type}
% end

  /// Encodes the elements of the given sequence.
  ///
  /// - parameter sequence: The sequences whose contents to encode.
  /// - throws: An error if any of the contained values throws an error.
  mutating func encode<T : Sequence>(
    contentsOf sequence: T) throws where T.Element : Encodable

  /// Encodes a nested container keyed by the given type and returns it.
  ///
  /// - parameter keyType: The key type to use for the container.
  /// - returns: A new keyed encoding container.
  mutating func nestedContainer<NestedKey>(
    keyedBy keyType: NestedKey.Type) -> KeyedEncodingContainer<NestedKey>

  /// Encodes an unkeyed encoding container and returns it.
  ///
  /// - returns: A new unkeyed encoding container.
  mutating func nestedUnkeyedContainer() -> UnkeyedEncodingContainer

  /// Encodes a nested container and returns an `Encoder` instance for encoding
  /// `super` into that container.
  ///
  /// - returns: A new encoder to pass to `super.encode(to:)`.
  mutating func superEncoder() -> Encoder
}

/// A type that provides a view into a decoder's storage and is used to hold
/// the encoded properties of a decodable type sequentially, without keys.
///
/// Decoders should provide types conforming to `UnkeyedDecodingContainer` for
/// their format.
public protocol UnkeyedDecodingContainer {
  /// The path of coding keys taken to get to this point in decoding.
  var codingPath: [CodingKey] { get }

  /// The number of elements contained within this container.
  ///
  /// If the number of elements is unknown, the value is `nil`.
  var count: Int? { get }

  /// A Boolean value indicating whether there are no more elements left to be
  /// decoded in the container.
  var isAtEnd: Bool { get }

  /// The current decoding index of the container (i.e. the index of the next
  /// element to be decoded.) Incremented after every successful decode call.
  var currentIndex: Int { get }

  /// Decodes a null value.
  ///
  /// If the value is not null, does not increment currentIndex.
  ///
  /// - returns: Whether the encountered value was null.
  /// - throws: `DecodingError.valueNotFound` if there are no more values to
  ///   decode.
  mutating func decodeNil() throws -> Bool

% for type in codable_types:
  /// Decodes a value of the given type.
  ///
  /// - parameter type: The type of value to decode.
  /// - returns: A value of the requested type, if present for the given key
  ///   and convertible to the requested type.
  /// - throws: `DecodingError.typeMismatch` if the encountered encoded value
  ///   is not convertible to the requested type.
  /// - throws: `DecodingError.valueNotFound` if the encountered encoded value
  ///   is null, or of there are no more values to decode.
  mutating func decode(_ type: ${type}.Type) throws -> ${type}
% end

  /// Decodes a value of the given type.
  ///
  /// - parameter type: The type of value to decode.
  /// - returns: A value of the requested type, if present for the given key
  ///   and convertible to the requested type.
  /// - throws: `DecodingError.typeMismatch` if the encountered encoded value
  ///   is not convertible to the requested type.
  /// - throws: `DecodingError.valueNotFound` if the encountered encoded value
  ///   is null, or of there are no more values to decode.
  mutating func decode<T : Decodable>(_ type: T.Type) throws -> T

% for type in codable_types:
  /// Decodes a value of the given type, if present.
  ///
  /// This method returns `nil` if the container has no elements left to
  /// decode, or if the value is null. The difference between these states can
  /// be distinguished by checking `isAtEnd`.
  ///
  /// - parameter type: The type of value to decode.
  /// - returns: A decoded value of the requested type, or `nil` if the value
  ///   is a null value, or if there are no more elements to decode.
  /// - throws: `DecodingError.typeMismatch` if the encountered encoded value
  ///   is not convertible to the requested type.
  mutating func decodeIfPresent(_ type: ${type}.Type) throws -> ${type}?
% end

  /// Decodes a value of the given type, if present.
  ///
  /// This method returns `nil` if the container has no elements left to
  /// decode, or if the value is null. The difference between these states can
  /// be distinguished by checking `isAtEnd`.
  ///
  /// - parameter type: The type of value to decode.
  /// - returns: A decoded value of the requested type, or `nil` if the value
  ///   is a null value, or if there are no more elements to decode.
  /// - throws: `DecodingError.typeMismatch` if the encountered encoded value
  ///   is not convertible to the requested type.
  mutating func decodeIfPresent<T : Decodable>(_ type: T.Type) throws -> T?

  /// Decodes a nested container keyed by the given type.
  ///
  /// - parameter type: The key type to use for the container.
  /// - returns: A keyed decoding container view into `self`.
  /// - throws: `DecodingError.typeMismatch` if the encountered stored value is
  ///   not a keyed container.
  mutating func nestedContainer<NestedKey>(
    keyedBy type: NestedKey.Type) throws -> KeyedDecodingContainer<NestedKey>

  /// Decodes an unkeyed nested container.
  ///
  /// - returns: An unkeyed decoding container view into `self`.
  /// - throws: `DecodingError.typeMismatch` if the encountered stored value is
  ///   not an unkeyed container.
  mutating func nestedUnkeyedContainer() throws -> UnkeyedDecodingContainer

  /// Decodes a nested container and returns a `Decoder` instance for decoding
  /// `super` from that container.
  ///
  /// - returns: A new `Decoder` to pass to `super.init(from:)`.
  /// - throws: `DecodingError.valueNotFound` if the encountered encoded value
  ///   is null, or of there are no more values to decode.
  mutating func superDecoder() throws -> Decoder
}

//===----------------------------------------------------------------------===//
// Single Value Encoding Containers
//===----------------------------------------------------------------------===//

/// A container that can support the storage and direct encoding of a single
/// non-keyed value.
public protocol SingleValueEncodingContainer {
  /// The path of coding keys taken to get to this point in encoding.
  var codingPath: [CodingKey] { get }

  /// Encodes a null value.
  ///
  /// - throws: `EncodingError.invalidValue` if a null value is invalid in the
  ///   current context for this format.
  /// - precondition: May not be called after a previous `self.encode(_:)`
  ///   call.
  mutating func encodeNil() throws

% for type in codable_types:
  /// Encodes a single value of the given type.
  ///
  /// - parameter value: The value to encode.
  /// - throws: `EncodingError.invalidValue` if the given value is invalid in
  ///   the current context for this format.
  /// - precondition: May not be called after a previous `self.encode(_:)`
  ///   call.
  mutating func encode(_ value: ${type}) throws
% end

  /// Encodes a single value of the given type.
  ///
  /// - parameter value: The value to encode.
  /// - throws: `EncodingError.invalidValue` if the given value is invalid in
  ///   the current context for this format.
  /// - precondition: May not be called after a previous `self.encode(_:)`
  ///   call.
  mutating func encode<T : Encodable>(_ value: T) throws
}

/// A container that can support the storage and direct decoding of a single
/// nonkeyed value.
public protocol SingleValueDecodingContainer {
  /// The path of coding keys taken to get to this point in encoding.
  var codingPath: [CodingKey] { get }

  /// Decodes a null value.
  ///
  /// - returns: Whether the encountered value was null.
  func decodeNil() -> Bool

% for type in codable_types:
  /// Decodes a single value of the given type.
  ///
  /// - parameter type: The type to decode as.
  /// - returns: A value of the requested type.
  /// - throws: `DecodingError.typeMismatch` if the encountered encoded value
  ///   cannot be converted to the requested type.
  /// - throws: `DecodingError.valueNotFound` if the encountered encoded value
  ///   is null.
  func decode(_ type: ${type}.Type) throws -> ${type}
% end

  /// Decodes a single value of the given type.
  ///
  /// - parameter type: The type to decode as.
  /// - returns: A value of the requested type.
  /// - throws: `DecodingError.typeMismatch` if the encountered encoded value
  ///   cannot be converted to the requested type.
  /// - throws: `DecodingError.valueNotFound` if the encountered encoded value
  ///   is null.
  func decode<T : Decodable>(_ type: T.Type) throws -> T
}

//===----------------------------------------------------------------------===//
// User Info
//===----------------------------------------------------------------------===//

/// A user-defined key for providing context during encoding and decoding.
public struct CodingUserInfoKey : RawRepresentable, Equatable, Hashable {
  public typealias RawValue = String

  /// The key's string value.
  public let rawValue: String

  /// Creates a new instance with the given raw value.
  ///
  /// - parameter rawValue: The value of the key.
  public init?(rawValue: String) {
    self.rawValue = rawValue
  }

  /// Returns a Boolean value indicating whether the given keys are equal.
  ///
  /// - parameter lhs: The key to compare against.
  /// - parameter rhs: The key to compare with.
  public static func ==(
    lhs: CodingUserInfoKey, rhs: CodingUserInfoKey) -> Bool
  {
    return lhs.rawValue == rhs.rawValue
  }

  /// The key's hash value.
  public var hashValue: Int {
    return self.rawValue.hashValue
  }

  /// Hashes the essential components of this value by feeding them into the
  /// given hasher.
  ///
  /// - Parameter hasher: The hasher to use when combining the components
  ///   of this instance.
  public func hash(into hasher: inout Hasher) {
    hasher.combine(self.rawValue)
  }
}

//===----------------------------------------------------------------------===//
// Errors
//===----------------------------------------------------------------------===//

/// An error that occurs during the encoding of a value.
public enum EncodingError : Error {
  /// The context in which the error occurred.
  public struct Context {
    /// The path of coding keys taken to get to the point of the failing encode
    /// call.
    public let codingPath: [CodingKey]

    /// A description of what went wrong, for debugging purposes.
    public let debugDescription: String

    /// The underlying error which caused this error, if any.
    public let underlyingError: Error?

    /// Creates a new context with the given path of coding keys and a
    /// description of what went wrong.
    ///
    /// - parameter codingPath: The path of coding keys taken to get to the
    ///   point of the failing encode call.
    /// - parameter debugDescription: A description of what went wrong, for
    ///   debugging purposes.
    /// - parameter underlyingError: The underlying error which caused this
    ///   error, if any.
    public init(
      codingPath: [CodingKey],
      debugDescription: String,
      underlyingError: Error? = nil)
    {
      self.codingPath = codingPath
      self.debugDescription = debugDescription
      self.underlyingError = underlyingError
    }
  }

  /// An indication that an encoder or its containers could not encode the
  /// given value.
  ///
  /// As associated values, this case contains the attempted value and context
  /// for debugging.
  case invalidValue(Any, Context)

  // MARK: - NSError Bridging

  // CustomNSError bridging applies only when the CustomNSError conformance is
  // applied in the same module as the declared error type. Since we cannot
  // access CustomNSError (which is defined in Foundation) from here, we can
  // use the "hidden" entry points.

  public var _domain: String {
    return "NSCocoaErrorDomain"
  }

  public var _code: Int {
    switch self {
    case .invalidValue: return 4866
    }
  }

  public var _userInfo: AnyObject? {
    // The error dictionary must be returned as an AnyObject. We can do this
    // only on platforms with bridging, unfortunately.
    #if _runtime(_ObjC)
      let context: Context
      switch self {
      case .invalidValue(_, let c): context = c
      }

      var userInfo: [String : Any] = [
        "NSCodingPath": context.codingPath,
        "NSDebugDescription": context.debugDescription
      ]

      if let underlyingError = context.underlyingError {
        userInfo["NSUnderlyingError"] = underlyingError
      }

      return userInfo as AnyObject
    #else
      return nil
    #endif
  }
}

/// An error that occurs during the decoding of a value.
public enum DecodingError : Error {
  /// The context in which the error occurred.
  public struct Context {
    /// The path of coding keys taken to get to the point of the failing decode
    /// call.
    public let codingPath: [CodingKey]

    /// A description of what went wrong, for debugging purposes.
    public let debugDescription: String

    /// The underlying error which caused this error, if any.
    public let underlyingError: Error?

    /// Creates a new context with the given path of coding keys and a
    /// description of what went wrong.
    ///
    /// - parameter codingPath: The path of coding keys taken to get to the
    ///   point of the failing decode call.
    /// - parameter debugDescription: A description of what went wrong, for
    ///   debugging purposes.
    /// - parameter underlyingError: The underlying error which caused this
    ///   error, if any.
    public init(
      codingPath: [CodingKey],
      debugDescription: String,
      underlyingError: Error? = nil)
    {
      self.codingPath = codingPath
      self.debugDescription = debugDescription
      self.underlyingError = underlyingError
    }
  }

  /// An indication that a value of the given type could not be decoded because
  /// it did not match the type of what was found in the encoded payload.
  ///
  /// As associated values, this case contains the attempted type and context
  /// for debugging.
  case typeMismatch(Any.Type, Context)

  /// An indication that a non-optional value of the given type was expected,
  /// but a null value was found.
  ///
  /// As associated values, this case contains the attempted type and context
  /// for debugging.
  case valueNotFound(Any.Type, Context)

  ///  An indication that a keyed decoding container was asked for an entry for
  ///  the given key, but did not contain one.
  ///
  /// As associated values, this case contains the attempted key and context
  /// for debugging.
  case keyNotFound(CodingKey, Context)

  /// An indication that the data is corrupted or otherwise invalid.
  ///
  /// As an associated value, this case contains the context for debugging.
  case dataCorrupted(Context)

  // MARK: - NSError Bridging

  // CustomNSError bridging applies only when the CustomNSError conformance is
  // applied in the same module as the declared error type. Since we cannot
  // access CustomNSError (which is defined in Foundation) from here, we can
  // use the "hidden" entry points.

  public var _domain: String {
    return "NSCocoaErrorDomain"
  }

  public var _code: Int {
    switch self {
    case .keyNotFound, .valueNotFound: return 4865
    case .typeMismatch, .dataCorrupted:  return 4864
    }
  }

  public var _userInfo: AnyObject? {
    // The error dictionary must be returned as an AnyObject. We can do this
    // only on platforms with bridging, unfortunately.
    #if _runtime(_ObjC)
      let context: Context
      switch self {
      case .keyNotFound(_,   let c): context = c
      case .valueNotFound(_, let c): context = c
      case .typeMismatch(_,  let c): context = c
      case .dataCorrupted(   let c): context = c
      }

      var userInfo: [String : Any] = [
        "NSCodingPath": context.codingPath,
        "NSDebugDescription": context.debugDescription
      ]

      if let underlyingError = context.underlyingError {
        userInfo["NSUnderlyingError"] = underlyingError
      }

      return userInfo as AnyObject
    #else
      return nil
    #endif
  }
}

// The following extensions allow for easier error construction.

internal struct _GenericIndexKey : CodingKey {
    internal var stringValue: String
    internal var intValue: Int?

  internal init?(stringValue: String) {
    return nil
  }

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

extension DecodingError {
  /// Returns a new `.dataCorrupted` error using a constructed coding path and
  /// the given debug description.
  ///
  /// The coding path for the returned error is constructed by appending the
  /// given key to the given container's coding path.
  ///
  /// - param key: The key which caused the failure.
  /// - param container: The container in which the corrupted data was
  ///   accessed.
  /// - param debugDescription: A description of the error to aid in debugging.
  ///
  /// - Returns: A new `.dataCorrupted` error with the given information.
  public static func dataCorruptedError<C : KeyedDecodingContainerProtocol>(
    forKey key: C.Key, in container: C, debugDescription: String
  ) -> DecodingError {
    let context = DecodingError.Context(
      codingPath: container.codingPath + [key],
      debugDescription: debugDescription)
    return .dataCorrupted(context)
  }

  /// Returns a new `.dataCorrupted` error using a constructed coding path and
  /// the given debug description.
  ///
  /// The coding path for the returned error is constructed by appending the
  /// given container's current index to its coding path.
  ///
  /// - param container: The container in which the corrupted data was
  ///   accessed.
  /// - param debugDescription: A description of the error to aid in debugging.
  ///
  /// - Returns: A new `.dataCorrupted` error with the given information.
  public static func dataCorruptedError(
    in container: UnkeyedDecodingContainer,
    debugDescription: String
  ) -> DecodingError {
    let context = DecodingError.Context(
      codingPath: container.codingPath +
        [_GenericIndexKey(intValue: container.currentIndex)!],
      debugDescription: debugDescription)
    return .dataCorrupted(context)
  }

  /// Returns a new `.dataCorrupted` error using a constructed coding path and
  /// the given debug description.
  ///
  /// The coding path for the returned error is the given container's coding
  /// path.
  ///
  /// - param container: The container in which the corrupted data was
  ///   accessed.
  /// - param debugDescription: A description of the error to aid in debugging.
  ///
  /// - Returns: A new `.dataCorrupted` error with the given information.
  public static func dataCorruptedError(
    in container: SingleValueDecodingContainer, debugDescription: String
  ) -> DecodingError {
    let context = DecodingError.Context(codingPath: container.codingPath,
      debugDescription: debugDescription)
    return .dataCorrupted(context)
  }
}

//===----------------------------------------------------------------------===//
// Keyed Encoding Container Implementations
//===----------------------------------------------------------------------===//

internal class _KeyedEncodingContainerBase<Key : CodingKey> {
  internal init(){}

  deinit {}

  // These must all be given a concrete implementation in _*Box.
  internal var codingPath: [CodingKey] {
    fatalError("_KeyedEncodingContainerBase cannot be used directly.")
  }

  internal func encodeNil(forKey key: Key) throws {
    fatalError("_KeyedEncodingContainerBase cannot be used directly.")
  }

% for type in codable_types:
  internal func encode(_ value: ${type}, forKey key: Key) throws {
    fatalError("_KeyedEncodingContainerBase cannot be used directly.")
  }
% end

  internal func encode<T : Encodable>(_ value: T, forKey key: Key) throws {
    fatalError("_KeyedEncodingContainerBase cannot be used directly.")
  }

  internal func encodeConditional<T : AnyObject & Encodable>(
    _ object: T, forKey key: Key) throws
  {
    fatalError("_KeyedEncodingContainerBase cannot be used directly.")
  }

% for type in codable_types:
  internal func encodeIfPresent(_ value: ${type}?, forKey key: Key) throws {
    fatalError("_KeyedEncodingContainerBase cannot be used directly.")
  }
% end

  internal func encodeIfPresent<T : Encodable>(
    _ value: T?, forKey key: Key) throws
  {
    fatalError("_KeyedEncodingContainerBase cannot be used directly.")
  }

  internal func nestedContainer<NestedKey>(
    keyedBy keyType: NestedKey.Type, forKey key: Key
  ) -> KeyedEncodingContainer<NestedKey> {
    fatalError("_KeyedEncodingContainerBase cannot be used directly.")
  }

  internal func nestedUnkeyedContainer(
    forKey key: Key) -> UnkeyedEncodingContainer {
    fatalError("_KeyedEncodingContainerBase cannot be used directly.")
  }

  internal func superEncoder() -> Encoder {
    fatalError("_KeyedEncodingContainerBase cannot be used directly.")
  }

  internal func superEncoder(forKey key: Key) -> Encoder {
    fatalError("_KeyedEncodingContainerBase cannot be used directly.")
  }
}

internal final class _KeyedEncodingContainerBox<
  Concrete : KeyedEncodingContainerProtocol
> : _KeyedEncodingContainerBase<Concrete.Key> {
  typealias Key = Concrete.Key

  internal var concrete: Concrete

  internal init(_ container: Concrete) {
    concrete = container
  }

  override internal var codingPath: [CodingKey] {
    return concrete.codingPath
  }

  override internal func encodeNil(forKey key: Key) throws {
    try concrete.encodeNil(forKey: key)
  }

% for type in codable_types:
  override internal func encode(_ value: ${type}, forKey key: Key) throws {
    try concrete.encode(value, forKey: key)
  }
% end

  override internal func encode<T : Encodable>(
    _ value: T, forKey key: Key) throws
  {
    try concrete.encode(value, forKey: key)
  }

  override internal func encodeConditional<T : AnyObject & Encodable>(
    _ object: T, forKey key: Key) throws
  {
    try concrete.encodeConditional(object, forKey: key)
  }

% for type in codable_types:
  override internal func encodeIfPresent(
    _ value: ${type}?, forKey key: Key) throws
  {
    try concrete.encodeIfPresent(value, forKey: key)
  }
% end

  override internal func encodeIfPresent<T : Encodable>(
    _ value: T?, forKey key: Key) throws
  {
    try concrete.encodeIfPresent(value, forKey: key)
  }

  override internal func nestedContainer<NestedKey>(
    keyedBy keyType: NestedKey.Type, forKey key: Key
  ) -> KeyedEncodingContainer<NestedKey> {
    return concrete.nestedContainer(keyedBy: NestedKey.self, forKey: key)
  }

  override internal func nestedUnkeyedContainer(
    forKey key: Key) -> UnkeyedEncodingContainer
  {
    return concrete.nestedUnkeyedContainer(forKey: key)
  }

  override internal func superEncoder() -> Encoder {
    return concrete.superEncoder()
  }

  override internal func superEncoder(forKey key: Key) -> Encoder {
    return concrete.superEncoder(forKey: key)
  }
}

internal class _KeyedDecodingContainerBase<Key : CodingKey> {
  internal init(){}

  deinit {}

  internal var codingPath: [CodingKey] {
    fatalError("_KeyedDecodingContainerBase cannot be used directly.")
  }

  internal var allKeys: [Key] {
    fatalError("_KeyedDecodingContainerBase cannot be used directly.")
  }

  internal func contains(_ key: Key) -> Bool {
    fatalError("_KeyedDecodingContainerBase cannot be used directly.")
  }

  internal func decodeNil(forKey key: Key) throws -> Bool {
    fatalError("_KeyedDecodingContainerBase cannot be used directly.")
  }

% for type in codable_types:
  internal func decode(
    _ type: ${type}.Type, forKey key: Key) throws -> ${type}
  {
    fatalError("_KeyedDecodingContainerBase cannot be used directly.")
  }
% end

  internal func decode<T : Decodable>(
    _ type: T.Type, forKey key: Key) throws -> T
  {
    fatalError("_KeyedDecodingContainerBase cannot be used directly.")
  }

% for type in codable_types:
  internal func decodeIfPresent(
    _ type: ${type}.Type, forKey key: Key) throws -> ${type}?
  {
    fatalError("_KeyedDecodingContainerBase cannot be used directly.")
  }
% end

  internal func decodeIfPresent<T : Decodable>(
    _ type: T.Type, forKey key: Key) throws -> T?
  {
    fatalError("_KeyedDecodingContainerBase cannot be used directly.")
  }

  internal func nestedContainer<NestedKey>(
    keyedBy type: NestedKey.Type, forKey key: Key
  ) throws -> KeyedDecodingContainer<NestedKey> {
    fatalError("_KeyedDecodingContainerBase cannot be used directly.")
  }

  internal func nestedUnkeyedContainer(
    forKey key: Key) throws -> UnkeyedDecodingContainer
  {
    fatalError("_KeyedDecodingContainerBase cannot be used directly.")
  }

  internal func superDecoder() throws -> Decoder {
    fatalError("_KeyedDecodingContainerBase cannot be used directly.")
  }

  internal func superDecoder(forKey key: Key) throws -> Decoder {
    fatalError("_KeyedDecodingContainerBase cannot be used directly.")
  }
}

internal final class _KeyedDecodingContainerBox<
  Concrete : KeyedDecodingContainerProtocol
> : _KeyedDecodingContainerBase<Concrete.Key> {
  typealias Key = Concrete.Key

  internal var concrete: Concrete

  internal init(_ container: Concrete) {
    concrete = container
  }

  override var codingPath: [CodingKey] {
    return concrete.codingPath
  }

  override var allKeys: [Key] {
    return concrete.allKeys
  }

  override internal func contains(_ key: Key) -> Bool {
    return concrete.contains(key)
  }

  override internal func decodeNil(forKey key: Key) throws -> Bool {
    return try concrete.decodeNil(forKey: key)
  }

% for type in codable_types:
  override internal func decode(
    _ type: ${type}.Type, forKey key: Key) throws -> ${type}
  {
    return try concrete.decode(${type}.self, forKey: key)
  }
% end

  override internal func decode<T : Decodable>(
    _ type: T.Type, forKey key: Key) throws -> T
  {
    return try concrete.decode(T.self, forKey: key)
  }

% for type in codable_types:
  override internal func decodeIfPresent(
    _ type: ${type}.Type, forKey key: Key) throws -> ${type}?
  {
    return try concrete.decodeIfPresent(${type}.self, forKey: key)
  }
% end

  override internal func decodeIfPresent<T : Decodable>(
    _ type: T.Type, forKey key: Key) throws -> T?
  {
    return try concrete.decodeIfPresent(T.self, forKey: key)
  }

  override internal func nestedContainer<NestedKey>(
    keyedBy type: NestedKey.Type, forKey key: Key
  ) throws -> KeyedDecodingContainer<NestedKey> {
    return try concrete.nestedContainer(keyedBy: NestedKey.self, forKey: key)
  }

  override internal func nestedUnkeyedContainer(
    forKey key: Key) throws -> UnkeyedDecodingContainer
  {
    return try concrete.nestedUnkeyedContainer(forKey: key)
  }

  override internal func superDecoder() throws -> Decoder {
    return try concrete.superDecoder()
  }

  override internal func superDecoder(forKey key: Key) throws -> Decoder {
    return try concrete.superDecoder(forKey: key)
  }
}

//===----------------------------------------------------------------------===//
// Primitive and RawRepresentable Extensions
//===----------------------------------------------------------------------===//

% for type in codable_types:
extension ${type} : Codable {
  /// Creates a new instance by decoding from the given decoder.
  ///
  /// This initializer throws an error if reading from the decoder fails, or
  /// if the data read is corrupted or otherwise invalid.
  ///
  /// - Parameter decoder: The decoder to read data from.
  public init(from decoder: Decoder) throws {
    self = try decoder.singleValueContainer().decode(${type}.self)
  }

  /// Encodes this value into the given encoder.
  ///
  /// This function throws an error if any values are invalid for the given
  /// encoder's format.
  ///
  /// - Parameter encoder: The encoder to write data to.
  public func encode(to encoder: Encoder) throws {
    var container = encoder.singleValueContainer()
    try container.encode(self)
  }
}

extension RawRepresentable where RawValue == ${type}, Self : Encodable {
  /// Encodes this value into the given encoder, when the type's `RawValue`
  /// is `${type}`.
  ///
  /// This function throws an error if any values are invalid for the given
  /// encoder's format.
  ///
  /// - Parameter encoder: The encoder to write data to.
  public func encode(to encoder: Encoder) throws {
    var container = encoder.singleValueContainer()
    try container.encode(self.rawValue)
  }
}

extension RawRepresentable where RawValue == ${type}, Self : Decodable {
  /// Creates a new instance by decoding from the given decoder, when the
  /// type's `RawValue` is `${type}`.
  ///
  /// This initializer throws an error if reading from the decoder fails, or
  /// if the data read is corrupted or otherwise invalid.
  ///
  /// - Parameter decoder: The decoder to read data from.
  public init(from decoder: Decoder) throws {
    let decoded = try decoder.singleValueContainer().decode(RawValue.self)
    guard let value = Self(rawValue: decoded) else {
      throw DecodingError.dataCorrupted(
        DecodingError.Context(
          codingPath: decoder.codingPath,
          debugDescription: "Cannot initialize \(Self.self) from invalid \(RawValue.self) value \(decoded)"))
    }

    self = value
  }
}
% end

//===----------------------------------------------------------------------===//
// Optional/Collection Type Conformances
//===----------------------------------------------------------------------===//

extension Optional : Encodable where Wrapped : Encodable {
  /// Encodes this optional value into the given encoder.
  ///
  /// This function throws an error if any values are invalid for the given
  /// encoder's format.
  ///
  /// - Parameter encoder: The encoder to write data to.
  public func encode(to encoder: Encoder) throws {
    var container = encoder.singleValueContainer()
    switch self {
    case .none: try container.encodeNil()
    case .some(let wrapped): try container.encode(wrapped)
    }
  }
}

extension Optional : Decodable where Wrapped : Decodable {
  /// Creates a new instance by decoding from the given decoder.
  ///
  /// This initializer throws an error if reading from the decoder fails, or
  /// if the data read is corrupted or otherwise invalid.
  ///
  /// - Parameter decoder: The decoder to read data from.
  public init(from decoder: Decoder) throws {
    let container = try decoder.singleValueContainer()
    if container.decodeNil() {
      self = .none
    }  else {
      let element = try container.decode(Wrapped.self)
      self = .some(element)
    }
  }
}

extension Array : Encodable where Element : Encodable {
  /// Encodes the elements of this array into the given encoder in an unkeyed
  /// container.
  ///
  /// This function throws an error if any values are invalid for the given
  /// encoder's format.
  ///
  /// - Parameter encoder: The encoder to write data to.
  public func encode(to encoder: Encoder) throws {
    var container = encoder.unkeyedContainer()
    for element in self {
      try container.encode(element)
    }
  }
}

extension Array : Decodable where Element : Decodable {
  /// Creates a new array by decoding from the given decoder.
  ///
  /// This initializer throws an error if reading from the decoder fails, or
  /// if the data read is corrupted or otherwise invalid.
  ///
  /// - Parameter decoder: The decoder to read data from.
  public init(from decoder: Decoder) throws {
    self.init()

    var container = try decoder.unkeyedContainer()
    while !container.isAtEnd {
      let element = try container.decode(Element.self)
      self.append(element)
    }
  }
}

extension ContiguousArray : Encodable where Element : Encodable {
  /// Encodes the elements of this contiguous array into the given encoder
  /// in an unkeyed container.
  ///
  /// This function throws an error if any values are invalid for the given
  /// encoder's format.
  ///
  /// - Parameter encoder: The encoder to write data to.
  public func encode(to encoder: Encoder) throws {
    var container = encoder.unkeyedContainer()
    for element in self {
      try container.encode(element)
    }
  }
}

extension ContiguousArray : Decodable where Element : Decodable {
  /// Creates a new contiguous array by decoding from the given decoder.
  ///
  /// This initializer throws an error if reading from the decoder fails, or
  /// if the data read is corrupted or otherwise invalid.
  ///
  /// - Parameter decoder: The decoder to read data from.
  public init(from decoder: Decoder) throws {
    self.init()

    var container = try decoder.unkeyedContainer()
    while !container.isAtEnd {
      let element = try container.decode(Element.self)
      self.append(element)
    }
  }
}

extension Set : Encodable where Element : Encodable {
  /// Encodes the elements of this set into the given encoder in an unkeyed
  /// container.
  ///
  /// This function throws an error if any values are invalid for the given
  /// encoder's format.
  ///
  /// - Parameter encoder: The encoder to write data to.
  public func encode(to encoder: Encoder) throws {
    var container = encoder.unkeyedContainer()
    for element in self {
      try container.encode(element)
    }
  }
}

extension Set : Decodable where Element : Decodable {
  /// Creates a new set by decoding from the given decoder.
  ///
  /// This initializer throws an error if reading from the decoder fails, or
  /// if the data read is corrupted or otherwise invalid.
  ///
  /// - Parameter decoder: The decoder to read data from.
  public init(from decoder: Decoder) throws {
    self.init()

    var container = try decoder.unkeyedContainer()
    while !container.isAtEnd {
      let element = try container.decode(Element.self)
      self.insert(element)
    }
  }
}

/// A wrapper for dictionary keys which are Strings or Ints.
internal struct _DictionaryCodingKey : CodingKey {
  internal let stringValue: String
  internal let intValue: Int?

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

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

extension Dictionary : Encodable where Key : Encodable, Value : Encodable {
  /// Encodes the contents of this dictionary into the given encoder.
  ///
  /// If the dictionary uses `String` or `Int` keys, the contents are encoded
  /// in a keyed container. Otherwise, the contents are encoded as alternating
  /// key-value pairs in an unkeyed container.
  ///
  /// This function throws an error if any values are invalid for the given
  /// encoder's format.
  ///
  /// - Parameter encoder: The encoder to write data to.
  public func encode(to encoder: Encoder) throws {
    if Key.self == String.self {
      // Since the keys are already Strings, we can use them as keys directly.
      var container = encoder.container(keyedBy: _DictionaryCodingKey.self)
      for (key, value) in self {
        let codingKey = _DictionaryCodingKey(stringValue: key as! String)!
        try container.encode(value, forKey: codingKey)
      }
    } else if Key.self == Int.self {
      // Since the keys are already Ints, we can use them as keys directly.
      var container = encoder.container(keyedBy: _DictionaryCodingKey.self)
      for (key, value) in self {
        let codingKey = _DictionaryCodingKey(intValue: key as! Int)!
        try container.encode(value, forKey: codingKey)
      }
    } else {
      // Keys are Encodable but not Strings or Ints, so we cannot arbitrarily
      // convert to keys. We can encode as an array of alternating key-value
      // pairs, though.
      var container = encoder.unkeyedContainer()
      for (key, value) in self {
        try container.encode(key)
        try container.encode(value)
      }
    }
  }
}

extension Dictionary : Decodable where Key : Decodable, Value : Decodable {
  /// Creates a new dictionary by decoding from the given decoder.
  ///
  /// This initializer throws an error if reading from the decoder fails, or
  /// if the data read is corrupted or otherwise invalid.
  ///
  /// - Parameter decoder: The decoder to read data from.
  public init(from decoder: Decoder) throws {
    self.init()

    if Key.self == String.self {
      // The keys are Strings, so we should be able to expect a keyed container.
      let container = try decoder.container(keyedBy: _DictionaryCodingKey.self)
      for key in container.allKeys {
        let value = try container.decode(Value.self, forKey: key)
        self[key.stringValue as! Key] = value
      }
    } else if Key.self == Int.self {
      // The keys are Ints, so we should be able to expect a keyed container.
      let container = try decoder.container(keyedBy: _DictionaryCodingKey.self)
      for key in container.allKeys {
        guard key.intValue != nil else {
          // We provide stringValues for Int keys; if an encoder chooses not to
          // use the actual intValues, we've encoded string keys.
          // So on init, _DictionaryCodingKey tries to parse string keys as
          // Ints. If that succeeds, then we would have had an intValue here.
          // We don't, so this isn't a valid Int key.
          var codingPath = decoder.codingPath
          codingPath.append(key)
          throw DecodingError.typeMismatch(
            Int.self, DecodingError.Context(
              codingPath: codingPath,
              debugDescription: "Expected Int key but found String key instead."))
        }

        let value = try container.decode(Value.self, forKey: key)
        self[key.intValue! as! Key] = value
      }
    } else {
      // We should have encoded as an array of alternating key-value pairs.
      var container = try decoder.unkeyedContainer()

      // We're expecting to get pairs. If the container has a known count, it
      // had better be even; no point in doing work if not.
      if let count = container.count {
        guard count % 2 == 0 else {
          throw DecodingError.dataCorrupted(
            DecodingError.Context(
              codingPath: decoder.codingPath,
              debugDescription: "Expected collection of key-value pairs; encountered odd-length array instead."))
        }
      }

      while !container.isAtEnd {
        let key = try container.decode(Key.self)

        guard !container.isAtEnd else {
          throw DecodingError.dataCorrupted(
            DecodingError.Context(
              codingPath: decoder.codingPath,
              debugDescription: "Unkeyed container reached end before value in key-value pair."))
        }

        let value = try container.decode(Value.self)
        self[key] = value
      }
    }
  }
}

//===----------------------------------------------------------------------===//
// Convenience Default Implementations
//===----------------------------------------------------------------------===//

// Default implementation of encodeConditional(_:forKey:) in terms of
// encode(_:forKey:)
extension KeyedEncodingContainerProtocol {
  public mutating func encodeConditional<T : AnyObject & Encodable>(
    _ object: T, forKey key: Key) throws
  {
    try encode(object, forKey: key)
  }
}

// Default implementation of encodeIfPresent(_:forKey:) in terms of
// encode(_:forKey:)
extension KeyedEncodingContainerProtocol {
% for type in codable_types:
  public mutating func encodeIfPresent(
    _ value: ${type}?, forKey key: Key) throws
  {
    guard let value = value else { return }
    try encode(value, forKey: key)
  }
% end

  public mutating func encodeIfPresent<T : Encodable>(
    _ value: T?, forKey key: Key) throws
  {
    guard let value = value else { return }
    try encode(value, forKey: key)
  }
}

// Default implementation of decodeIfPresent(_:forKey:) in terms of
// decode(_:forKey:) and decodeNil(forKey:)
extension KeyedDecodingContainerProtocol {
% for type in codable_types:
  public func decodeIfPresent(
    _ type: ${type}.Type, forKey key: Key) throws -> ${type}?
  {
    guard try self.contains(key) && !self.decodeNil(forKey: key)
      else { return nil }
    return try self.decode(${type}.self, forKey: key)
  }
% end

  public func decodeIfPresent<T : Decodable>(
    _ type: T.Type, forKey key: Key) throws -> T?
  {
    guard try self.contains(key) && !self.decodeNil(forKey: key)
      else { return nil }
    return try self.decode(T.self, forKey: key)
  }
}

// Default implementation of encodeConditional(_:) in terms of encode(_:),
// and encode(contentsOf:) in terms of encode(_:) loop.
extension UnkeyedEncodingContainer {
  public mutating func encodeConditional<T : AnyObject & Encodable>(
    _ object: T) throws
  {
    try self.encode(object)
  }

% for type in codable_types:
  public mutating func encode<T : Sequence>(
    contentsOf sequence: T) throws where T.Element == ${type}
  {
    for element in sequence {
      try self.encode(element)
    }
  }
% end

  public mutating func encode<T : Sequence>(
    contentsOf sequence: T) throws where T.Element : Encodable
  {
    for element in sequence {
      try self.encode(element)
    }
  }
}

// Default implementation of decodeIfPresent(_:) in terms of decode(_:) and
// decodeNil()
extension UnkeyedDecodingContainer {
% for type in codable_types:
  public mutating func decodeIfPresent(
    _ type: ${type}.Type) throws -> ${type}?
  {
    guard try !self.isAtEnd && !self.decodeNil() else { return nil }
    return try self.decode(${type}.self)
  }
% end

  public mutating func decodeIfPresent<T : Decodable>(
    _ type: T.Type) throws -> T?
  {
    guard try !self.isAtEnd && !self.decodeNil() else { return nil }
    return try self.decode(T.self)
  }
}

// ${'Local Variables'}:
// eval: (read-only-mode 1)
// End: