dictionaryCodable.swift

extension Dictionary : Encodable /* where Key : Encodable, Value : Encodable */ {
    public func encode(to encoder: Encoder) throws {
        assertTypeIsEncodable(Key.self, in: type(of: self))
        assertTypeIsEncodable(Value.self, in: type(of: self))

        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 (value as! Encodable).__encode(to: &container, 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 (value as! Encodable).__encode(to: &container, 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 (key as! Encodable).__encode(to: &container)
                try (value as! Encodable).__encode(to: &container)
            }
        }
    }
}