[Complex] Encode and decode using unkeyedContainer

[benrimmington]

The encoding would change from {"x":0,"y":0} to [0,0] in JSON.

SE-0239 was originally pitched with CodingKeys, but in a discussion of the implementation, Tony Parker wrote:

This is one of those cases where I think we should just use two unkeyed values. ClosedRange is well defined and I don't think it's likely for it to grow the need for more keys. Same goes for the other Range types.

This would probably also make the encoded range types more parsable by other languages when encoded to JSON.

... I feel very strongly that we should prefer a compact representation for fundamental and mathematically-defined types like this.

Instead of the synthesized conformance, we could use the following:

extension Complex: Decodable where RealType: Decodable {
  public init(from decoder: Decoder) throws {
    var unkeyedContainer = try decoder.unkeyedContainer()
    let x = try unkeyedContainer.decode(RealType.self)
    let y = try unkeyedContainer.decode(RealType.self)
    self.init(x, y)
  }
}

extension Complex: Encodable where RealType: Encodable {
  public func encode(to encoder: Encoder) throws {
    var unkeyedContainer = encoder.unkeyedContainer()
    try unkeyedContainer.encode(x)
    try unkeyedContainer.encode(y)
  }
}

[benrimmington]

Here's a simple test for the PropertyTests.swift file:

final class PropertyTests: XCTestCase {

  func testCodable<T: Codable & Real>(_ type: T.Type) throws {
    let encoder = JSONEncoder()
    encoder.nonConformingFloatEncodingStrategy = .convertToString(
      positiveInfinity: "inf",
      negativeInfinity: "-inf",
      nan: "nan")

    let decoder = JSONDecoder()
    decoder.nonConformingFloatDecodingStrategy = .convertFromString(
      positiveInfinity: "inf",
      negativeInfinity: "-inf",
      nan: "nan")

    for expected: Complex<T> in [.zero, .one, .i, .infinity] {
      let data = try encoder.encode(expected)
      // print("*** \(String(decoding: data, as: Unicode.UTF8.self)) ***")
      let actual = try decoder.decode(Complex<T>.self, from: data)
      XCTAssertEqual(actual, expected)
    }
  }

  func testCodable() throws {
    try testCodable(Float32.self)
    try testCodable(Float64.self)
    // Float80 doesn't conform to Codable.
  }
}

[stephentyrone]

Hi Ben! This makes good sense to me. If you throw up a PR, I'll review and merge.