SetCard abstraction

SetCard.swift

func combinations<S1: Sequence, S2: Sequence>(_ sequence1: S1, _ sequence2: S2) -> AnySequence<(S1.Element, S2.Element)> {
  AnySequence(
    sequence1.lazy.flatMap { element1 in
      sequence2.lazy.map { element2 in
        (element1, element2)
      }
    }
  )
}

func combinations<S1: Sequence, S2: Sequence, S3: Sequence>(_ sequence1: S1, _ sequence2: S2, _ sequence3: S3) -> AnySequence<(S1.Element, S2.Element, S3.Element)> {
  AnySequence (
    combinations(combinations(sequence1, sequence2), sequence3).lazy.map { ($0.0.0, $0.0.1, $0.1)}
  )
}

/// Produce all of the combinations of elements in each sequence.
/// - Note:
/// - Complexity: O(1)
/// - Parameters:
///   - sequence1: Input sequence.
///   - sequence2: Input sequence.
///   - sequence3: Input sequence.
///   - sequence4: Input sequence.
/// - Returns: All of the combinations.
func combinations<S1: Sequence, S2: Sequence, S3: Sequence, S4: Sequence>(_ sequence1: S1, _ sequence2: S2, _ sequence3: S3, _ sequence4: S4) -> AnySequence<(S1.Element, S2.Element, S3.Element, S4.Element)> {
  AnySequence (
    combinations(combinations(sequence1, sequence2, sequence3), sequence4).lazy.map { ($0.0.0, $0.0.1, $0.0.2, $0.1)}
  )
}


struct SetCard: Hashable {
  enum Number: CaseIterable, Hashable {
    case one, two, three
  }
  enum Shape: CaseIterable, Hashable {
    case diamond, squiggle, oval
  }
  enum Shading: CaseIterable, Hashable {
    case solid, stripped, open
  }
  enum Color: CaseIterable, Hashable {
    case red, green, purple
  }
  
  var number: Number
  var shape: Shape
  var shading: Shading
  var color: Color
}

extension SetCard {
  static func makeDeck() -> [SetCard] {
    combinations(Number.allCases, Shape.allCases,
                 Shading.allCases, Color.allCases).map(Self.init)
  }
}

extension SetCard {
  private static func pickAttribute<A: CaseIterable & Hashable>(_ a: A, _ b: A) -> A {
    a == b ? a : A.allCases.filter { $0 != a && $0 != b }.first!
  }
  
  static func complete(_ a: SetCard, _ b: SetCard) -> SetCard {
    SetCard(number: pickAttribute(a.number, b.number),
            shape: pickAttribute(a.shape, b.shape),
            shading: pickAttribute(a.shading, b.shading),
            color: pickAttribute(a.color, b.color))
  }
}

extension Collection where Element == SetCard {
  
  private func attributeIsSet<A: Hashable>(accessor: (SetCard) -> A) -> Bool {
    let attributes = self.reduce(into: Set<A>()) { $0.insert(accessor($1)) }
    return attributes.count == 1 || attributes.count == 3
  }

  func isSet() -> Bool {
    attributeIsSet(accessor: \.number) &&
    attributeIsSet(accessor: \.shape) &&
    attributeIsSet(accessor: \.shading) &&
    attributeIsSet(accessor: \.color)
  }
}