Model Tic-Tac-Toe board using proper type (#722)

* Model Tic-Tac-Toe board using proper type

* Update GameCore.swift

* wip

* wip

Author: stephencelis

Examples/TicTacToe/tic-tac-toe/Sources/GameCore/GameCore.swift

@@ -21,7 +21,7 @@ public enum Player: Equatable {
 }
 
 public struct GameState: Equatable {
-  public var board: [[Player?]] = .empty
+  public var board: Three<Three<Player?>> = .empty
   public var currentPlayer: Player = .x
   public var oPlayerName: String
   public var xPlayerName: String
@@ -74,12 +74,12 @@ public let gameReducer = Reducer<GameState, GameAction, GameEnvironment> { state
   }
 }
 
-extension Array where Element == [Player?] {
-  public static let empty: Self = [
-    [nil, nil, nil],
-    [nil, nil, nil],
-    [nil, nil, nil],
-  ]
+extension Three where Element == Three<Player?> {
+  public static let empty = Self(
+    .init(nil, nil, nil),
+    .init(nil, nil, nil),
+    .init(nil, nil, nil)
+  )
 
   public var isFilled: Bool {
     self.allSatisfy { row in row.allSatisfy { $0 != nil } }

Examples/TicTacToe/tic-tac-toe/Sources/GameCore/Three.swift

@@ -0,0 +1,46 @@
+/// A collection of three elements.
+public struct Three<Element> {
+  public var first: Element
+  public var second: Element
+  public var third: Element
+
+  public init(_ first: Element, _ second: Element, _ third: Element) {
+    self.first = first
+    self.second = second
+    self.third = third
+  }
+
+  public func map<T>(_ transform: (Element) -> T) -> Three<T> {
+    .init(transform(self.first), transform(self.second), transform(self.third))
+  }
+}
+
+extension Three: MutableCollection {
+  public subscript(offset: Int) -> Element {
+    _read {
+      switch offset {
+      case 0: yield self.first
+      case 1: yield self.second
+      case 2: yield self.third
+      default: fatalError()
+      }
+    }
+    _modify {
+      switch offset {
+      case 0: yield &self.first
+      case 1: yield &self.second
+      case 2: yield &self.third
+      default: fatalError()
+      }
+    }
+  }
+
+  public var startIndex: Int { 0 }
+  public var endIndex: Int { 3 }
+  public func index(after i: Int) -> Int { i + 1 }
+}
+
+extension Three: RandomAccessCollection {}
+
+extension Three: Equatable where Element: Equatable {}
+extension Three: Hashable where Element: Hashable {}

Examples/TicTacToe/tic-tac-toe/Sources/GameSwiftUI/GameView.swift

@@ -78,7 +78,7 @@ public struct GameView: View {
     proxy: GeometryProxy,
     viewStore: ViewStore<ViewState, GameAction>
   ) -> some View {
-    Button.init(action: {
+    Button(action: {
       viewStore.send(.cellTapped(row: row, column: column))
     }) {
       Text(viewStore.board[row][column])

Examples/TicTacToe/tic-tac-toe/Sources/GameUIKit/GameViewController.swift

@@ -8,7 +8,7 @@ public final class GameViewController: UIViewController {
   let viewStore: ViewStore<ViewState, GameAction>
 
   struct ViewState: Equatable {
-    let board: [[String]]
+    let board: Three<Three<String>>
     let isGameEnabled: Bool
     let isPlayAgainButtonHidden: Bool
     let title: String?