fix build by converting cxx backend to swift

Sources/SwiftFrontend/Ball.swift

@@ -0,0 +1,55 @@
+import Foundation
+import simd
+
+struct BallWorld {
+  var balls: [Ball] = []
+
+  mutating func add(position: SIMD2<Float>, angle: Float, size: Float) {
+    balls.append(Ball(position: position, angle: angle, size: size))
+  }
+
+  mutating func update(_ deltaTime: Float) {
+    for i in balls.indices {
+      balls[i].update(deltaTime)
+    }
+  }
+}
+
+struct Ball {
+  public static let speed: Float = 80.0
+  public static let worldSize: Float = 512.0
+
+  var _position: SIMD2<Float>, _velocity: SIMD2<Float>
+  var _size: Float
+
+  init(position: SIMD2<Float>, angle: Float, size ballSize: Float) {
+    _position = position
+    _velocity = SIMD2<Float>(
+       cos(angle * Float.pi * 2.0),
+      -sin(angle * Float.pi * 2.0)
+    )
+    _size = ballSize
+  }
+
+  public mutating func update(_ deltaTime: Float) {
+    _position += _velocity * Self.speed * deltaTime
+    if (_position.x < _size) {
+      _velocity.x = -_velocity.x
+      _position.x = _size
+    } else if (_position.x > Self.worldSize - _size) {
+      _velocity.x = -_velocity.x
+      _position.x = Self.worldSize - _size
+    }
+    if (_position.y < _size) {
+      _velocity.y = -_velocity.y
+      _position.y = _size
+    } else if (_position.y > Self.worldSize - _size) {
+      _velocity.y = -_velocity.y
+      _position.y = Self.worldSize - _size
+    }
+  }
+
+  var position: SIMD2<Float> { return _position }
+  var velocity: SIMD2<Float> { return _velocity }
+  var size: Float { return _size }
+}