Function to create a rounded polygon from the corner points and a corner radius

PlayButtonPost_RoundenPolygon.swift

public extension CGPath {
  static func createRoundedShapeWithinCornerPoints(points: [CGPoint],
                                                   cornerRadius radius: CGFloat,
                                                   shouldAddTwoArcsForIntermediatePoints: Bool = false) -> CGPath {
    assert(points.count > 2, "We need at least 3 points.")

    let path = CGMutablePath()
    
    for i in 0 ..< points.count {
      let firstIndex = i
      let secondIndex = mod(i + 1, points.count)
      let thirdIndex = mod(i + 2, points.count)

      let from = points[firstIndex]
      let via = points[secondIndex]
      let to = points[thirdIndex]

      let fromAngle = atan2f(Float(via.y - from.y), Float(via.x - from.x))
      let toAngle = atan2f(Float(to.y - via.y), Float(to.x - via.x))
      let fromOffset = CGVector(dx: CGFloat(-sinf(fromAngle)) * radius, dy: CGFloat(cosf(fromAngle)) * radius)
      let toOffset = CGVector(dx: CGFloat(-sinf(toAngle)) * radius, dy: CGFloat(cosf(toAngle)) * radius)

      let x1 = from.x + fromOffset.dx
      let y1 = from.y + fromOffset.dy
      let x2 = via.x + fromOffset.dx
      let y2 = via.y + fromOffset.dy
      let x3 = via.x + toOffset.dx
      let y3 = via.y + toOffset.dy
      let x4 = to.x + toOffset.dx
      let y4 = to.y + toOffset.dy

      let intersectionX = ((x1 * y2 - y1 * x2) * (x3 - x4) - (x1 - x2) * (x3 * y4 - y3 * x4)) / ((x1 - x2) * (y3 - y4) - (y1 - y2) * (x3 - x4))
      let intersectionY = ((x1 * y2 - y1 * x2) * (y3 - y4) - (y1 - y2) * (x3 * y4 - y3 * x4)) / ((x1 - x2) * (y3 - y4) - (y1 - y2) * (x3 - x4))

      let cornerCenter = CGPoint(x: floor(intersectionX), y: floor(intersectionY))
      let startAngle = fromAngle - (.pi / 2.0)
      let endAngle = toAngle - (.pi / 2.0)

      if shouldAddTwoArcsForIntermediatePoints && i != 0 && i != points.count - 1 {
        let intermediateStep = (startAngle + endAngle) / 2.0
        path.addArc(center: cornerCenter, radius: radius, startAngle: CGFloat(startAngle), endAngle: CGFloat(intermediateStep), clockwise: false)
        path.addArc(center: cornerCenter, radius: radius, startAngle: CGFloat(intermediateStep), endAngle: CGFloat(endAngle), clockwise: false)
      } else {
        path.addArc(center: cornerCenter, radius: radius, startAngle: CGFloat(startAngle), endAngle: CGFloat(endAngle), clockwise: false)
      }
    }
    path.closeSubpath()
    return path
  }
}