Polygon shape considered a corner radius.

Polygon.swift

/// 多角形のシェープ.
struct Polygon : Shape {
    
    /// 多角形の頂点の数. デフォルト値は`3`
    var numberOfVertex: Int
    
    /// 角の半径. デフォルト値は`3`.
    var cornerRadius: CGFloat
    
    /// 直線をトップに配置するかどうかのBool値. デフォルト値は`false`.
    ///
    /// `true`のときはは直線の中心がちょうど上にくる.
    /// `false`のときは。多角形の頂点がちょうど上にくる.
    var placesLineToTop: Bool
    
    init(_ numberOfVertex: Int = 3, cornerRadius: CGFloat = 3, placesLineToTop: Bool = false) {
        precondition(numberOfVertex >= 3)
        self.numberOfVertex = numberOfVertex
        self.cornerRadius = cornerRadius
        self.placesLineToTop = placesLineToTop
    }
    
    func path(in rect: CGRect) -> Path {
        var path = Path()
        
        let center = CGPoint(x: rect.midX, y: rect.midY)
        let radius = rect.size.width / 2
        let step = Double.pi * 2 / Double(self.numberOfVertex)
        
        let initialAngle = -(.pi / 2) + step / 2 * CGFloat(placesLineToTop ? 1 : 0)
        
        let points: [CGPoint] = (0..<self.numberOfVertex).map({ cornerNumber in
            let angle = initialAngle + step * CGFloat(cornerNumber)
            let x = center.x + cos(angle) * radius
            let y = center.y + sin(angle) * radius
            return CGPoint(x: x, y: y)
        })
        
        for i in (0..<points.count) {
            let nextPoint = i < points.count - 1 ? points[i + 1] : points[0]
            path.addArc(tangent1End: points[i], tangent2End: nextPoint, radius: cornerRadius)
        }
        path.closeSubpath()
        
        return path
    }
    
}

#Preview {
    Polygon(5, cornerRadius: 3, placesLineToTop: true)
        .stroke(lineWidth: 2)
        .frame(width: 25, height: 25)
}