RegularPolygon.swift

import SwiftUI

// Include this CGPoint extension: https://gist.github.com/Dev1an/7973cee9d960479b35b705f88b7f38c4

public struct RegularPolygon: Shape {

	let corners: [CGPoint]

	/**
	 Create a polygon shape with provided number of sides.
	 - Parameter sides: number of sides
	 */
	public init(sides: Int) {
		assert(sides > 2, "A polygon must have more than 2 sides")
		corners = .init(unsafeUninitializedCapacity: sides) { list, count in
			var corner = list.baseAddress!
			let endCorner = corner.advanced(by: sides)
			let angleIncrement = CGFloat.pi * 2 / CGFloat(sides)
			var angle = CGFloat.zero
			while corner < endCorner {
				corner.initialize(to: [ cos(angle), sin(angle) ] )
				corner = corner.advanced(by: 1)
				angle += angleIncrement
			}
			count = sides
		}
	}

	public func path(in rect: CGRect) -> Path {
		var path = Path()

		if let start = corners.first {
			let center = rect.origin + [rect.width, rect.height] / 2
			let diameter = min(rect.width, rect.height)
			let radius = diameter / 2.0

			path.move(to: start * radius + center)
			for unitCorner in corners.dropFirst() {
				path.addLine(to: unitCorner * radius + center)
			}

			path.closeSubpath()
		}

		return path
	}
}

struct RegularPolygon_Previews: PreviewProvider {
	static var previews: some View {
		VStack {
			ForEach(3..<9, content: RegularPolygon.init)
		}
	}
}