circle.swift

func circle(p1: CGPoint, _ p2: CGPoint, _ p3: CGPoint) -> (center: CGPoint, radius: CGFloat) {
    let p = ((p1.y - p3.y) * (p1.y * p1.y - p2.y * p2.y + p1.x * p1.x - p2.x * p2.x) - (p1.y - p2.y) * (p1.y * p1.y - p3.y * p3.y + p1.x * p1.x - p3.x * p3.x)) / (2 * (p1.y - p3.y) * (p1.x - p2.x) - 2 * (p1.y - p2.y) * (p1.x - p3.x))
    let q = ((p1.x - p3.x) * (p1.x * p1.x - p2.x * p2.x + p1.y * p1.y - p2.y * p2.y) - (p1.x - p2.x) * (p1.x * p1.x - p3.x * p3.x + p1.y * p1.y - p3.y * p3.y)) / (2 * (p1.x - p3.x) * (p1.y - p2.y) - 2 * (p1.x - p2.x) * (p1.y - p3.y))
    let center = CGPoint(x: p, y: q)
    let radius = hypot(p1.x - center.x, p1.y - center.y)
    return (center: center, radius: radius)
}