Circle Center from 3 Points

circle3.swift

func circleCenter(_ a: CGPoint, _ b: CGPoint, _ c: CGPoint) -> CGPoint {
    let dya = b.y - a.y
    let dxa = b.x - a.x
    let dyb = c.y - b.y
    let dxb = c.x - b.x
    let sa = dya / dxa
    let sb = dyb / dxb
    let x = (sa * sb * (a.y - c.y) + sb * (a.x + b.x) - sa * (b.x + c.x)) / (2 * (sb - sa))
    let y = -1 * (x - (a.x + b.x) / 2) / sa + (a.y + b.y) / 2
    return CGPoint(x: x, y: y)
}