CGRect+Lerp.swift

extension CGRect {
    /// 線形補間した矩形を返す
    /// - parameter rate: 変化割合。0 <= rate <= 1の範囲で指定する。
    func lerp(to rect: CGRect, rate: CGFloat) -> CGRect {
        let lerp = _lerpCurried(rate)
        return CGRect(
            x: lerp(origin.x, rect.origin.x),
            y: lerp(origin.y, rect.origin.y),
            width: lerp(width, rect.width),
            height: lerp(height, rect.height)
        )
    }
}

extension UIColor {
    /// 線形補間した色を返す
    func lerp(_ otherColor: UIColor, rate: CGFloat) -> UIColor {
        var r1: CGFloat = 0, g1: CGFloat = 0, b1: CGFloat = 0, a1: CGFloat = 0, r2: CGFloat = 0, g2: CGFloat = 0, b2: CGFloat = 0, a2: CGFloat = 0
        self.getRed(&r1, green: &g1, blue: &b1, alpha: &a1)
        otherColor.getRed(&r2, green: &g2, blue: &b2, alpha: &a2)
        
        let lerp = _lerpCurried(rate)
        return UIColor(
            red: lerp(r1, r2),
            green: lerp(g1, g2),
            blue: lerp(b1, b2),
            alpha: lerp(a1, a2)
        )
    }
}

private func _lerpCurried(_ rate: CGFloat) -> (CGFloat, CGFloat) -> CGFloat {
    let rateInversed = 1 - rate
    return { from, to in from * rateInversed + to * rate }
}