Swift: UIImage Crop Alpha

UIImageCropAlphaExtension.swift

import Foundation
import UIKit

/* Extension for croping transparent pixels
   example:
   let image: UIImage = UIImage(imageLiteral: "YOUR_IMAGE")
   let uiImageView = UIImageView(image: image.cropImageByAlpha())
   view.addSubview(uiImageView)
 
   Code was basically done here:
   http://stackoverflow.com/questions/9061800/how-do-i-autocrop-a-uiimage/13922413#13922413
   http://www.markj.net/iphone-uiimage-pixel-color/
 */
extension UIImage {
    func cropImageByAlpha() -> UIImage {
        let cgImage = self.CGImage
        let context = createARGBBitmapContextFromImage(cgImage!)
        let height = CGImageGetHeight(cgImage)
        let width = CGImageGetWidth(cgImage)
        
        var rect: CGRect = CGRectMake(0, 0, CGFloat(width), CGFloat(height))
        CGContextDrawImage(context, rect, cgImage)
        
        let data = UnsafePointer<CUnsignedChar>(CGBitmapContextGetData(context))
        
        var minX = width
        var minY = height
        var maxX: Int = 0
        var maxY: Int = 0
        
        //Filter through data and look for non-transparent pixels.
        for y in 0..<height {
            for x in 0..<width {
                let pixelIndex = (width * y + x) * 4 /* 4 for A, R, G, B */
                
                if data[Int(pixelIndex)] != 0 { //Alpha value is not zero pixel is not transparent.
                    if (x < minX) {
                        minX = x
                    }
                    if (x > maxX) {
                        maxX = x
                    }
                    if (y < minY) {
                        minY = y
                    }
                    if (y > maxY) {
                        maxY = y
                    }
                }
            }
        }
        
        rect = CGRectMake(CGFloat(minX), CGFloat(minY), CGFloat(maxX-minX), CGFloat(maxY-minY))
        let imageScale:CGFloat = self.scale
        let cgiImage = CGImageCreateWithImageInRect(self.CGImage, rect)
        return UIImage(CGImage: cgiImage!, scale: imageScale, orientation: self.imageOrientation)
    }
    
    private func createARGBBitmapContextFromImage(inImage: CGImageRef) -> CGContextRef? {
        
        let width = CGImageGetWidth(inImage)
        let height = CGImageGetHeight(inImage)
        
        let bitmapBytesPerRow = width * 4
        let bitmapByteCount = bitmapBytesPerRow * height
        
        let colorSpace = CGColorSpaceCreateDeviceRGB()
        if colorSpace == nil {
            return nil
        }
        
        let bitmapData = malloc(bitmapByteCount)
        if bitmapData == nil {
            return nil
        }
        
        let context = CGBitmapContextCreate (bitmapData, width, height, 8, bitmapBytesPerRow, colorSpace, CGImageAlphaInfo.PremultipliedFirst.rawValue)
        
        return context
    }
}

I needed to do this in Swift 4, so here's what I've hacked together:
https://gist.github.com/AdamLantz/d5d841e60583e740c0b5f515ba5064fb