RewriteChallenge.swift

//: Playground - noun: a place where people can play

import UIKit

// credit: http://stackoverflow.com/questions/603907/uiimage-resize-then-crop#605385

/// Cropping is a basic and generic operation on top of UIKit,
/// it deserves to be moved to some extension. No need to test it.
extension UIImage {
    func cropToFrame(frame: CGRect) -> UIImage {
        // Why use `UIGraphicsBeginImageContextWithOptions` over `UIGraphicsBeginImageContext`?
        // see: http://stackoverflow.com/questions/4334233/how-to-capture-uiview-to-uiimage-without-loss-of-quality-on-retina-display#4334902
        UIGraphicsBeginImageContextWithOptions(size, false, 0.0)
        self.drawInRect(frame)
        let scaledImage = UIGraphicsGetImageFromCurrentImageContext()!
        UIGraphicsEndImageContext()
        
        return scaledImage
    }
}

/// This is main logic of our code. Pure function. Testable. Only lets. :)
func scaleAndCropRect(from source: CGSize, to target:CGSize) -> CGRect {
    let widthFactor = target.width / source.width
    let heightFactor = target.height / source.height
    
    let scaled: CGSize; do {
        let scaleFactor = max(heightFactor, widthFactor)
        
        scaled = CGSize(
            width: source.width * scaleFactor,
            height: source.height * scaleFactor)
    }
    
    let origin: CGPoint; do {
        if widthFactor > heightFactor {
            origin = CGPoint(x: 0, y: (target.height - scaled.height) / 2.0)
        }
        else if widthFactor < heightFactor {
            origin = CGPoint(x: (target.width - scaled.width) / 2.0, y: 0)
        } else {
            origin = CGPoint.zero
        }
    }
    
    return CGRect(origin: origin, size: scaled)
}

/// Glue code to support old callsites. No need to test it.
func scaleAndCropImage(image:UIImage, toSize size: CGSize) -> UIImage {
    // Sanity check; make sure the image isn't already sized.
    guard image.size != size else { return image }
    
    return image.cropToFrame(scaleAndCropRect(from: image.size, to: size))
}