A simple solution for an interview question.

ReadAndMark.swift

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

import Cocoa
import CoreGraphics

//: Sample input: `100, 100, 200, 200, 300, 300`
let inputFilepath = "/Users/unixzii/Desktop/test.csv"
let outputFilepath = "/Users/unixzii/Desktop/test.png"

func pointsFromSequence(sequence: String) -> [CGPoint]? {
    var points = [CGPoint]()
    
    let numbers = sequence.characters.split(",").map { subSequence -> Double in
        let formattedSubSequence = String(subSequence).stringByTrimmingCharactersInSet(NSCharacterSet.whitespaceAndNewlineCharacterSet())
        return Double(formattedSubSequence) ?? 0
    }
    
    if (numbers.count % 2 == 0) {
        for i in 0..<Int(numbers.count / 2) {
            let x = CGFloat(numbers[i * 2])
            let y = CGFloat(numbers[i * 2 + 1])
            points.append(CGPoint(x: x, y: y))
        }
    } else {
        return nil
    }
    
    return points
}

func cgContextForBitmapWithSize(size: CGSize) -> CGContext? {
    let colorSpace = CGColorSpaceCreateDeviceRGB()
    let bitmapInfo = CGImageAlphaInfo.PremultipliedLast.rawValue    // FIXME: other flags may lead to undefined behaviors.
    return CGBitmapContextCreate(nil, Int(size.width), Int(size.height), 8, Int(size.width) * 4, colorSpace, bitmapInfo)
}

func main() {
    guard let fileContent = try? String(contentsOfFile: inputFilepath, encoding: NSUTF8StringEncoding) else {
        fatalError("Cannot open file.")
    }
    
    guard let points = pointsFromSequence(fileContent) else {
        fatalError("Invalid input.")
    }
    
    // Begin drawing the bitmap.
    let size = CGSize(width: 600, height: 600)
    let context = cgContextForBitmapWithSize(size)
    
    CGContextSetFillColorWithColor(context, NSColor.whiteColor().CGColor)
    CGContextFillRect(context, CGRect(origin: CGPointZero, size: size))
    points.forEach { point in
        CGContextSetFillColorWithColor(context, NSColor.redColor().CGColor)
        CGContextBeginPath(context)
        CGContextAddPath(context, CGPathCreateWithEllipseInRect(CGRect(origin: point, size: CGSize(width: 10, height: 10)), nil))
        CGContextFillPath(context)
    }
    
    // Finally, create and save the bitmap.
    if let cgImage = CGBitmapContextCreateImage(context) {
        let image = NSImage(CGImage: cgImage, size: size)
        image.lockFocus()
        let imageRep = NSBitmapImageRep(focusedViewRect: NSRect(origin: CGPointZero, size: size))
        image.unlockFocus()
        
        imageRep?.representationUsingType(.NSPNGFileType, properties: [:])?.writeToFile(outputFilepath, atomically: true)
    }
}

main()