String Parsing (objc.io #9) example 3

stringparsing3.swift

// objc.io Issue #9
// February, 2014
// String Parsing by Chris Eidhof
//
// Example 3
// Swift version by Rob Hudson

import UIKit

let FormatError = 100
let MyErrorDomain = "io.objc.parsing"

let string = "backgroundColor = #ff0000\ntextColor = #0000ff"

func parse(string: String, inout error: NSError?) -> [String : String]? {
    let scanner = NSScanner(string: string)
    scanner.charactersToBeSkipped = NSCharacterSet.whitespaceCharacterSet()
    let hexadecimalCharacterSet = NSCharacterSet(charactersInString: "0123456789abcdefABCDEF")
    var result = [String : String]()
    while !scanner.atEnd {
        var key: NSString?
        var value: NSString?
        let letters = NSCharacterSet.letterCharacterSet()
        let didScan = scanner.scanCharactersFromSet(letters, intoString: &key) &&
            scanner.scanString("=", intoString: nil) &&
            scanner.scanString("#", intoString: nil) &&
            scanner.scanCharactersFromSet(hexadecimalCharacterSet, intoString: &value) &&
            value?.length == 6
        if !didScan {
            let errorDetail = [NSLocalizedDescriptionKey : "Couldn't parse: \(scanner.scanLocation)"]
            error = NSError(domain: MyErrorDomain, code: FormatError, userInfo: errorDetail)
            return nil
        }
        if let key = key as? String, value = value as? String {
            result[key] = value
        }
        scanner.scanCharactersFromSet(NSCharacterSet.newlineCharacterSet(), intoString: nil) // scan an optional newline
    }
    return result
}

var error: NSError?
parse(string, &error)