ABTester.swift

public class ABTester {
  public init() {}
  
  /** Randomly chooses a weighted generic item from a list of item
   - item: a weighted item
   - more: more weighted items as a variadic parameter
   - randomNumberGenerator: random number generator
   - returns: the selected generic item
   - discussion:
   We force the first item to be provided so we ensure that we don't end up with an empty list of items
   */
  public func ABTest<T>(item: (item: Void -> T, weight: Int), _ more: (item: Void -> T, weight: Int) ..., randomNumberGenerator: RandomValueGenerator) -> T {
    return [[item], more].flatMap { $0 }.sorted {
      randomNumberGenerator.generateRandomValueBetween(1, maxValue: $0.weight) >
        randomNumberGenerator.generateRandomValueBetween(1, maxValue: $1.weight)
      }.first!.item() // force unwrapping is 100% secure since we will always have at least one item
  }
}

/// This protocol is to be implemented by those classes that generate a random number
public protocol RandomValueGenerator {
  /**
  Returns a random value between the specified parameters
  
  - parameter minValue: The min value the random can be
  - parameter maxValue: The max value the random can be
  */
  func generateRandomValueBetween(minValue: Int, maxValue: Int) -> Int
}

public class RandomNumberGeneratorImpl: RandomValueGenerator {
  public init() {}
  
  public func generateRandomValueBetween(minValue: Int, maxValue: Int) -> Int {
    return minValue + Int(arc4random_uniform(UInt32(maxValue - minValue + 1)))
  }
}