Predestination Generations

// Calculate the number of generations it takes before a man that goes back in time
// and has a baby with herself has identical genes in both sets of a rather small
// chromosome, that for some reason also complete resists random mutation.

import Foundation

struct individual {
    let alpha:String
    let beta:String
    func represent() -> String
    {
        return "\(self.alpha) and \(self.beta)"
    }
}


func combineStrings(alpha:String, beta:String) -> (String, String)
{
    assert(alpha.characters.count == beta.characters.count)

    var newAlpha = ""
    var newBeta = ""
    while newAlpha.characters.count < alpha.characters.count {
        let index = newAlpha.characters.count
        if (arc4random_uniform(100) < 50) {
            newAlpha.append(alpha[ alpha.startIndex.advancedBy(index) ])
            newBeta.append(beta[ beta.startIndex.advancedBy(index) ])
        }
        else {
            newAlpha.append(beta[ beta.startIndex.advancedBy(index) ])
            newBeta.append(alpha[ alpha.startIndex.advancedBy(index) ])
        }
    }
    return (newAlpha, newBeta)
}


func makeChild(male:individual, female:individual) -> individual
{
    let (sperm, _) = combineStrings(male.alpha, beta: male.beta)
    let (_, egg) = combineStrings(female.alpha, beta: female.beta)
    let (childAlpha, childBeta) = combineStrings(sperm, beta: egg)
    return individual(alpha: childAlpha, beta: childBeta)
}

let runs = 100
let tries = 20

var max = 0
var min = tries
var sum = 0


for run in 1...runs {

    // Initial individual
    var woman = individual(
        alpha: "0123456789!#€%&/()=.,¨'*_-ABCDEFGHIJKLM",
        beta:  "abcdefghijklmnopqrstuvwxyzNOPQRSTUVWXYZ"
    )

    for generation in 1...20 {
        // A woman has post delivery surgery and becomes a man
        let man = woman

        // The man travels back in time and impregnates himself as a woman
        let child = makeChild(man, female: woman)

        // The child is moved back in time by the bartender and becomes the woman
        woman = child

        if child.alpha == child.beta {
            print("\(generation): \(child.represent())")
            sum += generation
            if generation < min {
                min = generation
            }
            if generation > max {
                max = generation
            }
            break
        }
    }
}


print("Average: \(Float(sum) / Float(runs)). Max: \(max). Min: \(min)")