My solution for the Omni Group’s Swift Bike Shedding Club week two question: http://rubyquiz.com/quiz2.html

SecretSanta.swift

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

import Cocoa

class Person: Hashable {

    let firstName: String
    let lastName: String
    let email: String

    var hashValue: Int {
        return email.hashValue
    }

    init(firstName: String, lastName: String, email: String) {

        self.firstName = firstName
        self.lastName = lastName
        self.email = email
    }

    func canBeSantaFor(person: Person) -> Bool {

        return person.lastName != lastName
    }
}

func ==(lhs: Person, rhs: Person) -> Bool {

    return lhs.email == rhs.email
}

let input = "Luke Skywalker <[email protected]>\nLeia Skywalker <[email protected]>\nToula Portokalos <[email protected]>\nGus Portokalos <[email protected]>\nBruce Wayne <[email protected]>\nVirgil Brigman <[email protected]>\nLindsey Brigman <[email protected]>"

//let input = "Luke Skywalker <[email protected]>\nLeia Skywalker <[email protected]>\nToula Portokalos <[email protected]>\nGus Portokalos <[email protected]>\nBruce Wayne <[email protected]>\nVirgil Brigman <[email protected]>\nLindsey Brigman <[email protected]>\nFranz Kafka <[email protected]>\nCourtney Kafka <[email protected]>\nSkip Kafka <[email protected]>\nAloysius Kafka <[email protected]>"

// This one is not solvable.
//let input = "Luke Skywalker <[email protected]>\nLeia Skywalker <[email protected]>\nToula Portokalos <[email protected]>"

let lines = input.componentsSeparatedByString("\n")
let people = lines.map { personWithLine($0) }

if let assignments = assignSantas(people) {

    for oneSanta in assignments.keys {
        let oneRecipient = assignments[oneSanta]!
        print("\(oneSanta.firstName) \(oneSanta.lastName) is Santa for \(oneRecipient.firstName) \(oneRecipient.lastName).")
    }
}
else {
    print("There was no solution.")
}

extension String {

    mutating func removeThoseDamnCarets() {

        if self.hasPrefix("<") {
            self.removeAtIndex(self.startIndex)
        }
        if self.hasSuffix(">") {
            self.removeAtIndex(self.endIndex.advancedBy(-1))
        }
    }
}

private func personWithLine(line: String) -> Person {

    let components = line.componentsSeparatedByString(" ")
    let firstName = components[0]
    let lastName = components[1]
    var email = components[2]
    email.removeThoseDamnCarets()

    return Person(firstName: firstName, lastName: lastName, email: email)
}

private func possibleRecipientsForSanta(santa: Person, people: [Person]) -> [Person] {

    return people.filter { santa.canBeSantaFor($0) }
}

private func assignmentsAreValid(assignments: [Person: Person], people: [Person]) -> Bool {

    if (Set(people) != Set(assignments.keys)) {
        return false
    }

    if Set(assignments.values) != Set(assignments.keys) {
        return false
    }

    for oneSanta in assignments.keys {
        let recipient = assignments[oneSanta]!
        if !oneSanta.canBeSantaFor(recipient) {
            return false
        }
    }

    return true
}

private func assignSantas(people: [Person]) -> [Person: Person]? {

    let assignments = tryEveryCombination(people, assignments: [Person: Person](), index: 0)
    if assignmentsAreValid(assignments, people: people) {
        return assignments
    }
    return nil
}

private func tryEveryCombination(people: [Person], assignments: [Person: Person], index: Int) -> [Person: Person] {
    
    let santa = people[index]
    let possibleRecipients = possibleRecipientsForSanta(santa, people: people)
    var updatedAssignments = assignments
    for oneRecipient in possibleRecipients {

        if updatedAssignments.values.contains(oneRecipient) {
            continue
        }

        updatedAssignments[santa] = oneRecipient

        if index < people.count - 1 {
            updatedAssignments = tryEveryCombination(people, assignments: updatedAssignments, index: index + 1)
        }

        if assignmentsAreValid(updatedAssignments, people: people) {
            return updatedAssignments
        }
    }
    
    return updatedAssignments
}