SM-2 Algorithm

sm-2.swift

// Based on the SuperMemo 2 Algorithm:
// https://www.supermemo.com/english/ol/sm2.htm

import UIKit

struct Item {
    let id: UUID
    let prompt: String
    let explaination: String
    var score: Score

    init(prompt: String, explaination: String) {
        self.id = UUID()
        self.prompt = prompt
        self.explaination = explaination
        self.score = Score()
    }
}

struct Score {

    enum Grade: Int {
        case i // 0 complete blackout
        case f // 1 incorrect response; the correct one remembered
        case d // 2 incorrect response; where the correct one seemed easy to recall
        case c // 3 correct response recalled with serious difficulty
        case b // 4 correct response after a hesitation
        case a // 5 perfect response
        
        static var maxRawValue = 5
    }

    var easiness: Double // ≥ 1.3 as the hardest, defaults to 2.5
    var consecutiveCorrectAnswers: Int
    var nextDate: Date

    init(easiness: Double = 2.5, consecutiveCorrectAnswers: Int = 0) {
        self.easiness = easiness
        self.consecutiveCorrectAnswers = consecutiveCorrectAnswers
        self.nextDate = Date()
    }

    mutating func calculate(grade: Grade, for date: Date) {
        let isPassing = (grade.rawValue >= 3)
        let quality = Double(Grade.maxRawValue - grade.rawValue)

        // To calculate the Easiness Factor (EF), the viewer must assess the
        // quality of their response to the question using a graded scale of 0-5,
        // with 0 as failing:
        //     EF = EF + (0.1 - (5 - grade) * (0.08 + (5 - grade) * 0.02))

        self.easiness = easiness + (0.1 - quality * (0.08 + quality * 0.02))

        if isPassing {
            self.consecutiveCorrectAnswers += 1
            let days = (6 * pow(easiness, Double(consecutiveCorrectAnswers - 1)))
            self.nextDate = Calendar.current.date(byAdding: .day, value: Int(days), to: date)!
        } else {
            self.consecutiveCorrectAnswers = 0
            self.nextDate = Calendar.current.date(byAdding: .day, value: 1, to: date)!
        }
    }
}

var item = Item(prompt: "🏴󠁧󠁢󠁥󠁮󠁧󠁿", explaination: "Flag of England")

item.score.calculate(grade: .f, for: Date())
print(item.score)

item.score.calculate(grade: .b, for: Date())
print(item.score)

item.score.calculate(grade: .b, for: Date())
print(item.score)

// Output
// Score(easiness: 1.7000000000000002, consecutiveCorrectAnswers: 0, nextDate: 2018-02-11 19:56:15 +0000)
// Score(easiness: 1.5600000000000003, consecutiveCorrectAnswers: 1, nextDate: 2018-02-16 19:56:15 +0000)
// Score(easiness: 1.4200000000000004, consecutiveCorrectAnswers: 2, nextDate: 2018-02-18 19:56:15 +0000)