Simple review prompt point system for iOS apps, with points and half-lives assigned to in-app actions

StoreReviewPointsManager.swift

import UIKit
import StoreKit

enum StoreReviewValue: Int, Codable {
    case high
    case medium
    case low

    var value: NSDecimalNumber {
        switch self {
        case .high: return 100
        case .medium: return 10
        case .low: return 1
        }
    }
}

enum StoreReviewHalfLife {
    case hour
    case day
    case week
    case month

    var seconds: Int {
        switch self {
        case .hour: return 60*60
        case .day: return StoreReviewHalfLife.hour.seconds * 24
        case .week: return StoreReviewHalfLife.day.seconds * 7
        case .month: return StoreReviewHalfLife.week.seconds * 4
        }
    }
}

struct StoreReviewAction: Codable {
    var initialValue: StoreReviewValue
    var halfLife: Int
    var createdOn: Date

    var value: NSDecimalNumber {
        let now = Date()
        let interval = now.timeIntervalSince(createdOn)
        let exponent = NSDecimalNumber(value: pow(0.5, interval))
        return initialValue.value.multiplying(by: exponent)
    }

    init(value: StoreReviewValue, halfLife: Int) {
        self.initialValue = value
        self.halfLife = halfLife
        createdOn = Date()
    }
}

struct StoreReviewPointsManager {
    var actions: [StoreReviewAction] = []
    var promptThreshold: NSDecimalNumber!

    var data: Data? {
        let encoder = JSONEncoder()
        return try? encoder.encode(actions)
    }

    var value: NSDecimalNumber {
        return actions.reduce(NSDecimalNumber.zero, {
            $1.value.adding($0)
        })
    }

    let fileURL: URL = {
        let directories = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)
        guard let documentsDirectory = directories.first else {
            return URL(fileURLWithPath: "/tmp/points.txt")
        }

        return documentsDirectory.appendingPathComponent("/points.txt")
    }()

    // MARK: -

    init(threshold: NSDecimalNumber) {
        let decoder = JSONDecoder()
        if let data = try? Data(contentsOf: fileURL),
            let _actions = try? decoder.decode([StoreReviewAction].self, from: data) {
                actions = _actions
        }

        promptThreshold = threshold
    }

    // MARK: -

    func save() throws {
        try data?.write(to: fileURL)
    }

    mutating func addAction(value: StoreReviewValue, halfLife: StoreReviewHalfLife) {
        actions.append(StoreReviewAction(value: value, halfLife: halfLife.seconds))

        if self.value.compare(promptThreshold) == .orderedDescending {
            SKStoreReviewController.requestReview()

            actions.removeAll()
        }
    }
}

var manager = StoreReviewPointsManager(threshold: 10)
manager.addAction(value: .high, halfLife: .hour)

StoreReviewAction forgot to use its halfLife! Should look more like:

var value: NSDecimalNumber {
    let interval = Date().timeIntervalSince(createdOn)
    let halfLifeCount= interval/(Double(halfLife)*1000)
    let exponent = NSDecimalNumber(value: pow(0.5, interval))
    return initialValue.value.multiplying(by: exponent)
}