UUIDSerialNumberGenerator.swift

import Foundation

actor UUIDSerialNumberGenerator {
    static let shared: UUIDSerialNumberGenerator = .init()

    private var nextSerialNumber: Int = 0
    private var uuidToSerialNumber: [UUID: Int] = [:]

    private init() {}

    func serialNumber(for uuid: UUID) -> Int {
        if let serialNumber = uuidToSerialNumber[uuid] {
            return serialNumber
        }

        uuidToSerialNumber[uuid] = nextSerialNumber
        defer { nextSerialNumber += 1 }
        return nextSerialNumber
    }
}

let generator: UUIDSerialNumberGenerator = .shared
let a: UUID = .init()
let b: UUID = .init()
let c: UUID = .init()
let d: UUID = .init()
print(await generator.serialNumber(for: a))
print(await generator.serialNumber(for: b))
print(await generator.serialNumber(for: c))
print(await generator.serialNumber(for: d))
print(await generator.serialNumber(for: a))
print(await generator.serialNumber(for: b))
print(await generator.serialNumber(for: c))
print(await generator.serialNumber(for: d))