UpdateSpec.swift

// Copyright Spotify AB.
// SPDX-License-Identifier: Apache-2.0

import MobiusCore

public struct UpdateSpec<Model, Event, Effect> {
    public typealias Assert = (Result) -> Void

    private let update: Update<Model, Event, Effect>

    public init(_ update: Update<Model, Event, Effect>) {
        self.update = update
    }

    public init(_ update: @escaping (Model, Event) -> Next<Model, Effect>) {
        self.init(Update(update))
    }

    public func given(_ model: Model) -> When {
        return When(update, model)
    }

    public struct When {
        private let update: Update<Model, Event, Effect>
        private let model: Model

        init(_ update: Update<Model, Event, Effect>, _ model: Model) {
            self.update = update
            self.model = model
        }

        public func when(_ event: Event, _ moreEvents: Event...) -> Then {
            return Then(update, model, [event] + moreEvents)
        }
    }

    public struct Then {
        private let update: Update<Model, Event, Effect>
        private let model: Model
        private let events: [Event]

        init(_ update: Update<Model, Event, Effect>, _ model: Model, _ events: [Event]) {
            self.update = update
            self.model = model
            self.events = events
        }

        public func then(_ expression: Assert) {
            var lastNext: Next<Model, Effect>?
            var lastModel = model

            for event in events {
                lastNext = update.update(model: lastModel, event: event)
                lastModel = lastNext?.model ?? lastModel
            }

            // there will always be at least one event, so lastNext is guaranteed to have a value
            expression(Result(model: lastModel, lastNext: lastNext!))
        }
    }

    public struct Result {
        public let model: Model
        public let lastNext: Next<Model, Effect>
    }
}