module Client.Features.Quizzes.TakeQuiz.State

open System
open Elmish
open Fable.Core
open Browser.Dom
open Domain.Contracts
open Client.Features.Quizzes.TakeQuiz.Types

/// Ticks once a second for as long as the browser tab showing this page is
/// open. Nothing ever cancels the underlying `setInterval` (there's no page
/// lifecycle hook to hang it off in this app's minimal Elmish wiring) — once
/// the student finishes and the parent switches away from this page, these
/// dispatches just stop matching any case in `App.State.update` and are
/// silently dropped, so the only cost is a harmless once-a-second no-op.
let startTicking () : Cmd<Msg> = Cmd.ofEffect (fun dispatch -> JS.setInterval (fun () -> dispatch Tick) 1000 |> ignore)

/// Listens from the moment the attempt page mounts (matching DESIGN.md §3.6 —
/// a student who wanders off before answering anything still counts).
/// `blur`/`focus` catch switching to another window over the same tab;
/// `visibilitychange` catches switching tabs or minimizing. Both can fire for
/// the same departure, so `FocusLost`/`FocusRegained` in `update` de-dupe via
/// `AwayFromFocus` rather than reporting on every event.
let attachFocusTracking () : Cmd<Msg> =
    Cmd.ofEffect (fun dispatch ->
        document.addEventListener (
            "visibilitychange",
            fun _ -> dispatch (if document.hidden then FocusLost else FocusRegained)
        )

        window.addEventListener ("blur", fun _ -> dispatch FocusLost)
        window.addEventListener ("focus", fun _ -> dispatch FocusRegained))

let rec update (token: string option) (msg: Msg) (model: Model) : Model * Cmd<Msg> =
    match msg with
    | AnswerChanged(questionId, response) ->
        let updated = { model with Answers = model.Answers |> Map.add questionId response }

        let request: SubmitAnswerRequest =
            { AttemptId = model.Data.AttemptId
              QuestionId = questionId
              Response = response }

        let cmd =
            Cmd.OfAsync.either
                (Api.submitAnswer token)
                request
                (function
                 | Ok() -> AnswerSaved questionId
                 | Error err -> AnswerSaveFailed(questionId, err))
                (fun ex -> AnswerSaveFailed(questionId, ex.Message))

        updated, cmd
    | AnswerSaved _ -> { model with Error = None }, Cmd.none
    | AnswerSaveFailed(_, err) -> { model with Error = Some err }, Cmd.none
    | GoToQuestion index ->
        let clamped = index |> max 0 |> min (List.length model.Data.Questions - 1)
        { model with CurrentIndex = clamped }, Cmd.none
    | Tick ->
        let updated = { model with RemainingSeconds = remainingSeconds model.Data DateTimeOffset.UtcNow }
        // The server is the actual authority (it now rejects any answer
        // submitted past the deadline — see `SubmitAnswer.fs`); this just
        // saves the student a click once their own countdown reaches zero,
        // instead of leaving them stuck looking at a page that silently
        // stopped accepting changes.
        if updated.RemainingSeconds = Some 0 && not updated.IsFinishing then
            update token Finish updated
        // Backstop for `FocusLost`/`FocusRegained` below: `blur` and
        // `visibilitychange` are supposed to fire on every departure, but in
        // practice they can be unreliable — e.g. switching windows inside a
        // remote-desktop session doesn't always deliver them to the page.
        // `document.hasFocus()` is a direct, synchronous ground-truth check
        // that doesn't depend on any event actually being dispatched, so
        // piggybacking it onto the once-a-second tick catches a departure
        // within ~1s even when the events themselves go missing.
        elif not (document.hasFocus ()) && not updated.AwayFromFocus then
            update token FocusLost updated
        elif document.hasFocus () && updated.AwayFromFocus then
            update token FocusRegained updated
        else
            updated, Cmd.none
    | FocusLost ->
        if model.AwayFromFocus then
            model, Cmd.none
        else
            // Best-effort: whether this ping succeeds or fails, the student
            // shouldn't see an error banner over something that isn't their
            // action — the teacher-facing count is server-side and this is
            // just one report of it.
            let cmd =
                Cmd.OfAsync.either
                    (Api.reportFocusLoss token)
                    model.Data.AttemptId
                    (fun _ -> FocusLossReported)
                    (fun _ -> FocusLossReported)

            { model with AwayFromFocus = true }, cmd
    | FocusRegained -> { model with AwayFromFocus = false }, Cmd.none
    | FocusLossReported -> model, Cmd.none
    | Finish ->
        let updated = { model with IsFinishing = true }

        let cmd =
            Cmd.OfAsync.either
                (Api.finishAttempt token)
                model.Data.AttemptId
                (function
                 | Ok result -> Finished result
                 | Error err -> FinishFailed err)
                (fun ex -> FinishFailed ex.Message)

        updated, cmd
    | Finished _ -> model, Cmd.none // handled by the parent, which switches pages
    | FinishFailed err -> { model with Error = Some err; IsFinishing = false }, Cmd.none