/*
 * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
 * Copyright (c) 2022, the SerenityOS developers.
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <LibCore/EventLoop.h>
#include <LibCore/Timer.h>
#include <LibJS/Runtime/VM.h>
#include <LibWeb/Bindings/MainThreadVM.h>
#include <LibWeb/DOM/Document.h>
#include <LibWeb/HTML/BrowsingContext.h>
#include <LibWeb/HTML/EventLoop/EventLoop.h>
#include <LibWeb/HTML/Scripting/Environments.h>
#include <LibWeb/HTML/Window.h>
#include <LibWeb/HighResolutionTime/Performance.h>

namespace Web::HTML {

EventLoop::EventLoop()
    : m_task_queue(*this)
    , m_microtask_queue(*this)
{
}

EventLoop::~EventLoop() = default;

void EventLoop::schedule()
{
    if (!m_system_event_loop_timer) {
        m_system_event_loop_timer = Core::Timer::create_single_shot(0, [this] {
            process();
        });
    }

    if (!m_system_event_loop_timer->is_active())
        m_system_event_loop_timer->restart();
}

void EventLoop::set_vm(JS::VM& vm)
{
    VERIFY(!m_vm);
    m_vm = &vm;
}

EventLoop& main_thread_event_loop()
{
    return static_cast<Bindings::WebEngineCustomData*>(Bindings::main_thread_vm().custom_data())->event_loop;
}

// https://html.spec.whatwg.org/multipage/webappapis.html#spin-the-event-loop
void EventLoop::spin_until(Function<bool()> goal_condition)
{
    // FIXME: 1. Let task be the event loop's currently running task.

    // FIXME: 2. Let task source be task's source.

    // 3. Let old stack be a copy of the JavaScript execution context stack.
    // 4. Empty the JavaScript execution context stack.
    auto& vm = Bindings::main_thread_vm();
    vm.save_execution_context_stack();

    // 5. Perform a microtask checkpoint.
    perform_a_microtask_checkpoint();

    // 6. In parallel:
    // NOTE: We do these in reverse order here, but it shouldn't matter.

    //    2. Queue a task on task source to:
    //       1. Replace the JavaScript execution context stack with old stack.
    vm.restore_execution_context_stack();
    //       2. Perform any steps that appear after this spin the event loop instance in the original algorithm.
    //       NOTE: This is achieved by returning from the function.

    //    1. Wait until the condition goal is met.
    Core::EventLoop loop;
    loop.spin_until([&]() -> bool {
        if (goal_condition())
            return true;

        return goal_condition();
    });

    // 7. Stop task, allowing whatever algorithm that invoked it to resume.
    // NOTE: This is achieved by returning from the function.
}

// https://html.spec.whatwg.org/multipage/webappapis.html#event-loop-processing-model
void EventLoop::process()
{
    // An event loop must continually run through the following steps for as long as it exists:

    // 1. Let oldestTask be null.
    OwnPtr<Task> oldest_task;

    // 2. Let taskStartTime be the current high resolution time.
    // FIXME: 'current high resolution time' in hr-time-3 takes a global object,
    //        the HTML spec has not been updated to reflect this, let's use the shared timer.
    //        - https://github.com/whatwg/html/issues/7776
    double task_start_time = unsafe_shared_current_time();

    // 3. Let taskQueue be one of the event loop's task queues, chosen in an implementation-defined manner,
    //    with the constraint that the chosen task queue must contain at least one runnable task.
    //    If there is no such task queue, then jump to the microtasks step below.
    auto& task_queue = m_task_queue;

    // 4. Set oldestTask to the first runnable task in taskQueue, and remove it from taskQueue.
    oldest_task = task_queue.take_first_runnable();

    if (oldest_task) {
        // 5. Set the event loop's currently running task to oldestTask.
        m_currently_running_task = oldest_task.ptr();

        // 6. Perform oldestTask's steps.
        oldest_task->execute();

        // 7. Set the event loop's currently running task back to null.
        m_currently_running_task = nullptr;
    }

    // 8. Microtasks: Perform a microtask checkpoint.
    perform_a_microtask_checkpoint();

    // 9. Let hasARenderingOpportunity be false.
    [[maybe_unused]] bool has_a_rendering_opportunity = false;

    // FIXME: 10. Let now be the current high resolution time. [HRT]

    // FIXME: 11. If oldestTask is not null, then:

    // FIXME:     1. Let top-level browsing contexts be an empty set.

    // FIXME:     2. For each environment settings object settings of oldestTask's script evaluation environment settings object set, append setting's top-level browsing context to top-level browsing contexts.

    // FIXME:     3. Report long tasks, passing in taskStartTime, now (the end time of the task), top-level browsing contexts, and oldestTask.

    // FIXME: 12. Update the rendering: if this is a window event loop, then:

    // FIXME:     1. Let docs be all Document objects whose relevant agent's event loop is this event loop, sorted arbitrarily except that the following conditions must be met:
    //               - Any Document B whose browsing context's container document is A must be listed after A in the list.
    //               - If there are two documents A and B whose browsing contexts are both child browsing contexts whose container documents are another Document C, then the order of A and B in the list must match the shadow-including tree order of their respective browsing context containers in C's node tree.
    // FIXME: NOTE: The sort order specified above is missing here!
    NonnullRefPtrVector<DOM::Document> docs = documents_in_this_event_loop();

    auto for_each_fully_active_document_in_docs = [&](auto&& callback) {
        for (auto& document : docs) {
            if (document.is_fully_active())
                callback(document);
        }
    };

    // 2. Rendering opportunities: Remove from docs all Document objects whose browsing context do not have a rendering opportunity.
    docs.remove_all_matching([&](auto& document) {
        return document->browsing_context() && !document->browsing_context()->has_a_rendering_opportunity();
    });

    // 3. If docs is not empty, then set hasARenderingOpportunity to true
    //    and set this event loop's last render opportunity time to taskStartTime.
    if (!docs.is_empty()) {
        has_a_rendering_opportunity = true;
        m_last_render_opportunity_time = task_start_time;
    }

    // FIXME:     4. Unnecessary rendering: Remove from docs all Document objects which meet both of the following conditions:
    //               - The user agent believes that updating the rendering of the Document's browsing context would have no visible effect, and
    //               - The Document's map of animation frame callbacks is empty.

    // FIXME:     5. Remove from docs all Document objects for which the user agent believes that it's preferable to skip updating the rendering for other reasons.

    // FIXME:     6. For each fully active Document in docs, flush autofocus candidates for that Document if its browsing context is a top-level browsing context.

    // 7. For each fully active Document in docs, run the resize steps for that Document, passing in now as the timestamp. [CSSOMVIEW]
    for_each_fully_active_document_in_docs([&](DOM::Document& document) {
        document.run_the_resize_steps();
    });

    // FIXME:     8. For each fully active Document in docs, run the scroll steps for that Document, passing in now as the timestamp. [CSSOMVIEW]

    // 9. For each fully active Document in docs, evaluate media queries and report changes for that Document, passing in now as the timestamp. [CSSOMVIEW]
    for_each_fully_active_document_in_docs([&](DOM::Document& document) {
        document.evaluate_media_queries_and_report_changes();
    });

    // FIXME:     10. For each fully active Document in docs, update animations and send events for that Document, passing in now as the timestamp. [WEBANIMATIONS]

    // FIXME:     11. For each fully active Document in docs, run the fullscreen steps for that Document, passing in now as the timestamp. [FULLSCREEN]

    // FIXME:     12. For each fully active Document in docs, if the user agent detects that the backing storage associated with a CanvasRenderingContext2D or an OffscreenCanvasRenderingContext2D, context, has been lost, then it must run the context lost steps for each such context:

    // FIXME:     13. For each fully active Document in docs, run the animation frame callbacks for that Document, passing in now as the timestamp.
    for_each_fully_active_document_in_docs([&](DOM::Document& document) {
        run_animation_frame_callbacks(document, document.window().performance().now());
    });

    // FIXME:     14. For each fully active Document in docs, run the update intersection observations steps for that Document, passing in now as the timestamp. [INTERSECTIONOBSERVER]

    // FIXME:     15. Invoke the mark paint timing algorithm for each Document object in docs.

    // FIXME:     16. For each fully active Document in docs, update the rendering or user interface of that Document and its browsing context to reflect the current state.

    // 13. If all of the following are true
    // - this is a window event loop
    // - there is no task in this event loop's task queues whose document is fully active
    // - this event loop's microtask queue is empty
    // - hasARenderingOpportunity is false
    // FIXME: has_a_rendering_opportunity is always true
    if (m_type == Type::Window && !task_queue.has_runnable_tasks() && m_microtask_queue.is_empty() /*&& !has_a_rendering_opportunity*/) {
        // 1. Set this event loop's last idle period start time to the current high resolution time.
        m_last_idle_period_start_time = unsafe_shared_current_time();

        // 2. Let computeDeadline be the following steps:
        // NOTE: instead of passing around a function we use this event loop, which has compute_deadline()

        // 3. For each win of the same-loop windows for this event loop,
        //    perform the start an idle period algorithm for win with computeDeadline. [REQUESTIDLECALLBACK]
        for (auto& win : same_loop_windows())
            win.start_an_idle_period();
    }

    // FIXME: 14. If this is a worker event loop, then:

    // FIXME:     1. If this event loop's agent's single realm's global object is a supported DedicatedWorkerGlobalScope and the user agent believes that it would benefit from having its rendering updated at this time, then:
    // FIXME:        1. Let now be the current high resolution time. [HRT]
    // FIXME:        2. Run the animation frame callbacks for that DedicatedWorkerGlobalScope, passing in now as the timestamp.
    // FIXME:        3. Update the rendering of that dedicated worker to reflect the current state.

    // FIXME:     2. If there are no tasks in the event loop's task queues and the WorkerGlobalScope object's closing flag is true, then destroy the event loop, aborting these steps, resuming the run a worker steps described in the Web workers section below.

    // If there are tasks in the queue, schedule a new round of processing. :^)
    if (m_task_queue.has_runnable_tasks() || !m_microtask_queue.is_empty())
        schedule();
}

// FIXME: This is here to paper over an issue in the HTML parser where it'll create new interpreters (and thus ESOs) on temporary documents created for innerHTML if it uses Document::realm() to get the global object.
//        Use queue_global_task instead.
void old_queue_global_task_with_document(HTML::Task::Source source, DOM::Document& document, Function<void()> steps)
{
    main_thread_event_loop().task_queue().add(HTML::Task::create(source, &document, move(steps)));
}

// https://html.spec.whatwg.org/multipage/webappapis.html#queue-a-global-task
void queue_global_task(HTML::Task::Source source, JS::GlobalObject& global_object, Function<void()> steps)
{
    // 1. Let event loop be global's relevant agent's event loop.
    auto& global_custom_data = verify_cast<Bindings::WebEngineCustomData>(*global_object.vm().custom_data());
    auto& event_loop = global_custom_data.event_loop;

    // 2. Let document be global's associated Document, if global is a Window object; otherwise null.
    DOM::Document* document { nullptr };
    if (is<Bindings::WindowObject>(global_object)) {
        auto& window_object = verify_cast<Bindings::WindowObject>(global_object);
        document = &window_object.impl().associated_document();
    }

    // 3. Queue a task given source, event loop, document, and steps.
    event_loop.task_queue().add(HTML::Task::create(source, document, move(steps)));
}

// https://html.spec.whatwg.org/#queue-a-microtask
void queue_a_microtask(DOM::Document* document, Function<void()> steps)
{
    // 1. If event loop was not given, set event loop to the implied event loop.
    auto& event_loop = HTML::main_thread_event_loop();

    // FIXME: 2. If document was not given, set document to the implied document.

    // 3. Let microtask be a new task.
    // 4. Set microtask's steps to steps.
    // 5. Set microtask's source to the microtask task source.
    // 6. Set microtask's document to document.
    auto microtask = HTML::Task::create(HTML::Task::Source::Microtask, document, move(steps));

    // FIXME: 7. Set microtask's script evaluation environment settings object set to an empty set.

    // 8. Enqueue microtask on event loop's microtask queue.
    event_loop.microtask_queue().enqueue(move(microtask));
}

void perform_a_microtask_checkpoint()
{
    main_thread_event_loop().perform_a_microtask_checkpoint();
}

// https://html.spec.whatwg.org/#perform-a-microtask-checkpoint
void EventLoop::perform_a_microtask_checkpoint()
{
    // 1. If the event loop's performing a microtask checkpoint is true, then return.
    if (m_performing_a_microtask_checkpoint)
        return;

    // 2. Set the event loop's performing a microtask checkpoint to true.
    m_performing_a_microtask_checkpoint = true;

    // 3. While the event loop's microtask queue is not empty:
    while (!m_microtask_queue.is_empty()) {
        // 1. Let oldestMicrotask be the result of dequeuing from the event loop's microtask queue.
        auto oldest_microtask = m_microtask_queue.dequeue();

        // 2. Set the event loop's currently running task to oldestMicrotask.
        m_currently_running_task = oldest_microtask;

        // 3. Run oldestMicrotask.
        oldest_microtask->execute();

        // 4. Set the event loop's currently running task back to null.
        m_currently_running_task = nullptr;
    }

    // 4. For each environment settings object whose responsible event loop is this event loop, notify about rejected promises on that environment settings object.
    for (auto& environment_settings_object : m_related_environment_settings_objects)
        environment_settings_object.notify_about_rejected_promises({});

    // FIXME: 5. Cleanup Indexed Database transactions.

    // 6. Perform ClearKeptObjects().
    vm().finish_execution_generation();

    // 7. Set the event loop's performing a microtask checkpoint to false.
    m_performing_a_microtask_checkpoint = false;
}

NonnullRefPtrVector<DOM::Document> EventLoop::documents_in_this_event_loop() const
{
    NonnullRefPtrVector<DOM::Document> documents;
    for (auto& document : m_documents) {
        VERIFY(document);
        documents.append(*document);
    }
    return documents;
}

void EventLoop::register_document(Badge<DOM::Document>, DOM::Document& document)
{
    m_documents.append(&document);
}

void EventLoop::unregister_document(Badge<DOM::Document>, DOM::Document& document)
{
    bool did_remove = m_documents.remove_first_matching([&](auto& entry) { return entry.ptr() == &document; });
    VERIFY(did_remove);
}

void EventLoop::push_onto_backup_incumbent_settings_object_stack(Badge<EnvironmentSettingsObject>, EnvironmentSettingsObject& environment_settings_object)
{
    m_backup_incumbent_settings_object_stack.append(environment_settings_object);
}

void EventLoop::pop_backup_incumbent_settings_object_stack(Badge<EnvironmentSettingsObject>)
{
    m_backup_incumbent_settings_object_stack.take_last();
}

EnvironmentSettingsObject& EventLoop::top_of_backup_incumbent_settings_object_stack()
{
    return m_backup_incumbent_settings_object_stack.last();
}

void EventLoop::register_environment_settings_object(Badge<EnvironmentSettingsObject>, EnvironmentSettingsObject& environment_settings_object)
{
    m_related_environment_settings_objects.append(environment_settings_object);
}

void EventLoop::unregister_environment_settings_object(Badge<EnvironmentSettingsObject>, EnvironmentSettingsObject& environment_settings_object)
{
    bool did_remove = m_related_environment_settings_objects.remove_first_matching([&](auto& entry) { return &entry == &environment_settings_object; });
    VERIFY(did_remove);
}

// https://html.spec.whatwg.org/multipage/webappapis.html#same-loop-windows
NonnullRefPtrVector<Window> EventLoop::same_loop_windows() const
{
    NonnullRefPtrVector<Window> windows;
    for (auto& document : documents_in_this_event_loop())
        windows.append(document.window());
    return windows;
}

// https://w3c.github.io/hr-time/#dfn-unsafe-shared-current-time
double EventLoop::unsafe_shared_current_time() const
{
    return Time::now_monotonic().to_nanoseconds() / 1e6;
}

// https://html.spec.whatwg.org/multipage/webappapis.html#event-loop-processing-model:last-idle-period-start-time
double EventLoop::compute_deadline() const
{
    // 1. Let deadline be this event loop's last idle period start time plus 50.
    auto deadline = m_last_idle_period_start_time + 50;
    // 2. Let hasPendingRenders be false.
    auto has_pending_renders = false;
    // 3. For each windowInSameLoop of the same-loop windows for this event loop:
    for (auto& window : same_loop_windows()) {
        // 1. If windowInSameLoop's map of animation frame callbacks is not empty,
        //    or if the user agent believes that the windowInSameLoop might have pending rendering updates,
        //    set hasPendingRenders to true.
        if (window.has_animation_frame_callbacks())
            has_pending_renders = true;
        // FIXME: 2. Let timerCallbackEstimates be the result of getting the values of windowInSameLoop's map of active timers.
        // FIXME: 3. For each timeoutDeadline of timerCallbackEstimates, if timeoutDeadline is less than deadline, set deadline to timeoutDeadline.
    }
    // 4. If hasPendingRenders is true, then:
    if (has_pending_renders) {
        // 1. Let nextRenderDeadline be this event loop's last render opportunity time plus (1000 divided by the current refresh rate).
        // FIXME: Hardcoded to 60Hz
        auto next_render_deadline = m_last_render_opportunity_time + (1000.0 / 60.0);
        // 2. If nextRenderDeadline is less than deadline, then return nextRenderDeadline.
        if (next_render_deadline < deadline)
            return next_render_deadline;
    }
    // 5. Return deadline.
    return deadline;
}

}