/*

 * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>

 * Copyright (c) 2022, Andrew Kaster <akaster@serenityos.org>

 * Copyright (c) 2022, Luke Wilde <lukew@serenityos.org>

 *

 * SPDX-License-Identifier: BSD-2-Clause

 */

#include <AK/TypeCasts.h>

#include <AK/Vector.h>

#include <LibJS/Heap/Handle.h>

#include <LibWeb/DOM/Document.h>

#include <LibWeb/DOM/Element.h>

#include <LibWeb/DOM/ShadowRoot.h>

#include <LibWeb/HTML/Focus.h>

#include <LibWeb/HTML/HTMLInputElement.h>

#include <LibWeb/HTML/TraversableNavigable.h>

#include <LibWeb/UIEvents/FocusEvent.h>

namespace Web::HTML {

// https://html.spec.whatwg.org/multipage/interaction.html#fire-a-focus-event

static void fire_a_focus_event(JS::GCPtr<DOM::EventTarget> focus_event_target, JS::GCPtr<DOM::EventTarget> related_focus_target, FlyString const& event_name, bool bubbles)

{

    // To fire a focus event named e at an element t with a given related target r, fire an event named e at t, using FocusEvent,

    // with the relatedTarget attribute initialized to r, the view attribute initialized to t's node document's relevant global

    // object, and the composed flag set.

    UIEvents::FocusEventInit focus_event_init {};

    focus_event_init.related_target = related_focus_target;

    focus_event_init.view = verify_cast<HTML::Window>(focus_event_target->realm().global_object());

    auto focus_event = UIEvents::FocusEvent::create(focus_event_target->realm(), event_name, focus_event_init);

    // AD-HOC: support bubbling focus events, used for focusin & focusout.

    //         See: https://github.com/whatwg/html/issues/3514

    focus_event->set_bubbles(bubbles);

    focus_event->set_composed(true);

    focus_event_target->dispatch_event(focus_event);

}

// https://html.spec.whatwg.org/multipage/interaction.html#focus-update-steps

static void run_focus_update_steps(Vector<JS::Handle<DOM::Node>> old_chain, Vector<JS::Handle<DOM::Node>> new_chain, DOM::Node* new_focus_target)

{

    // 1. If the last entry in old chain and the last entry in new chain are the same,

    //    pop the last entry from old chain and the last entry from new chain and redo this step.

    while (!old_chain.is_empty()

        && !new_chain.is_empty()

        && &old_chain.last() == &new_chain.last()) {

        (void)old_chain.take_last();

        (void)new_chain.take_last();

    }

    // 2. For each entry entry in old chain, in order, run these substeps:

    for (auto& entry : old_chain) {

        // 1. If entry is an input element, and the change event applies to the element, and the element does not have

        //    a defined activation behavior, and the user has changed the element's value or its list of selected files

        //    while the control was focused without committing that change (such that it is different to what it was

        //    when the control was first focused), then fire an event named change at the element, with the bubbles

        //    attribute initialized to true.

        if (is<HTMLInputElement>(*entry)) {

            auto& input_element = static_cast<HTMLInputElement&>(*entry);

            // FIXME: Spec issue: It doesn't make sense to check if the element has a defined activation behavior, as

            //        that is always true. Instead, we check if it has an *input* activation behavior.

            //        https://github.com/whatwg/html/issues/9973

            if (input_element.change_event_applies() && !input_element.has_input_activation_behavior()) {

                input_element.commit_pending_changes();

            }

        }

        JS::GCPtr<DOM::EventTarget> blur_event_target;

        if (is<DOM::Element>(*entry)) {

            // 2. If entry is an element, let blur event target be entry.

            blur_event_target = entry.ptr();

        } else if (is<DOM::Document>(*entry)) {

            // If entry is a Document object, let blur event target be that Document object's relevant global object.

            blur_event_target = static_cast<DOM::Document&>(*entry).window();

        }

        // 3. If entry is the last entry in old chain, and entry is an Element,

        //    and the last entry in new chain is also an Element,

        //    then let related blur target be the last entry in new chain.

        //    Otherwise, let related blur target be null.

        JS::GCPtr<DOM::EventTarget> related_blur_target;

        if (!old_chain.is_empty()

            && &entry == &old_chain.last()

            && is<DOM::Element>(*entry)

            && !new_chain.is_empty()

            && is<DOM::Element>(*new_chain.last())) {

            related_blur_target = new_chain.last().ptr();

        }

        // 4. If blur event target is not null, fire a focus event named blur at blur event target,

        //    with related blur target as the related target.

        if (blur_event_target) {

            fire_a_focus_event(blur_event_target, related_blur_target, HTML::EventNames::blur, false);

            // AD-HOC: dispatch focusout

            fire_a_focus_event(blur_event_target, related_blur_target, HTML::EventNames::focusout, true);

        }

    }

    // FIXME: 3. Apply any relevant platform-specific conventions for focusing new focus target.

    //           (For example, some platforms select the contents of a text control when that control is focused.)

    (void)new_focus_target;

    // 4. For each entry entry in new chain, in reverse order, run these substeps:

    for (auto& entry : new_chain.in_reverse()) {

        // 1. If entry is a focusable area: designate entry as the focused area of the document.

        // FIXME: This isn't entirely right.

        if (is<DOM::Element>(*entry))

            entry->document().set_focused_element(&static_cast<DOM::Element&>(*entry));

        else if (is<DOM::Document>(*entry))

            entry->document().set_focused_element(static_cast<DOM::Document&>(*entry).document_element());

        JS::GCPtr<DOM::EventTarget> focus_event_target;

        if (is<DOM::Element>(*entry)) {

            // 2. If entry is an element, let focus event target be entry.

            focus_event_target = entry.ptr();

        } else if (is<DOM::Document>(*entry)) {

            // If entry is a Document object, let focus event target be that Document object's relevant global object.

            focus_event_target = static_cast<DOM::Document&>(*entry).window();

        }

        // 3. If entry is the last entry in new chain, and entry is an Element,

        //    and the last entry in old chain is also an Element,

        //    then let related focus target be the last entry in old chain.

        //    Otherwise, let related focus target be null.

        JS::GCPtr<DOM::EventTarget> related_focus_target;

        if (!new_chain.is_empty()

            && &entry == &new_chain.last()

            && is<DOM::Element>(*entry)

            && !old_chain.is_empty()

            && is<DOM::Element>(*old_chain.last())) {

            related_focus_target = old_chain.last().ptr();

        }

        // 4. If focus event target is not null, fire a focus event named focus at focus event target,

        //    with related focus target as the related target.

        if (focus_event_target) {

            fire_a_focus_event(focus_event_target, related_focus_target, HTML::EventNames::focus, false);

            // AD-HOC: dispatch focusin

            fire_a_focus_event(focus_event_target, related_focus_target, HTML::EventNames::focusin, true);

        }

    }

}

// https://html.spec.whatwg.org/multipage/interaction.html#focus-chain

static Vector<JS::Handle<DOM::Node>> focus_chain(DOM::Node* subject)

{

    // FIXME: Move this somewhere more spec-friendly.

    if (!subject)

        return {};

    // 1. Let output be an empty list.

    Vector<JS::Handle<DOM::Node>> output;

    // 2. Let currentObject be subject.

    auto* current_object = subject;

    // 3. While true:

    while (true) {

        // 1. Append currentObject to output.

        output.append(JS::make_handle(*current_object));

        // FIXME: 2. If currentObject is an area element's shape, then append that area element to output.

        // FIXME:    Otherwise, if currentObject's DOM anchor is an element that is not currentObject itself, then append currentObject's DOM anchor to output.

        // FIXME: Everything below needs work. The conditions are not entirely right.

        if (!is<DOM::Document>(*current_object)) {

            // 3. If currentObject is a focusable area, then set currentObject to currentObject's DOM anchor's node document.

            current_object = &current_object->document();

        } else if (is<DOM::Document>(*current_object)

            && current_object->navigable()

            && current_object->navigable()->parent()) {

            // Otherwise, if currentObject is a Document whose node navigable's parent is non-null, then set currentObject to currentObject's node navigable's parent.

            current_object = current_object->navigable()->container();

        } else {

            // Otherwise, break.

            break;

        }

    }

    // 4. Return output.

    return output;

}

// https://html.spec.whatwg.org/multipage/interaction.html#focusing-steps

// FIXME: This should accept more types.

void run_focusing_steps(DOM::Node* new_focus_target, DOM::Node* fallback_target, [[maybe_unused]] Optional<ByteString> focus_trigger)

{

    // FIXME: 1. If new focus target is not a focusable area, then set new focus target

    //           to the result of getting the focusable area for new focus target,

    //           given focus trigger if it was passed.

    // 2. If new focus target is null, then:

    if (!new_focus_target) {

        // 1. If no fallback target was specified, then return.

        if (!fallback_target)

            return;

        // 2. Otherwise, set new focus target to the fallback target.

        new_focus_target = fallback_target;

    }

    // 3. If new focus target is a navigable container with non-null nested browsing context,

    //    then set new focus target to the nested browsing context's active document.

    if (is<HTML::NavigableContainer>(*new_focus_target)) {

        auto& navigable_container = static_cast<HTML::NavigableContainer&>(*new_focus_target);

        if (auto* nested_browsing_context = navigable_container.nested_browsing_context())

            new_focus_target = nested_browsing_context->active_document();

    }

    // FIXME: 4. If new focus target is a focusable area and its DOM anchor is inert, then return.

    // 5. If new focus target is the currently focused area of a top-level browsing context, then return.

    if (!new_focus_target->document().browsing_context())

        return;

    auto top_level_traversable = new_focus_target->document().browsing_context()->top_level_traversable();

    if (new_focus_target == top_level_traversable->currently_focused_area().ptr())

        return;

    // 6. Let old chain be the current focus chain of the top-level browsing context in which

    //    new focus target finds itself.

    auto old_chain = focus_chain(top_level_traversable->currently_focused_area());

    // 7. Let new chain be the focus chain of new focus target.

    auto new_chain = focus_chain(new_focus_target);

    // 8. Run the focus update steps with old chain, new chain, and new focus target respectively.

    run_focus_update_steps(old_chain, new_chain, new_focus_target);

}

// https://html.spec.whatwg.org/multipage/interaction.html#unfocusing-steps

void run_unfocusing_steps(DOM::Node* old_focus_target)

{

    // NOTE: The unfocusing steps do not always result in the focus changing, even when applied to the currently focused

    // area of a top-level browsing context. For example, if the currently focused area of a top-level browsing context

    // is a viewport, then it will usually keep its focus regardless until another focusable area is explicitly focused

    // with the focusing steps.

    auto is_shadow_host = [](DOM::Node* node) {

        return is<DOM::Element>(node) && static_cast<DOM::Element*>(node)->is_shadow_host();

    };

    // 1. If old focus target is a shadow host whose shadow root's delegates focus is true, and old focus target's

    //    shadow root is a shadow-including inclusive ancestor of the currently focused area of a top-level browsing

    //    context's DOM anchor, then set old focus target to that currently focused area of a top-level browsing

    //    context.

    if (is_shadow_host(old_focus_target)) {

        auto shadow_root = static_cast<DOM::Element*>(old_focus_target)->shadow_root();

        if (shadow_root->delegates_focus()) {

            auto top_level_traversable = old_focus_target->document().browsing_context()->top_level_traversable();

            if (auto currently_focused_area = top_level_traversable->currently_focused_area()) {

                if (shadow_root->is_shadow_including_ancestor_of(*currently_focused_area)) {

                    old_focus_target = currently_focused_area;

                }

            }

        }

    }

    // FIXME: 2. If old focus target is inert, then return.

    // FIXME: 3. If old focus target is an area element and one of its shapes is the currently focused area of a

    //    top-level browsing context, or, if old focus target is an element with one or more scrollable regions, and one

    //    of them is the currently focused area of a top-level browsing context, then let old focus target be that

    //    currently focused area of a top-level browsing context.

    // NOTE: HTMLAreaElement is currently missing the shapes property

    auto top_level_traversable = old_focus_target->document().browsing_context()->top_level_traversable();

    // 4. Let old chain be the current focus chain of the top-level browsing context in which old focus target finds itself.

    auto old_chain = focus_chain(top_level_traversable->currently_focused_area());

    // 5. If old focus target is not one of the entries in old chain, then return.

    auto it = old_chain.find_if([&](auto const& node) { return old_focus_target == node; });

    if (it == old_chain.end())

        return;

    // 6. If old focus target is not a focusable area, then return.

    if (!old_focus_target->is_focusable())

        return;

    // 7. Let topDocument be old chain's last entry.

    auto* top_document = verify_cast<DOM::Document>(old_chain.last().ptr());

    // 8. If topDocument's node navigable has system focus, then run the focusing steps for topDocument's viewport.

    if (top_document->navigable()->traversable_navigable()->system_visibility_state() == HTML::VisibilityState::Visible) {

        run_focusing_steps(top_document);

    } else {

        // FIXME: Otherwise, apply any relevant platform-specific conventions for removing system focus from

        // topDocument's browsing context, and run the focus update steps with old chain, an empty list, and null

        // respectively.

        // What? It already doesn't have system focus, what possible platform-specific conventions are there?

        run_focus_update_steps(old_chain, {}, nullptr);

    }

    // FIXME: When the currently focused area of a top-level browsing context is somehow unfocused without another

    // element being explicitly focused in its stead, the user agent must immediately run the unfocusing steps for that

    // object.

    // What? How are we supposed to detect when something is "somehow unfocused without another element being explicitly focused"?

}

}