ladybird/Userland/Libraries/LibWeb/Layout/FlexFormattingContext.cpp

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

#include "InlineFormattingContext.h"
#include <AK/Function.h>
#include <AK/QuickSort.h>
#include <AK/StdLibExtras.h>
#include <LibWeb/Layout/BlockContainer.h>
#include <LibWeb/Layout/BlockFormattingContext.h>
#include <LibWeb/Layout/Box.h>
#include <LibWeb/Layout/FlexFormattingContext.h>
#include <LibWeb/Layout/InitialContainingBlock.h>
#include <LibWeb/Layout/ReplacedBox.h>
#include <LibWeb/Layout/TextNode.h>

namespace Web::Layout {

// NOTE: We use a custom clamping function here instead of AK::clamp(), since the AK version
//       will VERIFY(max >= min) and CSS explicitly allows that (see css-values-4.)
template<typename T>
constexpr T css_clamp(T const& value, T const& min, T const& max)
{
    return ::max(min, ::min(value, max));
}

static float get_pixel_size(FormattingState const& state, Box const& box, Optional<CSS::LengthPercentage> const& length_percentage)
{
    if (!length_percentage.has_value())
        return 0;
    auto inner_main_size = CSS::Length::make_px(state.get(*box.containing_block()).content_width);
    return length_percentage->resolved(box, inner_main_size).to_px(box);
}

static bool is_undefined_or_auto(Optional<CSS::LengthPercentage> const& length_percentage)
{
    if (!length_percentage.has_value())
        return true;
    return length_percentage->is_length() && length_percentage->length().is_auto();
}

FlexFormattingContext::FlexFormattingContext(FormattingState& state, Box const& flex_container, FormattingContext* parent)
    : FormattingContext(Type::Flex, state, flex_container, parent)
    , m_flex_container_state(m_state.get_mutable(flex_container))
    , m_flex_direction(flex_container.computed_values().flex_direction())
{
}

FlexFormattingContext::~FlexFormattingContext() = default;

void FlexFormattingContext::run(Box const& run_box, LayoutMode layout_mode)
{
    VERIFY(&run_box == &flex_container());

    // This implements https://www.w3.org/TR/css-flexbox-1/#layout-algorithm

    // 1. Generate anonymous flex items
    generate_anonymous_flex_items();

    // 2. Determine the available main and cross space for the flex items
    float main_max_size = NumericLimits<float>::max();
    float main_min_size = 0;
    float cross_max_size = NumericLimits<float>::max();
    float cross_min_size = 0;
    bool main_is_constrained = false;
    bool cross_is_constrained = false;
    determine_available_main_and_cross_space(main_is_constrained, cross_is_constrained, main_min_size, main_max_size, cross_min_size, cross_max_size);

    // 3. Determine the flex base size and hypothetical main size of each item
    for (auto& flex_item : m_flex_items) {
        if (flex_item.box.is_replaced_box()) {
            // FIXME: Get rid of prepare_for_replaced_layout() and make replaced elements figure out their intrinsic size lazily.
            static_cast<ReplacedBox&>(flex_item.box).prepare_for_replaced_layout();
        }
        determine_flex_base_size_and_hypothetical_main_size(flex_item);
    }

    if (layout_mode == LayoutMode::MinContent || layout_mode == LayoutMode::MaxContent) {
        // We're computing intrinsic size for the flex container.
        determine_intrinsic_size_of_flex_container(layout_mode);

        // Our caller is only interested in the content-width and content-height results,
        // which have now been set on m_flex_container_state, so there's no need to continue
        // the main layout algorithm after this point.
        return;
    }

    // 4. Determine the main size of the flex container
    determine_main_size_of_flex_container(main_is_constrained, main_min_size, main_max_size);

    // 5. Collect flex items into flex lines:
    // After this step no additional items are to be added to flex_lines or any of its items!
    collect_flex_items_into_flex_lines();

    // 6. Resolve the flexible lengths
    resolve_flexible_lengths();

    // Cross Size Determination
    // 7. Determine the hypothetical cross size of each item
    for (auto& flex_item : m_flex_items) {
        determine_hypothetical_cross_size_of_item(flex_item);
    }

    // 8. Calculate the cross size of each flex line.
    calculate_cross_size_of_each_flex_line(cross_min_size, cross_max_size);

    // 9. Handle 'align-content: stretch'.
    // FIXME: This

    // 10. Collapse visibility:collapse items.
    // FIXME: This

    // 11. Determine the used cross size of each flex item.
    determine_used_cross_size_of_each_flex_item();

    // 12. Distribute any remaining free space.
    distribute_any_remaining_free_space();

    // 13. Resolve cross-axis auto margins.
    // FIXME: This

    // 14. Align all flex items along the cross-axis
    align_all_flex_items_along_the_cross_axis();

    // 15. Determine the flex container’s used cross size:
    determine_flex_container_used_cross_size(cross_min_size, cross_max_size);

    // 16. Align all flex lines (per align-content)
    align_all_flex_lines();

    // AD-HOC: Layout the inside of all flex items.
    copy_dimensions_from_flex_items_to_boxes();
    for (auto& flex_item : m_flex_items) {
        if (auto independent_formatting_context = layout_inside(flex_item.box, LayoutMode::Normal))
            independent_formatting_context->parent_context_did_dimension_child_root_box();
    }

    // FIXME: We run the "copy dimensions" step *again* here, in order to override any sizes
    //        assigned to the flex item by the "layout inside" step above. This is definitely not
    //        part of the spec, and simply covering up the fact that our inside layout currently
    //        mutates the height of BFC roots.
    copy_dimensions_from_flex_items_to_boxes();

    flex_container().for_each_child_of_type<Box>([&](Layout::Box& box) {
        if (box.is_absolutely_positioned())
            layout_absolutely_positioned_element(box);
    });
}

void FlexFormattingContext::populate_specified_margins(FlexItem& item, CSS::FlexDirection flex_direction) const
{
    auto width_of_containing_block = m_state.get(*item.box.containing_block()).content_width;
    auto width_of_containing_block_as_length = CSS::Length::make_px(width_of_containing_block);
    // FIXME: This should also take reverse-ness into account
    if (flex_direction == CSS::FlexDirection::Row || flex_direction == CSS::FlexDirection::RowReverse) {
        item.borders.main_before = item.box.computed_values().border_left().width;
        item.borders.main_after = item.box.computed_values().border_right().width;
        item.borders.cross_before = item.box.computed_values().border_top().width;
        item.borders.cross_after = item.box.computed_values().border_bottom().width;

        item.padding.main_before = item.box.computed_values().padding().left.resolved(item.box, width_of_containing_block_as_length).to_px(item.box);
        item.padding.main_after = item.box.computed_values().padding().right.resolved(item.box, width_of_containing_block_as_length).to_px(item.box);
        item.padding.cross_before = item.box.computed_values().padding().top.resolved(item.box, width_of_containing_block_as_length).to_px(item.box);
        item.padding.cross_after = item.box.computed_values().padding().bottom.resolved(item.box, width_of_containing_block_as_length).to_px(item.box);

        item.margins.main_before = item.box.computed_values().margin().left.resolved(item.box, width_of_containing_block_as_length).to_px(item.box);
        item.margins.main_after = item.box.computed_values().margin().right.resolved(item.box, width_of_containing_block_as_length).to_px(item.box);
        item.margins.cross_before = item.box.computed_values().margin().top.resolved(item.box, width_of_containing_block_as_length).to_px(item.box);
        item.margins.cross_after = item.box.computed_values().margin().bottom.resolved(item.box, width_of_containing_block_as_length).to_px(item.box);
    } else {
        item.borders.main_before = item.box.computed_values().border_top().width;
        item.borders.main_after = item.box.computed_values().border_bottom().width;
        item.borders.cross_before = item.box.computed_values().border_left().width;
        item.borders.cross_after = item.box.computed_values().border_right().width;

        item.padding.main_before = item.box.computed_values().padding().top.resolved(item.box, width_of_containing_block_as_length).to_px(item.box);
        item.padding.main_after = item.box.computed_values().padding().bottom.resolved(item.box, width_of_containing_block_as_length).to_px(item.box);
        item.padding.cross_before = item.box.computed_values().padding().left.resolved(item.box, width_of_containing_block_as_length).to_px(item.box);
        item.padding.cross_after = item.box.computed_values().padding().right.resolved(item.box, width_of_containing_block_as_length).to_px(item.box);

        item.margins.main_before = item.box.computed_values().margin().top.resolved(item.box, width_of_containing_block_as_length).to_px(item.box);
        item.margins.main_after = item.box.computed_values().margin().bottom.resolved(item.box, width_of_containing_block_as_length).to_px(item.box);
        item.margins.cross_before = item.box.computed_values().margin().left.resolved(item.box, width_of_containing_block_as_length).to_px(item.box);
        item.margins.cross_after = item.box.computed_values().margin().right.resolved(item.box, width_of_containing_block_as_length).to_px(item.box);
    }
};

// https://www.w3.org/TR/css-flexbox-1/#flex-items
void FlexFormattingContext::generate_anonymous_flex_items()
{
    // More like, sift through the already generated items.
    // After this step no items are to be added or removed from flex_items!
    // It holds every item we need to consider and there should be nothing in the following
    // calculations that could change that.
    // This is particularly important since we take references to the items stored in flex_items
    // later, whose addresses won't be stable if we added or removed any items.
    HashMap<int, Vector<FlexItem>> order_item_bucket;

    flex_container().for_each_child_of_type<Box>([&](Box& child_box) {
        // Skip anonymous text runs that are only whitespace.
        if (child_box.is_anonymous() && !child_box.first_child_of_type<BlockContainer>()) {
            bool contains_only_white_space = true;
            child_box.for_each_in_subtree([&](auto const& node) {
                if (!is<TextNode>(node) || !static_cast<TextNode const&>(node).dom_node().data().is_whitespace()) {
                    contains_only_white_space = false;
                    return IterationDecision::Break;
                }
                return IterationDecision::Continue;
            });
            if (contains_only_white_space)
                return IterationDecision::Continue;
        }

        // Skip any "out-of-flow" children
        if (child_box.is_out_of_flow(*this))
            return IterationDecision::Continue;

        child_box.set_flex_item(true);
        FlexItem flex_item = { child_box };
        populate_specified_margins(flex_item, m_flex_direction);

        auto& order_bucket = order_item_bucket.ensure(child_box.computed_values().order());
        order_bucket.append(move(flex_item));

        return IterationDecision::Continue;
    });

    auto keys = order_item_bucket.keys();

    if (is_direction_reverse()) {
        quick_sort(keys, [](auto& a, auto& b) { return a > b; });
    } else {
        quick_sort(keys, [](auto& a, auto& b) { return a < b; });
    }

    for (auto key : keys) {
        auto order_bucket = order_item_bucket.get(key);
        if (order_bucket.has_value()) {
            auto items = order_bucket.value();
            if (is_direction_reverse()) {
                for (auto flex_item : items.in_reverse()) {
                    m_flex_items.append(move(flex_item));
                }
            } else {
                for (auto flex_item : items) {
                    m_flex_items.append(move(flex_item));
                }
            }
        }
    }
}

bool FlexFormattingContext::has_definite_main_size(Box const& box) const
{
    return is_row_layout() ? box.has_definite_width() : box.has_definite_height();
}

float FlexFormattingContext::specified_main_size(Box const& box) const
{
    auto const& box_state = m_state.get(box);
    return is_row_layout() ? box_state.content_width : box_state.content_height;
}

float FlexFormattingContext::specified_cross_size(Box const& box) const
{
    auto const& box_state = m_state.get(box);
    return is_row_layout() ? box_state.content_height : box_state.content_width;
}

float FlexFormattingContext::resolved_definite_cross_size(Box const& box) const
{
    if (is_row_layout())
        VERIFY(box.has_definite_height());
    else
        VERIFY(box.has_definite_width());
    auto const& cross_value = is_row_layout() ? box.computed_values().height() : box.computed_values().width();
    if (cross_value->is_length())
        return cross_value->length().to_px(box);
    return cross_value->resolved(box, CSS::Length::make_px(specified_cross_size(flex_container()))).to_px(box);
}

float FlexFormattingContext::resolved_definite_main_size(Box const& box) const
{
    if (is_row_layout())
        VERIFY(box.has_definite_width());
    else
        VERIFY(box.has_definite_height());
    auto const& cross_value = is_row_layout() ? box.computed_values().width() : box.computed_values().height();
    if (cross_value->is_length())
        return cross_value->length().to_px(box);
    return cross_value->resolved(box, CSS::Length::make_px(specified_main_size(flex_container()))).to_px(box);
}

bool FlexFormattingContext::has_main_min_size(Box const& box) const
{
    auto value = is_row_layout() ? box.computed_values().min_width() : box.computed_values().min_height();
    return !is_undefined_or_auto(value);
}

bool FlexFormattingContext::has_cross_min_size(Box const& box) const
{
    auto value = is_row_layout() ? box.computed_values().min_height() : box.computed_values().min_width();
    return !is_undefined_or_auto(value);
}

bool FlexFormattingContext::has_definite_cross_size(Box const& box) const
{
    return is_row_layout() ? box.has_definite_height() : box.has_definite_width();
}

float FlexFormattingContext::specified_main_size_of_child_box(Box const& child_box) const
{
    auto main_size_of_parent = specified_main_size(flex_container());
    auto& value = is_row_layout() ? child_box.computed_values().width() : child_box.computed_values().height();
    if (!value.has_value())
        return 0;
    return value->resolved(child_box, CSS::Length::make_px(main_size_of_parent)).to_px(child_box);
}

float FlexFormattingContext::specified_main_min_size(Box const& box) const
{
    return is_row_layout()
        ? get_pixel_size(m_state, box, box.computed_values().min_width())
        : get_pixel_size(m_state, box, box.computed_values().min_height());
}

float FlexFormattingContext::specified_cross_min_size(Box const& box) const
{
    return is_row_layout()
        ? get_pixel_size(m_state, box, box.computed_values().min_height())
        : get_pixel_size(m_state, box, box.computed_values().min_width());
}

bool FlexFormattingContext::has_main_max_size(Box const& box) const
{
    return is_row_layout()
        ? !is_undefined_or_auto(box.computed_values().max_width())
        : !is_undefined_or_auto(box.computed_values().max_height());
}

bool FlexFormattingContext::has_cross_max_size(Box const& box) const
{
    return is_row_layout()
        ? !is_undefined_or_auto(box.computed_values().max_height())
        : !is_undefined_or_auto(box.computed_values().max_width());
}

float FlexFormattingContext::specified_main_max_size(Box const& box) const
{
    return is_row_layout()
        ? get_pixel_size(m_state, box, box.computed_values().max_width())
        : get_pixel_size(m_state, box, box.computed_values().max_height());
}

float FlexFormattingContext::specified_cross_max_size(Box const& box) const
{
    return is_row_layout()
        ? get_pixel_size(m_state, box, box.computed_values().max_height())
        : get_pixel_size(m_state, box, box.computed_values().max_width());
}

float FlexFormattingContext::calculated_main_size(Box const& box) const
{
    auto const& box_state = m_state.get(box);
    return is_row_layout() ? box_state.content_width : box_state.content_height;
}

bool FlexFormattingContext::is_cross_auto(Box const& box) const
{
    auto& cross_length = is_row_layout() ? box.computed_values().height() : box.computed_values().width();
    return cross_length.has_value() && cross_length->is_length() && cross_length->length().is_auto();
}

bool FlexFormattingContext::is_main_axis_margin_first_auto(Box const& box) const
{
    if (is_row_layout())
        return box.computed_values().margin().left.is_length() && box.computed_values().margin().left.length().is_auto();
    return box.computed_values().margin().top.is_length() && box.computed_values().margin().top.length().is_auto();
}

bool FlexFormattingContext::is_main_axis_margin_second_auto(Box const& box) const
{
    if (is_row_layout())
        return box.computed_values().margin().right.is_length() && box.computed_values().margin().right.length().is_auto();
    return box.computed_values().margin().bottom.is_length() && box.computed_values().margin().bottom.length().is_auto();
}

void FlexFormattingContext::set_main_size(Box const& box, float size)
{
    if (is_row_layout())
        m_state.get_mutable(box).content_width = size;
    else
        m_state.get_mutable(box).content_height = size;
}

void FlexFormattingContext::set_cross_size(Box const& box, float size)
{
    if (is_row_layout())
        m_state.get_mutable(box).content_height = size;
    else
        m_state.get_mutable(box).content_width = size;
}

void FlexFormattingContext::set_offset(Box const& box, float main_offset, float cross_offset)
{
    if (is_row_layout())
        m_state.get_mutable(box).offset = Gfx::FloatPoint { main_offset, cross_offset };
    else
        m_state.get_mutable(box).offset = Gfx::FloatPoint { cross_offset, main_offset };
}

void FlexFormattingContext::set_main_axis_first_margin(FlexItem& item, float margin)
{
    item.margins.main_before = margin;
    if (is_row_layout())
        m_state.get_mutable(item.box).margin_left = margin;
    else
        m_state.get_mutable(item.box).margin_top = margin;
}

void FlexFormattingContext::set_main_axis_second_margin(FlexItem& item, float margin)
{
    item.margins.main_after = margin;
    if (is_row_layout())
        m_state.get_mutable(item.box).margin_right = margin;
    else
        m_state.get_mutable(item.box).margin_bottom = margin;
}

float FlexFormattingContext::sum_of_margin_padding_border_in_main_axis(Box const& box) const
{
    auto const& box_state = m_state.get(box);

    if (is_row_layout()) {
        return box_state.margin_left + box_state.margin_right
            + box_state.padding_left + box_state.padding_right
            + box_state.border_left + box_state.border_right;
    } else {
        return box_state.margin_top + box_state.margin_bottom
            + box_state.padding_top + box_state.padding_bottom
            + box_state.border_top + box_state.border_bottom;
    }
}

// https://www.w3.org/TR/css-flexbox-1/#algo-available
void FlexFormattingContext::determine_available_main_and_cross_space(bool& main_is_constrained, bool& cross_is_constrained, float& main_min_size, float& main_max_size, float& cross_min_size, float& cross_max_size)
{
    auto containing_block_effective_main_size = [&](Box const& box) -> Optional<float> {
        auto& containing_block = *box.containing_block();
        if (has_definite_main_size(containing_block))
            return resolved_definite_main_size(containing_block);
        return {};
    };

    Optional<float> main_available_space;
    main_is_constrained = false;

    // For each dimension,
    //     if that dimension of the flex container’s content box is a definite size, use that;
    //     if that dimension of the flex container is being sized under a min or max-content constraint, the available space in that dimension is that constraint;
    //     otherwise, subtract the flex container’s margin, border, and padding from the space available to the flex container in that dimension and use that value. (This might result in an infinite value.)

    if (has_definite_main_size(flex_container())) {
        main_is_constrained = true;
        main_available_space = specified_main_size(flex_container());
    } else {
        if (has_main_max_size(flex_container())) {
            main_max_size = specified_main_max_size(flex_container());
            main_available_space = main_max_size;
            main_is_constrained = true;
        }
        if (has_main_min_size(flex_container())) {
            main_min_size = specified_main_min_size(flex_container());
            main_is_constrained = true;
        }

        if (!main_is_constrained) {
            auto available_main_size = containing_block_effective_main_size(flex_container());
            main_available_space = available_main_size.value_or(NumericLimits<float>::max()) - sum_of_margin_padding_border_in_main_axis(flex_container());
            if (flex_container().computed_values().flex_wrap() == CSS::FlexWrap::Wrap || flex_container().computed_values().flex_wrap() == CSS::FlexWrap::WrapReverse) {
                main_available_space = specified_main_size(*flex_container().containing_block());
                main_is_constrained = true;
            }
        }
    }

    Optional<float> cross_available_space;
    cross_is_constrained = false;

    if (has_definite_cross_size(flex_container())) {
        cross_available_space = specified_cross_size(flex_container());
    } else {
        if (has_cross_max_size(flex_container())) {
            cross_max_size = specified_cross_max_size(flex_container());
            cross_is_constrained = true;
        }
        if (has_cross_min_size(flex_container())) {
            cross_min_size = specified_cross_min_size(flex_container());
            cross_is_constrained = true;
        }

        // FIXME: Is this right? Probably not.
        if (!cross_is_constrained)
            cross_available_space = cross_max_size;
    }

    m_available_space = AvailableSpace { .main = main_available_space, .cross = cross_available_space };
}

float FlexFormattingContext::calculate_indefinite_main_size(FlexItem const& item)
{
    VERIFY(!has_definite_main_size(item.box));

    if (has_definite_cross_size(item.box)) {
        // For indefinite main sizes, we perform a throwaway layout and then measure it.
        FormattingState throwaway_state(&m_state);
        auto& box_state = throwaway_state.get_mutable(item.box);

        // Item has definite cross size, layout with that as the used cross size.
        auto independent_formatting_context = create_independent_formatting_context_if_needed(throwaway_state, item.box);
        // NOTE: Flex items should always create an independent formatting context!
        VERIFY(independent_formatting_context);

        if (is_row_layout()) {
            box_state.content_height = resolved_definite_cross_size(item.box);
        } else {
            box_state.content_width = resolved_definite_cross_size(item.box);
        }
        independent_formatting_context->run(item.box, LayoutMode::Normal);

        if (is_row_layout())
            return box_state.content_width;
        return BlockFormattingContext::compute_theoretical_height(throwaway_state, item.box);
    }

    // Item has indefinite cross size, layout with "fit-content"

    // If we're in a row layout and looking for the width, just use the fit-content width.
    if (is_row_layout())
        return calculate_fit_content_width(item.box, m_available_space->main);

    // We're in a column layout, looking for the height. Figure out the fit-content width,
    // then layout with that and see what height comes out of it.
    float fit_content_cross_size = calculate_fit_content_width(item.box, m_available_space->cross);

    FormattingState throwaway_state(&m_state);
    auto& box_state = throwaway_state.get_mutable(item.box);

    // Item has definite cross size, layout with that as the used cross size.
    auto independent_formatting_context = create_independent_formatting_context_if_needed(throwaway_state, item.box);
    // NOTE: Flex items should always create an independent formatting context!
    VERIFY(independent_formatting_context);

    box_state.content_width = fit_content_cross_size;
    independent_formatting_context->run(item.box, LayoutMode::Normal);

    return BlockFormattingContext::compute_theoretical_height(throwaway_state, item.box);
}

// https://www.w3.org/TR/css-flexbox-1/#algo-main-item
void FlexFormattingContext::determine_flex_base_size_and_hypothetical_main_size(FlexItem& flex_item)
{
    auto& child_box = flex_item.box;

    flex_item.flex_base_size = [&] {
        auto const& used_flex_basis = child_box.computed_values().flex_basis();

        // A. If the item has a definite used flex basis, that’s the flex base size.
        if (used_flex_basis.is_definite()) {
            auto specified_base_size = get_pixel_size(m_state, child_box, used_flex_basis.length_percentage.value());
            if (specified_base_size == 0)
                return calculated_main_size(flex_item.box);
            return specified_base_size;
        }

        // B. If the flex item has ...
        //    - an intrinsic aspect ratio,
        //    - a used flex basis of content, and
        //    - a definite cross size,
        if (flex_item.box.has_intrinsic_aspect_ratio()
            && used_flex_basis.type == CSS::FlexBasis::Content
            && has_definite_cross_size(child_box)) {
            TODO();
            // flex_base_size is calculated from definite cross size and intrinsic aspect ratio
        }

        // C. If the used flex basis is content or depends on its available space,
        //    and the flex container is being sized under a min-content or max-content constraint
        //    (e.g. when performing automatic table layout [CSS21]), size the item under that constraint.
        //    The flex base size is the item’s resulting main size.
        if (used_flex_basis.type == CSS::FlexBasis::Content
            // FIXME: && sized under min-content or max-content constraints
            && false) {
            TODO();
            // Size child_box under the constraints, flex_base_size is then the resulting main_size.
        }

        // D. Otherwise, if the used flex basis is content or depends on its available space,
        //    the available main size is infinite, and the flex item’s inline axis is parallel to the main axis,
        //    lay the item out using the rules for a box in an orthogonal flow [CSS3-WRITING-MODES].
        //    The flex base size is the item’s max-content main size.
        if (used_flex_basis.type == CSS::FlexBasis::Content
            // FIXME: && main_size is infinite && inline axis is parallel to the main axis
            && false && false) {
            TODO();
            // Use rules for a flex_container in orthogonal flow
        }

        // E. Otherwise, size the item into the available space using its used flex basis in place of its main size,
        //    treating a value of content as max-content. If a cross size is needed to determine the main size
        //    (e.g. when the flex item’s main size is in its block axis) and the flex item’s cross size is auto and not definite,
        //    in this calculation use fit-content as the flex item’s cross size.
        //    The flex base size is the item’s resulting main size.
        // FIXME: This is probably too naive.
        // FIXME: Care about FlexBasis::Auto
        if (has_definite_main_size(child_box))
            return specified_main_size_of_child_box(child_box);

        return calculate_indefinite_main_size(flex_item);
    }();

    // The hypothetical main size is the item’s flex base size clamped according to its used min and max main sizes (and flooring the content box size at zero).
    auto clamp_min = has_main_min_size(child_box) ? specified_main_min_size(child_box) : automatic_minimum_size(flex_item);
    auto clamp_max = has_main_max_size(child_box) ? specified_main_max_size(child_box) : NumericLimits<float>::max();
    flex_item.hypothetical_main_size = css_clamp(flex_item.flex_base_size, clamp_min, clamp_max);
}

// https://drafts.csswg.org/css-flexbox-1/#min-size-auto
float FlexFormattingContext::automatic_minimum_size(FlexItem const& item) const
{
    // FIXME: Deal with scroll containers.
    return content_based_minimum_size(item);
}

// https://drafts.csswg.org/css-flexbox-1/#specified-size-suggestion
Optional<float> FlexFormattingContext::specified_size_suggestion(FlexItem const& item) const
{
    // If the item’s preferred main size is definite and not automatic,
    // then the specified size suggestion is that size. It is otherwise undefined.
    if (has_definite_main_size(item.box))
        return specified_main_size(item.box);
    return {};
}

// https://drafts.csswg.org/css-flexbox-1/#content-size-suggestion
float FlexFormattingContext::content_size_suggestion(FlexItem const& item) const
{
    // FIXME: Apply clamps
    if (is_row_layout())
        return calculate_min_and_max_content_width(item.box).min_content_size;
    return calculate_min_and_max_content_height(item.box).min_content_size;
}

// https://drafts.csswg.org/css-flexbox-1/#transferred-size-suggestion
Optional<float> FlexFormattingContext::transferred_size_suggestion(FlexItem const& item) const
{
    // If the item has a preferred aspect ratio and its preferred cross size is definite,
    // then the transferred size suggestion is that size
    // (clamped by its minimum and maximum cross sizes if they are definite), converted through the aspect ratio.
    if (item.box.has_intrinsic_aspect_ratio() && has_definite_cross_size(item.box)) {
        auto aspect_ratio = item.box.intrinsic_aspect_ratio().value();
        // FIXME: Clamp cross size to min/max cross size before this conversion.
        return resolved_definite_cross_size(item.box) * aspect_ratio;
    }

    // It is otherwise undefined.
    return {};
}

// https://drafts.csswg.org/css-flexbox-1/#content-based-minimum-size
float FlexFormattingContext::content_based_minimum_size(FlexItem const& item) const
{
    auto unclamped_size = [&] {
        // The content-based minimum size of a flex item is the smaller of its specified size suggestion
        // and its content size suggestion if its specified size suggestion exists;
        if (auto specified_size_suggestion = this->specified_size_suggestion(item); specified_size_suggestion.has_value()) {
            return min(specified_size_suggestion.value(), content_size_suggestion(item));
        }

        // otherwise, the smaller of its transferred size suggestion and its content size suggestion
        // if the element is replaced and its transferred size suggestion exists;
        if (item.box.is_replaced_box()) {
            if (auto transferred_size_suggestion = this->transferred_size_suggestion(item); transferred_size_suggestion.has_value()) {
                return min(transferred_size_suggestion.value(), content_size_suggestion(item));
            }
        }

        // otherwise its content size suggestion.
        return content_size_suggestion(item);
    }();

    // In all cases, the size is clamped by the maximum main size if it’s definite.
    if (has_main_max_size(item.box)) {
        return min(unclamped_size, specified_main_max_size(item.box));
    }
    return unclamped_size;
}

// https://www.w3.org/TR/css-flexbox-1/#algo-main-container
void FlexFormattingContext::determine_main_size_of_flex_container(bool const main_is_constrained, float const main_min_size, float const main_max_size)
{
    // FIXME: This function should make use of our ability to calculate the flex container's
    //        intrinsic max-content sizes via LayoutMode::MaxContent.

    if (!main_is_constrained || !m_available_space->main.has_value()) {
        // Uses https://www.w3.org/TR/css-flexbox-1/#intrinsic-main-sizes
        // 9.9.1
        // 1.
        float largest_max_content_flex_fraction = 0;
        for (auto& flex_item : m_flex_items) {
            // FIXME: This needs some serious work.
            float max_content_contribution = calculated_main_size(flex_item.box);
            float max_content_flex_fraction = max_content_contribution - (flex_item.flex_base_size + flex_item.margins.main_before + flex_item.margins.main_after + flex_item.borders.main_before + flex_item.borders.main_after + flex_item.padding.main_before + flex_item.padding.main_after);
            if (max_content_flex_fraction > 0) {
                max_content_flex_fraction /= max(flex_item.box.computed_values().flex_grow(), 1.0f);
            } else {
                max_content_flex_fraction /= max(flex_item.box.computed_values().flex_shrink(), 1.0f) * flex_item.flex_base_size;
            }
            flex_item.max_content_flex_fraction = max_content_flex_fraction;

            if (max_content_flex_fraction > largest_max_content_flex_fraction)
                largest_max_content_flex_fraction = max_content_flex_fraction;
        }

        // 2. Omitted
        // 3.
        float result = 0;
        for (auto& flex_item : m_flex_items) {
            auto product = 0;
            if (flex_item.max_content_flex_fraction > 0) {
                product = largest_max_content_flex_fraction * flex_item.box.computed_values().flex_grow();
            } else {
                product = largest_max_content_flex_fraction * max(flex_item.box.computed_values().flex_shrink(), 1.0f) * flex_item.flex_base_size;
            }
            result += flex_item.flex_base_size + flex_item.margins.main_before + flex_item.margins.main_after + flex_item.borders.main_before + flex_item.borders.main_after + flex_item.padding.main_before + flex_item.padding.main_after + product;
        }
        m_available_space->main = css_clamp(result, main_min_size, main_max_size);
    }
    set_main_size(flex_container(), m_available_space->main.value_or(NumericLimits<float>::max()));
}

// https://www.w3.org/TR/css-flexbox-1/#algo-line-break
void FlexFormattingContext::collect_flex_items_into_flex_lines()
{
    // FIXME: Also support wrap-reverse

    // If the flex container is single-line, collect all the flex items into a single flex line.
    if (is_single_line()) {
        FlexLine line;
        for (auto& flex_item : m_flex_items) {
            line.items.append(&flex_item);
        }
        m_flex_lines.append(move(line));
        return;
    }

    // Otherwise, starting from the first uncollected item, collect consecutive items one by one
    // until the first time that the next collected item would not fit into the flex container’s inner main size
    // (or until a forced break is encountered, see §10 Fragmenting Flex Layout).
    // If the very first uncollected item wouldn't fit, collect just it into the line.

    // For this step, the size of a flex item is its outer hypothetical main size. (Note: This can be negative.)

    // Repeat until all flex items have been collected into flex lines.

    FlexLine line;
    float line_main_size = 0;
    for (auto& flex_item : m_flex_items) {
        auto outer_hypothetical_main_size = flex_item.hypothetical_main_size + flex_item.margins.main_before + flex_item.margins.main_after + flex_item.borders.main_before + flex_item.borders.main_after + flex_item.padding.main_before + flex_item.padding.main_after;
        if ((line_main_size + outer_hypothetical_main_size) > m_available_space->main.value_or(NumericLimits<float>::max())) {
            m_flex_lines.append(move(line));
            line = {};
            line_main_size = 0;
        }
        line.items.append(&flex_item);
        line_main_size += outer_hypothetical_main_size;
    }
    m_flex_lines.append(move(line));
}

// https://www.w3.org/TR/css-flexbox-1/#resolve-flexible-lengths
void FlexFormattingContext::resolve_flexible_lengths()
{
    enum FlexFactor {
        FlexGrowFactor,
        FlexShrinkFactor
    };

    FlexFactor used_flex_factor;
    // 6.1. Determine used flex factor
    for (auto& flex_line : m_flex_lines) {
        size_t number_of_unfrozen_items_on_line = flex_line.items.size();

        float sum_of_hypothetical_main_sizes = 0;
        for (auto& flex_item : flex_line.items) {
            sum_of_hypothetical_main_sizes += (flex_item->hypothetical_main_size + flex_item->margins.main_before + flex_item->margins.main_after + flex_item->borders.main_before + flex_item->borders.main_after + flex_item->padding.main_before + flex_item->padding.main_after);
        }
        if (sum_of_hypothetical_main_sizes < m_available_space->main.value_or(NumericLimits<float>::max()))
            used_flex_factor = FlexFactor::FlexGrowFactor;
        else
            used_flex_factor = FlexFactor::FlexShrinkFactor;

        for (auto& flex_item : flex_line.items) {
            if (used_flex_factor == FlexFactor::FlexGrowFactor)
                flex_item->flex_factor = flex_item->box.computed_values().flex_grow();
            else if (used_flex_factor == FlexFactor::FlexShrinkFactor)
                flex_item->flex_factor = flex_item->box.computed_values().flex_shrink();
        }

        // 6.2. Size inflexible items
        auto freeze_item_setting_target_main_size_to_hypothetical_main_size = [&number_of_unfrozen_items_on_line](FlexItem& item) {
            item.target_main_size = item.hypothetical_main_size;
            number_of_unfrozen_items_on_line--;
            item.frozen = true;
        };
        for (auto& flex_item : flex_line.items) {
            if (flex_item->flex_factor.has_value() && flex_item->flex_factor.value() == 0) {
                freeze_item_setting_target_main_size_to_hypothetical_main_size(*flex_item);
            } else if (used_flex_factor == FlexFactor::FlexGrowFactor) {
                // FIXME: Spec doesn't include the == case, but we take a too basic approach to calculating the values used so this is appropriate
                if (flex_item->flex_base_size > flex_item->hypothetical_main_size) {
                    freeze_item_setting_target_main_size_to_hypothetical_main_size(*flex_item);
                }
            } else if (used_flex_factor == FlexFactor::FlexShrinkFactor) {
                if (flex_item->flex_base_size < flex_item->hypothetical_main_size) {
                    freeze_item_setting_target_main_size_to_hypothetical_main_size(*flex_item);
                }
            }
        }

        // 6.3. Calculate initial free space
        auto calculate_free_space = [&]() {
            float sum_of_items_on_line = 0;
            for (auto& flex_item : flex_line.items) {
                if (flex_item->frozen)
                    sum_of_items_on_line += flex_item->target_main_size + flex_item->margins.main_before + flex_item->margins.main_after + flex_item->borders.main_before + flex_item->borders.main_after + flex_item->padding.main_before + flex_item->padding.main_after;
                else
                    sum_of_items_on_line += flex_item->flex_base_size + flex_item->margins.main_before + flex_item->margins.main_after + flex_item->borders.main_before + flex_item->borders.main_after + flex_item->padding.main_before + flex_item->padding.main_after;
            }
            return specified_main_size(flex_container()) - sum_of_items_on_line;
        };

        float initial_free_space = calculate_free_space();

        // 6.4 Loop
        auto for_each_unfrozen_item = [&flex_line](auto callback) {
            for (auto& flex_item : flex_line.items) {
                if (!flex_item->frozen)
                    callback(flex_item);
            }
        };

        while (number_of_unfrozen_items_on_line > 0) {
            // b Calculate the remaining free space
            auto remaining_free_space = calculate_free_space();
            float sum_of_unfrozen_flex_items_flex_factors = 0;
            for_each_unfrozen_item([&](FlexItem* item) {
                sum_of_unfrozen_flex_items_flex_factors += item->flex_factor.value_or(1);
            });

            if (sum_of_unfrozen_flex_items_flex_factors < 1) {
                auto intermediate_free_space = initial_free_space * sum_of_unfrozen_flex_items_flex_factors;
                if (AK::abs(intermediate_free_space) < AK::abs(remaining_free_space))
                    remaining_free_space = intermediate_free_space;
            }

            // c Distribute free space proportional to the flex factors
            if (remaining_free_space != 0) {
                if (used_flex_factor == FlexFactor::FlexGrowFactor) {
                    float sum_of_flex_grow_factor_of_unfrozen_items = sum_of_unfrozen_flex_items_flex_factors;
                    for_each_unfrozen_item([&](FlexItem* flex_item) {
                        float ratio = flex_item->flex_factor.value_or(1) / sum_of_flex_grow_factor_of_unfrozen_items;
                        flex_item->target_main_size = flex_item->flex_base_size + (remaining_free_space * ratio);
                    });
                } else if (used_flex_factor == FlexFactor::FlexShrinkFactor) {
                    float sum_of_scaled_flex_shrink_factor_of_unfrozen_items = 0;
                    for_each_unfrozen_item([&](FlexItem* flex_item) {
                        flex_item->scaled_flex_shrink_factor = flex_item->flex_factor.value_or(1) * flex_item->flex_base_size;
                        sum_of_scaled_flex_shrink_factor_of_unfrozen_items += flex_item->scaled_flex_shrink_factor;
                    });

                    for_each_unfrozen_item([&](FlexItem* flex_item) {
                        float ratio = 1.0f;
                        if (sum_of_scaled_flex_shrink_factor_of_unfrozen_items != 0.0f)
                            ratio = flex_item->scaled_flex_shrink_factor / sum_of_scaled_flex_shrink_factor_of_unfrozen_items;
                        flex_item->target_main_size = flex_item->flex_base_size - (AK::abs(remaining_free_space) * ratio);
                    });
                }
            } else {
                // This isn't spec but makes sense.
                for_each_unfrozen_item([&](FlexItem* flex_item) {
                    flex_item->target_main_size = flex_item->flex_base_size;
                });
            }
            // d Fix min/max violations.
            float adjustments = 0.0f;
            for_each_unfrozen_item([&](FlexItem* item) {
                auto min_main = has_main_min_size(item->box)
                    ? specified_main_min_size(item->box)
                    : automatic_minimum_size(*item);
                auto max_main = has_main_max_size(item->box)
                    ? specified_main_max_size(item->box)
                    : NumericLimits<float>::max();

                float original_target_size = item->target_main_size;

                if (item->target_main_size < min_main) {
                    item->target_main_size = min_main;
                    item->is_min_violation = true;
                }

                if (item->target_main_size > max_main) {
                    item->target_main_size = max_main;
                    item->is_max_violation = true;
                }
                float delta = item->target_main_size - original_target_size;
                adjustments += delta;
            });
            // e Freeze over-flexed items
            float total_violation = adjustments;
            if (total_violation == 0) {
                for_each_unfrozen_item([&](FlexItem* item) {
                    --number_of_unfrozen_items_on_line;
                    item->frozen = true;
                });
            } else if (total_violation > 0) {
                for_each_unfrozen_item([&](FlexItem* item) {
                    if (item->is_min_violation) {
                        --number_of_unfrozen_items_on_line;
                        item->frozen = true;
                    }
                });
            } else if (total_violation < 0) {
                for_each_unfrozen_item([&](FlexItem* item) {
                    if (item->is_max_violation) {
                        --number_of_unfrozen_items_on_line;
                        item->frozen = true;
                    }
                });
            }
        }

        // 6.5.
        for (auto& flex_item : flex_line.items) {
            flex_item->main_size = flex_item->target_main_size;
        }
    }
}

// https://drafts.csswg.org/css-flexbox-1/#algo-cross-item
void FlexFormattingContext::determine_hypothetical_cross_size_of_item(FlexItem& item)
{
    // Determine the hypothetical cross size of each item by performing layout
    // as if it were an in-flow block-level box with the used main size
    // and the given available space, treating auto as fit-content.

    // If we have a definite cross size, this is easy! No need to perform layout, we can just use it as-is.
    if (has_definite_cross_size(item.box)) {
        auto clamp_min = has_cross_min_size(item.box) ? specified_cross_min_size(item.box) : 0;
        auto clamp_max = has_cross_max_size(item.box) ? specified_cross_max_size(item.box) : NumericLimits<float>::max();
        item.hypothetical_cross_size = css_clamp(resolved_definite_cross_size(item.box), clamp_min, clamp_max);
        return;
    }

    if (has_definite_main_size(item.box)) {
        // For indefinite cross sizes, we perform a throwaway layout and then measure it.
        FormattingState throwaway_state(&m_state);
        auto& box_state = throwaway_state.get_mutable(item.box);

        // Item has definite main size, layout with that as the used main size.
        auto independent_formatting_context = create_independent_formatting_context_if_needed(throwaway_state, item.box);
        // NOTE: Flex items should always create an independent formatting context!
        VERIFY(independent_formatting_context);

        if (is_row_layout()) {
            box_state.content_width = resolved_definite_main_size(item.box);
        } else {
            box_state.content_height = resolved_definite_main_size(item.box);
        }
        independent_formatting_context->run(item.box, LayoutMode::Normal);

        if (is_row_layout())
            item.hypothetical_cross_size = BlockFormattingContext::compute_theoretical_height(throwaway_state, item.box);
        else
            item.hypothetical_cross_size = box_state.content_width;
    } else {
        // Item has indefinite main size, layout with "fit-content"

        // If we're in a column layout and looking for the width, just use the fit-content width.
        if (!is_row_layout()) {
            item.hypothetical_cross_size = calculate_fit_content_width(item.box, m_available_space->cross);
            return;
        }

        // We're in a row layout, looking for the height. Figure out the fit-content width,
        // then layout with that and see what height comes out of it.

        float fit_content_main_size = calculate_fit_content_width(item.box, m_available_space->main);

        FormattingState throwaway_state(&m_state);
        auto& box_state = throwaway_state.get_mutable(item.box);

        auto independent_formatting_context = create_independent_formatting_context_if_needed(throwaway_state, item.box);
        // NOTE: Flex items should always create an independent formatting context!
        VERIFY(independent_formatting_context);

        box_state.content_width = fit_content_main_size;
        independent_formatting_context->run(item.box, LayoutMode::Normal);

        item.hypothetical_cross_size = BlockFormattingContext::compute_theoretical_height(throwaway_state, item.box);
    }
}

// https://www.w3.org/TR/css-flexbox-1/#algo-cross-line
void FlexFormattingContext::calculate_cross_size_of_each_flex_line(float const cross_min_size, float const cross_max_size)
{
    // If the flex container is single-line and has a definite cross size, the cross size of the flex line is the flex container’s inner cross size.
    if (is_single_line() && has_definite_cross_size(flex_container())) {
        m_flex_lines[0].cross_size = specified_cross_size(flex_container());
        return;
    }

    // Otherwise, for each flex line:
    for (auto& flex_line : m_flex_lines) {
        // FIXME: 1. Collect all the flex items whose inline-axis is parallel to the main-axis, whose align-self is baseline,
        //           and whose cross-axis margins are both non-auto. Find the largest of the distances between each item’s baseline
        //           and its hypothetical outer cross-start edge, and the largest of the distances between each item’s baseline
        //           and its hypothetical outer cross-end edge, and sum these two values.

        // FIXME: This isn't spec but makes sense here
        if (has_definite_cross_size(flex_container()) && flex_container().computed_values().align_items() == CSS::AlignItems::Stretch) {
            flex_line.cross_size = specified_cross_size(flex_container()) / m_flex_lines.size();
            continue;
        }

        // 2. Among all the items not collected by the previous step, find the largest outer hypothetical cross size.
        float largest_hypothetical_cross_size = 0;
        for (auto& flex_item : flex_line.items) {
            if (largest_hypothetical_cross_size < flex_item->hypothetical_cross_size_with_margins())
                largest_hypothetical_cross_size = flex_item->hypothetical_cross_size_with_margins();
        }

        // 3. The used cross-size of the flex line is the largest of the numbers found in the previous two steps and zero.
        flex_line.cross_size = max(0.0f, largest_hypothetical_cross_size);
    }

    // If the flex container is single-line, then clamp the line’s cross-size to be within the container’s computed min and max cross sizes.
    // Note that if CSS 2.1’s definition of min/max-width/height applied more generally, this behavior would fall out automatically.
    if (is_single_line())
        css_clamp(m_flex_lines[0].cross_size, cross_min_size, cross_max_size);
}

// https://www.w3.org/TR/css-flexbox-1/#algo-stretch
void FlexFormattingContext::determine_used_cross_size_of_each_flex_item()
{
    // FIXME: Get the alignment via "align-self" of the item (which accesses "align-items" of the parent if unset)
    for (auto& flex_line : m_flex_lines) {
        for (auto& flex_item : flex_line.items) {
            if (is_cross_auto(flex_item->box) && flex_container().computed_values().align_items() == CSS::AlignItems::Stretch) {
                flex_item->cross_size = flex_line.cross_size;
            } else {
                flex_item->cross_size = flex_item->hypothetical_cross_size;
            }
        }
    }
}

// https://www.w3.org/TR/css-flexbox-1/#algo-main-align
void FlexFormattingContext::distribute_any_remaining_free_space()
{
    for (auto& flex_line : m_flex_lines) {
        // 12.1.
        float used_main_space = 0;
        size_t auto_margins = 0;
        for (auto& flex_item : flex_line.items) {
            used_main_space += flex_item->main_size;
            if (is_main_axis_margin_first_auto(flex_item->box))
                ++auto_margins;
            else
                used_main_space += flex_item->margins.main_before + flex_item->borders.main_before + flex_item->padding.main_before;

            if (is_main_axis_margin_second_auto(flex_item->box))
                ++auto_margins;
            else
                used_main_space += flex_item->margins.main_after + flex_item->borders.main_after + flex_item->padding.main_after;
        }
        float remaining_free_space = m_available_space->main.value_or(NumericLimits<float>::max()) - used_main_space;
        if (remaining_free_space > 0) {
            float size_per_auto_margin = remaining_free_space / (float)auto_margins;
            for (auto& flex_item : flex_line.items) {
                if (is_main_axis_margin_first_auto(flex_item->box))
                    set_main_axis_first_margin(*flex_item, size_per_auto_margin);
                if (is_main_axis_margin_second_auto(flex_item->box))
                    set_main_axis_second_margin(*flex_item, size_per_auto_margin);
            }
        } else {
            for (auto& flex_item : flex_line.items) {
                if (is_main_axis_margin_first_auto(flex_item->box))
                    set_main_axis_first_margin(*flex_item, 0);
                if (is_main_axis_margin_second_auto(flex_item->box))
                    set_main_axis_second_margin(*flex_item, 0);
            }
        }

        // 12.2.
        float space_between_items = 0;
        float space_before_first_item = 0;
        auto number_of_items = flex_line.items.size();

        switch (flex_container().computed_values().justify_content()) {
        case CSS::JustifyContent::FlexStart:
            break;
        case CSS::JustifyContent::FlexEnd:
            space_before_first_item = m_available_space->main.value_or(NumericLimits<float>::max()) - used_main_space;
            break;
        case CSS::JustifyContent::Center:
            space_before_first_item = (m_available_space->main.value_or(NumericLimits<float>::max()) - used_main_space) / 2.0f;
            break;
        case CSS::JustifyContent::SpaceBetween:
            space_between_items = remaining_free_space / (number_of_items - 1);
            break;
        case CSS::JustifyContent::SpaceAround:
            space_between_items = remaining_free_space / number_of_items;
            space_before_first_item = space_between_items / 2.0f;
            break;
        }

        // FIXME: Support reverse
        float main_offset = space_before_first_item;
        for (auto& flex_item : flex_line.items) {
            flex_item->main_offset = main_offset + flex_item->margins.main_before + flex_item->borders.main_before + flex_item->padding.main_before;
            main_offset += flex_item->margins.main_before + flex_item->borders.main_before + flex_item->padding.main_before + flex_item->main_size + flex_item->margins.main_after + flex_item->borders.main_after + flex_item->padding.main_after + space_between_items;
        }
    }
}

void FlexFormattingContext::dump_items() const
{
    dbgln("\033[34;1mflex-container\033[0m {}, direction: {}, current-size: {}x{}", flex_container().debug_description(), is_row_layout() ? "row" : "column", m_flex_container_state.content_width, m_flex_container_state.content_height);
    for (size_t i = 0; i < m_flex_lines.size(); ++i) {
        dbgln("{} flex-line #{}:", flex_container().debug_description(), i);
        for (size_t j = 0; j < m_flex_lines[i].items.size(); ++j) {
            auto& item = *m_flex_lines[i].items[j];
            dbgln("{}   flex-item #{}: {} (main:{}, cross:{})", flex_container().debug_description(), j, item.box.debug_description(), item.main_size, item.cross_size);
        }
    }
}

void FlexFormattingContext::align_all_flex_items_along_the_cross_axis()
{
    // FIXME: Get the alignment via "align-self" of the item (which accesses "align-items" of the parent if unset)
    // FIXME: Take better care of margins
    float line_cross_offset = 0;
    for (auto& flex_line : m_flex_lines) {
        for (auto* flex_item : flex_line.items) {
            switch (flex_container().computed_values().align_items()) {
            case CSS::AlignItems::Baseline:
                // FIXME: Implement this
                //  Fallthrough
            case CSS::AlignItems::FlexStart:
            case CSS::AlignItems::Stretch:
                flex_item->cross_offset = line_cross_offset + flex_item->margins.cross_before + flex_item->borders.cross_before + flex_item->padding.cross_before;
                break;
            case CSS::AlignItems::FlexEnd:
                flex_item->cross_offset = line_cross_offset + flex_line.cross_size - flex_item->cross_size;
                break;
            case CSS::AlignItems::Center:
                flex_item->cross_offset = line_cross_offset + (flex_line.cross_size / 2.0f) - (flex_item->cross_size / 2.0f);
                break;
            default:
                break;
            }
        }

        line_cross_offset += flex_line.cross_size;
    }
}

// https://www.w3.org/TR/css-flexbox-1/#algo-cross-container
void FlexFormattingContext::determine_flex_container_used_cross_size(float const cross_min_size, float const cross_max_size)
{
    float cross_size = 0;
    if (has_definite_cross_size(flex_container())) {
        // Flex container has definite cross size: easy-peasy.
        cross_size = specified_cross_size(flex_container());
    } else {
        // Flex container has indefinite cross size.
        auto cross_size_value = is_row_layout() ? flex_container().computed_values().height() : flex_container().computed_values().width();
        if (!cross_size_value.has_value() || (cross_size_value->is_length() && cross_size_value->length().is_auto()) || cross_size_value->is_percentage()) {
            // If a content-based cross size is needed, use the sum of the flex lines' cross sizes.
            float sum_of_flex_lines_cross_sizes = 0;
            for (auto& flex_line : m_flex_lines) {
                sum_of_flex_lines_cross_sizes += flex_line.cross_size;
            }
            cross_size = sum_of_flex_lines_cross_sizes;

            if (cross_size_value->is_percentage()) {
                // FIXME: Handle percentage values here! Right now we're just treating them as "auto"
            }
        } else {
            // Otherwise, resolve the indefinite size at this point.
            cross_size = cross_size_value->resolved(flex_container(), CSS::Length::make_px(specified_cross_size(*flex_container().containing_block()))).to_px(flex_container());
        }
    }
    set_cross_size(flex_container(), css_clamp(cross_size, cross_min_size, cross_max_size));
}

// https://www.w3.org/TR/css-flexbox-1/#algo-line-align
void FlexFormattingContext::align_all_flex_lines()
{
    // FIXME: Support reverse
    if (is_single_line()) {
        // For single-line flex containers, we only need to center the line along the cross axis.
        auto& flex_line = m_flex_lines[0];
        float cross_size_of_flex_container = specified_cross_size(flex_container());
        for (auto* flex_item : flex_line.items)
            flex_item->cross_offset += (cross_size_of_flex_container / 2.0f) - (flex_line.cross_size / 2.0f);
    } else {
        // FIXME: Support align-content
    }
}

void FlexFormattingContext::copy_dimensions_from_flex_items_to_boxes()
{
    for (auto& flex_item : m_flex_items) {
        auto const& box = flex_item.box;
        auto& box_state = m_state.get_mutable(box);

        box_state.padding_left = box.computed_values().padding().left.resolved(box, CSS::Length::make_px(m_flex_container_state.content_width)).to_px(box);
        box_state.padding_right = box.computed_values().padding().right.resolved(box, CSS::Length::make_px(m_flex_container_state.content_width)).to_px(box);
        box_state.padding_top = box.computed_values().padding().top.resolved(box, CSS::Length::make_px(m_flex_container_state.content_width)).to_px(box);
        box_state.padding_bottom = box.computed_values().padding().bottom.resolved(box, CSS::Length::make_px(m_flex_container_state.content_width)).to_px(box);

        box_state.margin_left = box.computed_values().margin().left.resolved(box, CSS::Length::make_px(m_flex_container_state.content_width)).to_px(box);
        box_state.margin_right = box.computed_values().margin().right.resolved(box, CSS::Length::make_px(m_flex_container_state.content_width)).to_px(box);
        box_state.margin_top = box.computed_values().margin().top.resolved(box, CSS::Length::make_px(m_flex_container_state.content_width)).to_px(box);
        box_state.margin_bottom = box.computed_values().margin().bottom.resolved(box, CSS::Length::make_px(m_flex_container_state.content_width)).to_px(box);

        box_state.border_left = box.computed_values().border_left().width;
        box_state.border_right = box.computed_values().border_right().width;
        box_state.border_top = box.computed_values().border_top().width;
        box_state.border_bottom = box.computed_values().border_bottom().width;

        set_main_size(box, flex_item.main_size);
        set_cross_size(box, flex_item.cross_size);
        set_offset(box, flex_item.main_offset, flex_item.cross_offset);
    }
}

// https://drafts.csswg.org/css-flexbox-1/#intrinsic-sizes
void FlexFormattingContext::determine_intrinsic_size_of_flex_container(LayoutMode layout_mode)
{
    VERIFY(layout_mode != LayoutMode::Normal);

    float main_size = calculate_intrinsic_main_size_of_flex_container(layout_mode);
    float cross_size = calculate_intrinsic_cross_size_of_flex_container(layout_mode);
    if (is_row_layout()) {
        m_flex_container_state.content_width = main_size;
        m_flex_container_state.content_height = cross_size;
    } else {
        m_flex_container_state.content_height = main_size;
        m_flex_container_state.content_width = cross_size;
    }
}

// https://drafts.csswg.org/css-flexbox-1/#intrinsic-main-sizes
float FlexFormattingContext::calculate_intrinsic_main_size_of_flex_container(LayoutMode layout_mode)
{
    VERIFY(layout_mode != LayoutMode::Normal);

    // The min-content main size of a single-line flex container is calculated identically to the max-content main size,
    // except that the flex items’ min-content contributions are used instead of their max-content contributions.
    // However, for a multi-line container, it is simply the largest min-content contribution of all the non-collapsed flex items in the flex container.
    if (!is_single_line() && layout_mode == LayoutMode::MinContent) {
        float largest_contribution = 0;
        for (auto const& flex_item : m_flex_items) {
            // FIXME: Skip collapsed flex items.
            largest_contribution = max(largest_contribution, calculate_main_min_content_contribution(flex_item));
        }
        return largest_contribution;
    }

    // The max-content main size of a flex container is, fundamentally, the smallest size the flex container
    // can take such that when flex layout is run with that container size, each flex item ends up at least
    // as large as its max-content contribution, to the extent allowed by the items’ flexibility.
    // It is calculated, considering only non-collapsed flex items, by:

    // 1. For each flex item, subtract its outer flex base size from its max-content contribution size.
    //    If that result is positive, divide by its flex grow factor floored at 1;
    //    if negative, divide by its scaled flex shrink factor having floored the flex shrink factor at 1.
    //    This is the item’s max-content flex fraction.
    for (auto& flex_item : m_flex_items) {
        float contribution;
        if (layout_mode == LayoutMode::MinContent)
            contribution = calculate_main_min_content_contribution(flex_item);
        else
            contribution = calculate_main_max_content_contribution(flex_item);

        float flex_fraction = contribution - flex_item.flex_base_size;
        if (flex_fraction >= 0)
            flex_fraction /= max(flex_item.box.computed_values().flex_grow(), 1.0f);
        else
            flex_fraction /= max(flex_item.box.computed_values().flex_shrink(), 1.0f) * flex_item.flex_base_size;

        // FIXME: The name max_content_flex_fraction here is misleading, since we also use this code path for min-content sizing.
        flex_item.max_content_flex_fraction = flex_fraction;
    }

    // 2. Place all flex items into lines of infinite length.
    m_flex_lines.clear();
    if (!m_flex_items.is_empty())
        m_flex_lines.append(FlexLine {});
    for (auto& flex_item : m_flex_items) {
        // FIXME: Honor breaking requests.
        m_flex_lines.last().items.append(&flex_item);
    }

    // 3. Within each line, find the largest max-content flex fraction among all the flex items.
    //    Add each item’s flex base size to the product of its flex grow factor
    //    (or scaled flex shrink factor, if the chosen max-content flex fraction was negative)
    //    and the chosen max-content flex fraction, then clamp that result by the max main size floored by the min main size.
    float largest_sum = 0;
    for (auto& flex_line : m_flex_lines) {
        float largest_flex_fraction = 0;
        for (auto& flex_item : flex_line.items) {
            // FIXME: The name max_content_flex_fraction here is misleading, since we also use this code path for min-content sizing.
            largest_flex_fraction = max(largest_flex_fraction, flex_item->max_content_flex_fraction);
        }

        float sum = 0;
        for (auto& flex_item : flex_line.items) {
            auto product = 0;
            if (flex_item->max_content_flex_fraction >= 0) {
                product = largest_flex_fraction * flex_item->box.computed_values().flex_grow();
            } else {
                product = largest_flex_fraction * max(flex_item->box.computed_values().flex_shrink(), 1.0f) * flex_item->flex_base_size;
            }
            sum += flex_item->flex_base_size + flex_item->margins.main_before + flex_item->margins.main_after + flex_item->borders.main_before + flex_item->borders.main_after + flex_item->padding.main_before + flex_item->padding.main_after + product;
        }

        largest_sum = max(largest_sum, sum);
    }

    // 4. The flex container’s max-content size is the largest sum of the afore-calculated sizes of all items within a single line.
    return largest_sum;
}

// https://drafts.csswg.org/css-flexbox-1/#intrinsic-cross-sizes
float FlexFormattingContext::calculate_intrinsic_cross_size_of_flex_container(LayoutMode layout_mode)
{
    VERIFY(layout_mode != LayoutMode::Normal);

    // The min-content/max-content cross size of a single-line flex container
    // is the largest min-content contribution/max-content contribution (respectively) of its flex items.
    if (is_single_line()) {
        float largest_contribution = 0;
        for (auto& flex_item : m_flex_items) {
            float contribution;
            if (layout_mode == LayoutMode::MinContent)
                contribution = calculate_cross_min_content_contribution(flex_item);
            else if (layout_mode == LayoutMode::MaxContent)
                contribution = calculate_cross_max_content_contribution(flex_item);
            largest_contribution = max(largest_contribution, contribution);
        }
        return largest_contribution;
    }

    // For a multi-line flex container, the min-content/max-content cross size is the sum of the flex line cross sizes
    // resulting from sizing the flex container under a cross-axis min-content constraint/max-content constraint (respectively).
    // FIXME: However, if the flex container is flex-flow: column wrap;, then it’s sized by first finding the largest
    //        min-content/max-content cross-size contribution among the flex items (respectively), then using that size
    //        as the available space in the cross axis for each of the flex items during layout.
    float sum_of_flex_line_cross_sizes = 0;
    for (auto& flex_line : m_flex_lines) {
        sum_of_flex_line_cross_sizes += flex_line.cross_size;
    }
    return sum_of_flex_line_cross_sizes;
}

float FlexFormattingContext::calculate_main_min_content_contribution(FlexItem const& flex_item) const
{
    auto intrinsic_sizes = FormattingContext::calculate_intrinsic_sizes(flex_item.box);
    auto const& box_state = m_state.get(flex_item.box);
    if (is_row_layout())
        return box_state.margin_box_left() + intrinsic_sizes.min_content_size.width() + box_state.margin_box_right();
    return box_state.margin_box_top() + intrinsic_sizes.min_content_size.height() + box_state.margin_box_bottom();
}

float FlexFormattingContext::calculate_main_max_content_contribution(FlexItem const& flex_item) const
{
    auto intrinsic_sizes = FormattingContext::calculate_intrinsic_sizes(flex_item.box);
    auto const& box_state = m_state.get(flex_item.box);
    if (is_row_layout())
        return box_state.margin_box_left() + intrinsic_sizes.max_content_size.width() + box_state.margin_box_right();
    return box_state.margin_box_top() + intrinsic_sizes.max_content_size.height() + box_state.margin_box_bottom();
}

float FlexFormattingContext::calculate_cross_min_content_contribution(FlexItem const& flex_item) const
{
    auto intrinsic_sizes = FormattingContext::calculate_intrinsic_sizes(flex_item.box);
    auto const& box_state = m_state.get(flex_item.box);
    if (is_row_layout())
        return box_state.margin_box_top() + intrinsic_sizes.min_content_size.height() + box_state.margin_box_bottom();
    return box_state.margin_box_left() + intrinsic_sizes.min_content_size.width() + box_state.margin_box_right();
}

float FlexFormattingContext::calculate_cross_max_content_contribution(FlexItem const& flex_item) const
{
    auto intrinsic_sizes = FormattingContext::calculate_intrinsic_sizes(flex_item.box);
    auto const& box_state = m_state.get(flex_item.box);
    if (is_row_layout())
        return box_state.margin_box_top() + intrinsic_sizes.max_content_size.height() + box_state.margin_box_bottom();
    return box_state.margin_box_left() + intrinsic_sizes.max_content_size.width() + box_state.margin_box_right();
}

}