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

/*
 * Copyright (c) 2021, 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/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/TextNode.h>

namespace Web::Layout {

static float get_pixel_size(Box const& box, CSS::Length const& length)
{
    return length.resolved(CSS::Length::make_px(0), box, box.containing_block()->width()).to_px(box);
}

FlexFormattingContext::FlexFormattingContext(Box& flex_container, FormattingContext* parent)
    : FormattingContext(flex_container, parent)
    , m_flex_direction(flex_container.computed_values().flex_direction())
{
}

FlexFormattingContext::~FlexFormattingContext()
{
}

struct DirectionAgnosticMargins {
    float main_before { 0 };
    float main_after { 0 };
    float cross_before { 0 };
    float cross_after { 0 };
};

struct FlexItem {
    Box& box;
    float flex_base_size { 0 };
    float hypothetical_main_size { 0 };
    float hypothetical_cross_size { 0 };
    float hypothetical_cross_size_with_margins() { return hypothetical_cross_size + margins.cross_before + margins.cross_after; }
    float target_main_size { 0 };
    bool frozen { false };
    Optional<float> flex_factor {};
    float scaled_flex_shrink_factor { 0 };
    float max_content_flex_fraction { 0 };
    float main_size { 0 };
    float cross_size { 0 };
    float main_offset { 0 };
    float cross_offset { 0 };
    DirectionAgnosticMargins margins {};
    bool is_min_violation { false };
    bool is_max_violation { false };
};

struct FlexLine {
    Vector<FlexItem*> items;
    float cross_size { 0 };
};

void FlexFormattingContext::run(Box& flex_container, LayoutMode)
{
    // This implements https://www.w3.org/TR/css-flexbox-1/#layout-algorithm

    // FIXME: Implement reverse and ordering.

    bool is_row = is_row_layout();

    // Determine main/cross direction
    auto main_size_is_infinite = false;
    auto layout_for_maximum_main_size = [&](Box& box) {
        bool main_constrained = false;
        if (is_row) {
            if (!box.computed_values().width().is_undefined_or_auto() || !box.computed_values().min_width().is_undefined_or_auto()) {
                main_constrained = true;
            }
        } else {
            if (!box.computed_values().height().is_undefined_or_auto() || !box.computed_values().min_height().is_undefined_or_auto()) {
                main_constrained = true;
            }
        }

        if (!main_constrained && box.children_are_inline()) {
            auto& block_container = verify_cast<BlockContainer>(box);
            BlockFormattingContext bfc(block_container, this);
            bfc.run(box, LayoutMode::Default);
            InlineFormattingContext ifc(block_container, &bfc);

            if (is_row) {
                ifc.run(box, LayoutMode::OnlyRequiredLineBreaks);
                return box.width();
            } else {
                ifc.run(box, LayoutMode::AllPossibleLineBreaks);
                return box.height();
            }
        }
        if (is_row) {
            layout_inside(box, LayoutMode::OnlyRequiredLineBreaks);
            return box.width();
        } else {
            return BlockFormattingContext::compute_theoretical_height(box);
        }
    };
    auto containing_block_effective_main_size = [&is_row, &main_size_is_infinite](Box& box) {
        if (is_row) {
            if (box.containing_block()->has_definite_width())
                return box.containing_block()->width();
            main_size_is_infinite = true;
            return NumericLimits<float>::max();
        } else {
            if (box.containing_block()->has_definite_height())
                return box.containing_block()->height();
            main_size_is_infinite = true;
            return NumericLimits<float>::max();
        }
    };
    auto calculate_hypothetical_cross_size = [&is_row, this](Box& box) {
        bool cross_constrained = false;
        if (is_row) {
            if (!box.computed_values().height().is_undefined_or_auto() || !box.computed_values().min_height().is_undefined_or_auto()) {
                cross_constrained = true;
            }
        } else {
            if (!box.computed_values().width().is_undefined_or_auto() || !box.computed_values().min_width().is_undefined_or_auto()) {
                cross_constrained = true;
            }
        }

        if (!cross_constrained && box.children_are_inline()) {
            auto& block_container = verify_cast<BlockContainer>(box);
            BlockFormattingContext bfc(block_container, this);
            bfc.run(box, LayoutMode::Default);
            InlineFormattingContext ifc(block_container, &bfc);
            ifc.run(box, LayoutMode::OnlyRequiredLineBreaks);

            if (is_row)
                return box.height();
            return box.width();
        }
        if (is_row) {
            return BlockFormattingContext::compute_theoretical_height(box);
        } else {
            BlockFormattingContext context(verify_cast<BlockContainer>(box), this);
            context.compute_width(box);
            return box.width();
        }
    };

    Vector<FlexItem> flex_items;

    // 1. Generate anonymous flex items
    generate_anonymous_flex_items(flex_container, flex_items);

    // 2. Determine the available main and cross space for the flex items
    float main_available_size = 0;
    [[maybe_unused]] float cross_available_size = 0;
    [[maybe_unused]] float main_max_size = NumericLimits<float>::max();
    [[maybe_unused]] 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;

    if (has_definite_main_size(flex_container)) {
        main_is_constrained = true;
        main_available_size = specified_main_size(flex_container);
    } else {
        if (has_main_max_size(flex_container)) {
            main_max_size = specified_main_max_size(flex_container);
            main_available_size = 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_size = available_main_size - 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_size = specified_main_size(*flex_container.containing_block());
                main_is_constrained = true;
            }
        }
    }

    if (has_definite_cross_size(flex_container)) {
        cross_available_size = 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_size = cross_max_size;
    }

    // 3. Determine the flex base size and hypothetical main size of each item
    for (auto& flex_item : flex_items) {
        auto& child_box = flex_item.box;
        auto flex_basis = child_box.computed_values().flex_basis();
        if (flex_basis.type == CSS::FlexBasis::Length) {
            // A
            auto specified_base_size = get_pixel_size(child_box, flex_basis.length);
            if (specified_base_size == 0)
                flex_item.flex_base_size = calculated_main_size(flex_item.box);
            else
                flex_item.flex_base_size = specified_base_size;
        } else if (flex_basis.type == CSS::FlexBasis::Content
            && has_definite_cross_size(child_box)
            // FIXME: && has intrinsic aspect ratio.
            && false) {
            // B
            TODO();
            // flex_base_size is calculated from definite cross size and intrinsic aspect ratio
        } else if (flex_basis.type == CSS::FlexBasis::Content
            // FIXME: && sized under min-content or max-content contstraints
            && false) {
            // C
            TODO();
            // Size child_box under the constraints, flex_base_size is then the resulting main_size.
        } else if (flex_basis.type == CSS::FlexBasis::Content
            // FIXME: && main_size is infinite && inline axis is parallel to the main axis
            && false && false) {
            // D
            TODO();
            // Use rules for a flex_container in orthogonal flow
        } else {
            // E
            // FIXME: This is probably too naive.
            // FIXME: Care about FlexBasis::Auto
            if (has_definite_main_size(child_box)) {
                flex_item.flex_base_size = specified_main_size_of_child_box(flex_container, child_box);
            } else {
                flex_item.flex_base_size = layout_for_maximum_main_size(child_box);
            }
        }
        auto clamp_min = has_main_min_size(child_box)
            ? specified_main_min_size(child_box)
            : 0;
        auto clamp_max = has_main_max_size(child_box)
            ? specified_main_max_size(child_box)
            : NumericLimits<float>::max();

        flex_item.hypothetical_main_size = clamp(flex_item.flex_base_size, clamp_min, clamp_max);
    }

    // 4. Determine the main size of the flex container
    if ((!main_is_constrained && main_size_is_infinite) || main_available_size == 0) {
        // 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 : 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;
            if (max_content_flex_fraction > 0) {
                max_content_flex_fraction /= max(flex_item.box.computed_values().flex_grow_factor().value_or(1), 1.0f);
            } else {
                max_content_flex_fraction /= max(flex_item.box.computed_values().flex_shrink_factor().value_or(1), 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 : 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_factor().value_or(1);
            } else {
                product = largest_max_content_flex_fraction * max(flex_item.box.computed_values().flex_shrink_factor().value_or(1), 1.0f) * flex_item.flex_base_size;
            }
            result += flex_item.flex_base_size + product;
        }
        main_available_size = clamp(result, main_min_size, main_max_size);
    }
    set_main_size(flex_container, main_available_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!
    Vector<FlexLine> flex_lines;
    // FIXME: Also support wrap-reverse
    if (flex_container.computed_values().flex_wrap() == CSS::FlexWrap::Nowrap) {
        FlexLine line;
        for (auto& flex_item : flex_items) {
            line.items.append(&flex_item);
        }
        flex_lines.append(line);
    } else {
        FlexLine line;
        float line_main_size = 0;
        for (auto& flex_item : flex_items) {
            if ((line_main_size + flex_item.hypothetical_main_size) > main_available_size) {
                flex_lines.append(line);
                line = {};
                line_main_size = 0;
            }
            line.items.append(&flex_item);
            line_main_size += flex_item.hypothetical_main_size;
        }
        flex_lines.append(line);
    }

    // 6. Resolve the flexible lengths https://www.w3.org/TR/css-flexbox-1/#resolve-flexible-lengths
    enum FlexFactor {
        FlexGrowFactor,
        FlexShrinkFactor
    };

    FlexFactor used_flex_factor;
    // 6.1. Determine used flex factor
    for (auto& flex_line : 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;
        }
        if (sum_of_hypothetical_main_sizes < main_available_size)
            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_factor();
            else if (used_flex_factor == FlexFactor::FlexShrinkFactor)
                flex_item->flex_factor = flex_item->box.computed_values().flex_shrink_factor();
        }

        // 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;
                else
                    sum_of_items_on_line += flex_item->flex_base_size;
            }
            return main_available_size - 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)
                    : 0;
                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;
        };
    }

    // Cross Size Determination
    // 7. Determine the hypothetical cross size of each item
    for (auto& flex_item : flex_items) {
        flex_item.hypothetical_cross_size = calculate_hypothetical_cross_size(flex_item.box);
    }

    // 8. Calculate the cross size of each flex line.
    if (flex_lines.size() == 1 && has_definite_cross_size(flex_container)) {
        flex_lines[0].cross_size = specified_cross_size(flex_container);
    } else {
        for (auto& flex_line : flex_lines) {
            // FIXME: Implement 8.1

            // 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) / flex_lines.size();
                continue;
            }

            // 8.2
            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();
            }

            // 8.3
            flex_line.cross_size = max(0.0f, largest_hypothetical_cross_size);
        }

        if (flex_lines.size() == 1) {
            clamp(flex_lines[0].cross_size, 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.
    // FIXME: Get the alignment via "align-self" of the item (which accesses "align-items" of the parent if unset)
    for (auto& flex_line : 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;
            }
        }
    }

    // 12. Distribute any remaining free space.
    for (auto& flex_line : 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;
            if (is_main_axis_margin_second_auto(flex_item->box))
                ++auto_margins;
        }
        float remaining_free_space = main_available_size - 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->box, size_per_auto_margin);
                if (is_main_axis_margin_second_auto(flex_item->box))
                    set_main_axis_second_margin(flex_item->box, 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->box, 0);
                if (is_main_axis_margin_second_auto(flex_item->box))
                    set_main_axis_second_margin(flex_item->box, 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 = main_available_size - used_main_space;
            break;
        case CSS::JustifyContent::Center:
            space_before_first_item = (main_available_size - 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;
            main_offset += flex_item->main_size + space_between_items;
        }
    }

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

    // 14. 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 : 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;
                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;
    }

    // 15. Determine the flex container’s used cross size:
    if (has_definite_cross_size(flex_container)) {
        float clamped_cross_size = clamp(specified_cross_size(flex_container), cross_min_size, cross_max_size);
        set_cross_size(flex_container, clamped_cross_size);
    } else {
        float sum_of_flex_lines_cross_sizes = 0;
        for (auto& flex_line : flex_lines) {
            sum_of_flex_lines_cross_sizes += flex_line.cross_size;
        }
        float clamped_cross_size = clamp(sum_of_flex_lines_cross_sizes, cross_min_size, cross_max_size);
        set_cross_size(flex_container, clamped_cross_size);
    }

    // 16. Align all flex lines
    // FIXME: Support align-content
    // FIXME: Support reverse
    for (auto& flex_line : flex_lines) {
        for (auto* flex_item : flex_line.items) {
            set_main_size(flex_item->box, flex_item->main_size);
            set_cross_size(flex_item->box, flex_item->cross_size);
            set_offset(flex_item->box, flex_item->main_offset, flex_item->cross_offset);
        }
    }
}

static void populate_specified_margins(FlexItem& item, CSS::FlexDirection flex_direction)
{
    auto width_of_containing_block = item.box.width_of_logical_containing_block();
    // FIXME: This should also take reverse-ness into account
    if (flex_direction == CSS::FlexDirection::Row || flex_direction == CSS::FlexDirection::RowReverse) {
        item.margins.main_before = item.box.computed_values().margin().left.resolved_or_zero(item.box, width_of_containing_block).to_px(item.box);
        item.margins.main_after = item.box.computed_values().margin().right.resolved_or_zero(item.box, width_of_containing_block).to_px(item.box);
        item.margins.cross_before = item.box.computed_values().margin().top.resolved_or_zero(item.box, width_of_containing_block).to_px(item.box);
        item.margins.cross_after = item.box.computed_values().margin().bottom.resolved_or_zero(item.box, width_of_containing_block).to_px(item.box);
    } else {
        item.margins.main_before = item.box.computed_values().margin().top.resolved_or_zero(item.box, width_of_containing_block).to_px(item.box);
        item.margins.main_after = item.box.computed_values().margin().bottom.resolved_or_zero(item.box, width_of_containing_block).to_px(item.box);
        item.margins.cross_before = item.box.computed_values().margin().left.resolved_or_zero(item.box, width_of_containing_block).to_px(item.box);
        item.margins.cross_after = item.box.computed_values().margin().right.resolved_or_zero(item.box, width_of_containing_block).to_px(item.box);
    }
};

// https://www.w3.org/TR/css-flexbox-1/#flex-items
void FlexFormattingContext::generate_anonymous_flex_items(Box& flex_container, Vector<FlexItem>& 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.
    if (!flex_container.has_definite_width()) {
        flex_container.set_width(flex_container.containing_block()->width());
    } else {
        flex_container.set_width(flex_container.computed_values().width().resolved_or_zero(flex_container, flex_container.containing_block()->width()).to_px(flex_container));
    }

    if (!flex_container.has_definite_height()) {
        flex_container.set_height(flex_container.containing_block()->height());
    } else {
        flex_container.set_height(flex_container.computed_values().height().resolved_or_zero(flex_container, flex_container.containing_block()->height()).to_px(flex_container));
    }

    flex_container.for_each_child_of_type<Box>([&](Box& child_box) {
        layout_inside(child_box, LayoutMode::Default);
        // 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_inclusive_subtree_of_type<TextNode>([&contains_only_white_space](auto& text_node) {
                if (!text_node.text_for_rendering().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);
        flex_items.append(move(flex_item));
        return IterationDecision::Continue;
    });
}

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
{
    return is_row_layout() ? box.width() : box.height();
}

float FlexFormattingContext::specified_cross_size(Box const& box) const
{
    return is_row_layout() ? box.height() : box.width();
}

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 !value.is_undefined_or_auto();
}

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 !value.is_undefined_or_auto();
}

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

bool FlexFormattingContext::cross_size_is_absolute_or_resolved_nicely(NodeWithStyle const& box) const
{
    auto length = is_row_layout() ? box.computed_values().height() : box.computed_values().width();

    if (length.is_absolute() || length.is_relative())
        return true;
    if (length.is_undefined_or_auto())
        return false;

    if (!box.parent())
        return false;
    if (length.is_percentage() && cross_size_is_absolute_or_resolved_nicely(*box.parent()))
        return true;
    return false;
}

float FlexFormattingContext::specified_main_size_of_child_box(Box const& flex_container, 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();
    return value.resolved_or_zero(child_box, 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(box, box.computed_values().min_width())
        : get_pixel_size(box, box.computed_values().min_height());
}

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

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

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

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

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

float FlexFormattingContext::calculated_main_size(Box const& box) const
{
    return is_row_layout() ? box.width() : box.height();
}

bool FlexFormattingContext::is_cross_auto(Box const& box) const
{
    return is_row_layout() ? box.computed_values().height().is_auto() : box.computed_values().width().is_auto();
}

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

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

void FlexFormattingContext::set_main_size(Box& box, float size)
{
    if (is_row_layout())
        box.set_width(size);
    else
        box.set_height(size);
}

void FlexFormattingContext::set_cross_size(Box& box, float size)
{
    if (is_row_layout())
        box.set_height(size);
    else
        box.set_width(size);
}

void FlexFormattingContext::set_offset(Box& box, float main_offset, float cross_offset)
{
    if (is_row_layout())
        box.set_offset(main_offset, cross_offset);
    else
        box.set_offset(cross_offset, main_offset);
}

void FlexFormattingContext::set_main_axis_first_margin(Box& box, float margin)
{
    if (is_row_layout())
        box.box_model().margin.left = margin;
    else
        box.box_model().margin.top = margin;
}

void FlexFormattingContext::set_main_axis_second_margin(Box& box, float margin)
{
    if (is_row_layout())
        box.box_model().margin.right = margin;
    else
        box.box_model().margin.bottom = margin;
}

float FlexFormattingContext::sum_of_margin_padding_border_in_main_axis(Box const& box) const
{
    auto& margin = box.box_model().margin;
    auto& padding = box.box_model().padding;
    auto& border = box.box_model().border;

    if (is_row_layout()) {
        return margin.left + margin.right
            + padding.left + padding.right
            + border.left + border.right;
    } else {
        return margin.top + margin.bottom
            + padding.top + padding.bottom
            + border.top + border.bottom;
    }
}

}