def columns_layout(context, box, bottom_space, skip_stack, containing_block,
                   page_is_empty, absolute_boxes, fixed_boxes,
                   adjoining_margins):
    """Lay out a multi-column ``box``."""
    from .block import (  # isort:skip
        block_box_layout, block_level_layout, block_level_width,
        collapse_margin, remove_placeholders)

    style = box.style
    width = style['column_width']
    count = style['column_count']
    height = style['height']
    original_bottom_space = bottom_space
    context.in_column = True

    if style['position'] == 'relative':
        # New containing block, use a new absolute list
        absolute_boxes = []

    box = box.copy_with_children(box.children)
    box.position_y += collapse_margin(adjoining_margins) - box.margin_top

    # Set height if defined
    if height != 'auto' and height.unit != '%':
        assert height.unit == 'px'
        height_defined = True
        empty_space = context.page_bottom - box.content_box_y() - height.value
        bottom_space = max(bottom_space, empty_space)
    else:
        height_defined = False

    # TODO: the columns container width can be unknown if the containing block
    # needs the size of this block to know its own size
    block_level_width(box, containing_block)

    if style['column_gap'] == 'normal':
        # 1em because in column context
        gap = style['font_size']
    else:
        gap = percentage(style['column_gap'], box.width)

    # Define the number of columns and their widths
    if width == 'auto' and count != 'auto':
        width = max(0, box.width - (count - 1) * gap) / count
    elif width != 'auto' and count == 'auto':
        count = max(1, int(floor((box.width + gap) / (width + gap))))
        width = (box.width + gap) / count - gap
    else:  # overconstrained, with width != 'auto' and count != 'auto'
        count = max(
            1, min(count, int(floor((box.width + gap) / (width + gap)))))
        width = (box.width + gap) / count - gap

    # Handle column-span property with the following structure:
    # columns_and_blocks = [
    #     [column_child_1, column_child_2],
    #     spanning_block,
    #     …
    # ]
    columns_and_blocks = []
    column_children = []
    skip, = skip_stack.keys() if skip_stack else (0,)
    for i, child in enumerate(box.children[skip:], start=skip):
        if child.style['column_span'] == 'all':
            if column_children:
                columns_and_blocks.append(
                    (i - len(column_children), column_children))
            columns_and_blocks.append((i, child.copy()))
            column_children = []
            continue
        column_children.append(child.copy())
    if column_children:
        columns_and_blocks.append(
            (i + 1 - len(column_children), column_children))

    if skip_stack:
        skip_stack = {0: skip_stack[skip]}

    if not box.children:
        next_page = {'break': 'any', 'page': None}
        skip_stack = None

    # Find height and balance.
    #
    # The current algorithm starts from the total available height, to check
    # whether the whole content can fit. If it doesn’t fit, we keep the partial
    # rendering. If it fits, we try to balance the columns starting from the
    # ideal height (the total height divided by the number of columns). We then
    # iterate until the last column is not the highest one. At the end of each
    # loop, we add the minimal height needed to make one direct child at the
    # top of one column go to the end of the previous column.
    #
    # We rely on a real rendering for each loop, and with a stupid algorithm
    # like this it can last minutes…

    adjoining_margins = []
    current_position_y = box.content_box_y()
    new_children = []
    column_skip_stack = None
    last_loop = False
    break_page = False
    footnote_area_heights = [
        0 if context.current_footnote_area.height == 'auto'
        else context.current_footnote_area.margin_height()]
    last_footnotes_height = 0
    for index, column_children_or_block in columns_and_blocks:
        if not isinstance(column_children_or_block, list):
            # We have a spanning block, we display it like other blocks
            block = column_children_or_block
            resolve_percentages(block, containing_block)
            block.position_x = box.content_box_x()
            block.position_y = current_position_y
            new_child, resume_at, next_page, adjoining_margins, _, _ = (
                block_level_layout(
                    context, block, original_bottom_space, skip_stack,
                    containing_block, page_is_empty, absolute_boxes,
                    fixed_boxes, adjoining_margins))
            skip_stack = None
            if new_child is None:
                last_loop = True
                break_page = True
                break
            new_children.append(new_child)
            current_position_y = (
                new_child.border_height() + new_child.border_box_y())
            adjoining_margins.append(new_child.margin_bottom)
            if resume_at:
                last_loop = True
                break_page = True
                column_skip_stack = resume_at
                break
            page_is_empty = False
            continue

        # We have a list of children that we have to balance between columns
        column_children = column_children_or_block

        # Find the total height available for the first run
        current_position_y += collapse_margin(adjoining_margins)
        adjoining_margins = []
        column_box = _create_column_box(
            box, containing_block, column_children, width, current_position_y)
        height = max_height = (
            context.page_bottom - current_position_y - original_bottom_space)

        # Try to render columns until the content fits, increase the column
        # height step by step
        column_skip_stack = skip_stack
        lost_space = inf
        original_excluded_shapes = context.excluded_shapes[:]
        original_page_is_empty = page_is_empty
        page_is_empty = stop_rendering = balancing = False
        while True:
            # Remove extra excluded shapes introduced during the previous loop
            while len(context.excluded_shapes) > len(original_excluded_shapes):
                context.excluded_shapes.pop()

            # Render the columns
            column_skip_stack = skip_stack
            consumed_heights = []
            new_boxes = []
            for i in range(count):
                # Render one column
                new_box, resume_at, next_page, _, _, _ = block_box_layout(
                    context, column_box,
                    context.page_bottom - current_position_y - height,
                    column_skip_stack, containing_block,
                    page_is_empty or not balancing, [], [], [],
                    discard=False, max_lines=None)
                if new_box is None:
                    # We didn't render anything, retry
                    column_skip_stack = {0: None}
                    break
                new_boxes.append(new_box)
                column_skip_stack = resume_at

                # Calculate consumed height, empty space and next box height
                in_flow_children = [
                    child for child in new_box.children
                    if child.is_in_normal_flow()]
                if in_flow_children:
                    # Get the empty space at the bottom of the column box
                    consumed_height = (
                        in_flow_children[-1].margin_height() +
                        in_flow_children[-1].position_y - current_position_y)
                    empty_space = height - consumed_height

                    # Get the minimum size needed to render the next box
                    if column_skip_stack:
                        next_box = block_box_layout(
                            context, column_box, inf, column_skip_stack,
                            containing_block, True, [], [], [],
                            discard=False, max_lines=None)[0]
                        for child in next_box.children:
                            if child.is_in_normal_flow():
                                next_box_height = child.margin_height()
                                break
                        remove_placeholders(context, [next_box], [], [])
                    else:
                        next_box_height = 0
                else:
                    consumed_height = empty_space = next_box_height = 0

                consumed_heights.append(consumed_height)

                # Append the size needed to render the next box in this
                # column.
                #
                # The next box size may be smaller than the empty space, for
                # example when the next box can't be separated from its own
                # next box. In this case we don't try to find the real value
                # and let the workaround below fix this for us.
                #
                # We also want to avoid very small values that may have been
                # introduced by rounding errors. As the workaround below at
                # least adds 1 pixel for each loop, we can ignore lost spaces
                # lower than 1px.
                if next_box_height - empty_space > 1:
                    lost_space = min(lost_space, next_box_height - empty_space)

                # Stop if we already rendered the whole content
                if resume_at is None:
                    break

            # Remove placeholders but keep the current footnote area height
            last_footnotes_height = (
                0 if context.current_footnote_area.height == 'auto'
                else context.current_footnote_area.margin_height())
            remove_placeholders(context, new_boxes, [], [])

            if last_loop:
                break

            if balancing:
                if column_skip_stack is None:
                    # We rendered the whole content, stop
                    break

                # Increase the column heights and render them again
                add_height = 1 if lost_space == inf else lost_space
                height += add_height

                if height > max_height:
                    # We reached max height, stop rendering
                    height = max_height
                    stop_rendering = True
                    break
            else:
                if last_footnotes_height not in footnote_area_heights:
                    # Footnotes have been rendered, try to re-render with the
                    # new footnote area height
                    height -= last_footnotes_height - footnote_area_heights[-1]
                    footnote_area_heights.append(last_footnotes_height)
                    continue

                everything_fits = (
                    not column_skip_stack and
                    max(consumed_heights) <= max_height)
                if everything_fits:
                    # Everything fits, start expanding columns at the average
                    # of the column heights
                    max_height -= last_footnotes_height
                    if (style['column_fill'] == 'balance' or
                            index < columns_and_blocks[-1][0]):
                        balancing = True
                        height = sum(consumed_heights) / count
                    else:
                        break
                else:
                    # Content overflows even at maximum height, stop now and
                    # let the columns continue on the next page
                    height += footnote_area_heights[-1]
                    if len(footnote_area_heights) > 2:
                        last_footnotes_height = min(
                            last_footnotes_height, footnote_area_heights[-1])
                    height -= last_footnotes_height
                    stop_rendering = True
                    break

        # TODO: check style['max']-height
        bottom_space = max(
            bottom_space, context.page_bottom - current_position_y - height)

        # Replace the current box children with real columns
        i = 0
        max_column_height = 0
        columns = []
        while True:
            column_box = _create_column_box(
                box, containing_block, column_children, width,
                current_position_y)
            if style['direction'] == 'rtl':
                column_box.position_x += box.width - (i + 1) * width - i * gap
            else:
                column_box.position_x += i * (width + gap)
            new_child, column_skip_stack, column_next_page, _, _, _ = (
                block_box_layout(
                    context, column_box, bottom_space, skip_stack,
                    containing_block, original_page_is_empty, absolute_boxes,
                    fixed_boxes, None, discard=False, max_lines=None))
            if new_child is None:
                columns = []
                break_page = True
                break
            next_page = column_next_page
            skip_stack = column_skip_stack
            columns.append(new_child)
            max_column_height = max(
                max_column_height, new_child.margin_height())
            if skip_stack is None:
                bottom_space = original_bottom_space
                break
            i += 1
            if i == count and not height_defined:
                # [If] a declaration that constrains the column height
                # (e.g., using height or max-height). In this case,
                # additional column boxes are created in the inline
                # direction.
                break

        # Update the current y position and set the columns’ height
        current_position_y += min(max_height, max_column_height)
        for column in columns:
            column.height = max_column_height
            new_children.append(column)

        skip_stack = None
        page_is_empty = False

        if stop_rendering:
            break

    # Report footnotes above the defined footnotes height
    _report_footnotes(context, last_footnotes_height)

    if box.children and not new_children:
        # The box has children but none can be drawn, let's skip the whole box
        context.in_column = False
        return None, (0, None), {'break': 'any', 'page': None}, [], False

    # Set the height of the containing box
    box.children = new_children
    current_position_y += collapse_margin(adjoining_margins)
    height = current_position_y - box.content_box_y()
    if box.height == 'auto':
        box.height = height
        height_difference = 0
    else:
        height_difference = box.height - height

    # Update the latest columns’ height to respect min-height
    if box.min_height != 'auto' and box.min_height > box.height:
        height_difference += box.min_height - box.height
        box.height = box.min_height
    for child in new_children[::-1]:
        if child.is_column:
            child.height += height_difference
        else:
            break

    if style['position'] == 'relative':
        # New containing block, resolve the layout of the absolute descendants
        for absolute_box in absolute_boxes:
            absolute_layout(
                context, absolute_box, box, fixed_boxes, bottom_space,
                skip_stack=None)

    # Calculate skip stack
    if column_skip_stack:
        skip, = column_skip_stack.keys()
        skip_stack = {index + skip: column_skip_stack[skip]}
    elif break_page:
        skip_stack = {index: None}

    # Update page bottom according to the new footnotes
    if context.current_footnote_area.height != 'auto':
        context.page_bottom += footnote_area_heights[0]
        context.page_bottom -= context.current_footnote_area.margin_height()

    context.in_column = False
    return box, skip_stack, next_page, [], False
Lay out a multi-column ``box``.
weasyprint/layout/column.py