WeasyPrint/weasyprint/layout/inlines.py

# coding: utf8
"""
    weasyprint.layout.inline
    ------------------------

    Line breaking and layout for inline-level boxes.

    :copyright: Copyright 2011-2014 Simon Sapin and contributors, see AUTHORS.
    :license: BSD, see LICENSE for details.

"""

from __future__ import division, unicode_literals

from .absolute import absolute_layout, AbsolutePlaceholder
from .float import avoid_collisions, float_layout
from .replaced import image_marker_layout
from .min_max import handle_min_max_width, handle_min_max_height
from .percentages import resolve_percentages, resolve_one_percentage
from .preferred import (shrink_to_fit, inline_preferred_minimum_width,
                        trailing_whitespace_size)
from .tables import find_in_flow_baseline, table_wrapper_width
from ..text import split_first_line
from ..formatting_structure import boxes
from ..css.computed_values import strut_layout, ex_ratio


def iter_line_boxes(context, box, position_y, skip_stack, containing_block,
                    device_size, absolute_boxes, fixed_boxes):
    """Return an iterator of ``(line, resume_at)``.

    ``line`` is a laid-out LineBox with as much content as possible that
    fits in the available width.

    :param linebox: a non-laid-out :class:`LineBox`
    :param position_y: vertical top position of the line box on the page
    :param skip_stack: ``None`` to start at the beginning of ``linebox``,
                       or a ``resume_at`` value to continue just after an
                       already laid-out line.
    :param containing_block: Containing block of the line box:
                             a :class:`BlockContainerBox`
    :param device_size: ``(width, height)`` of the current page.

    """
    while 1:
        line, resume_at = get_next_linebox(
            context, box, position_y, skip_stack, containing_block,
            device_size, absolute_boxes, fixed_boxes)
        if line:
            position_y = line.position_y + line.height
        if line is None:
            return
        yield line, resume_at
        if resume_at is None:
            return
        skip_stack = resume_at


def get_next_linebox(context, linebox, position_y, skip_stack,
                     containing_block, device_size, absolute_boxes,
                     fixed_boxes):
    """Return ``(line, resume_at)``."""
    resolve_percentages(linebox, containing_block)
    if skip_stack is None:
        # text-indent only at the start of the first line
        # Other percentages (margins, width, ...) do not apply.
        resolve_one_percentage(linebox, 'text_indent', containing_block.width)
    else:
        linebox.text_indent = 0

    skip_stack = skip_first_whitespace(linebox, skip_stack)
    if skip_stack == 'continue':
        return None, None

    linebox.width = inline_preferred_minimum_width(
        context, linebox, skip_stack=skip_stack, first_line=True)

    linebox.height, _ = strut_layout(linebox.style, context.enable_hinting)
    linebox.position_y = position_y
    position_x, position_y, available_width = avoid_collisions(
        context, linebox, containing_block, outer=False)
    candidate_height = linebox.height

    excluded_shapes = context.excluded_shapes[:]

    while 1:
        linebox.position_x = position_x
        linebox.position_y = position_y
        max_x = position_x + available_width
        position_x += linebox.text_indent

        line_placeholders = []
        line_absolutes = []
        line_fixed = []
        waiting_floats = []

        line, resume_at, preserved_line_break = split_inline_box(
            context, linebox, position_x, max_x, skip_stack,
            containing_block, device_size, line_absolutes,
            line_fixed, line_placeholders, waiting_floats)

        if is_phantom_linebox(line) and not preserved_line_break:
            line.height = 0
            break

        remove_last_whitespace(context, line)

        bottom, top = line_box_verticality(line)
        assert top is not None
        assert bottom is not None
        line.baseline = -top
        line.position_y = top
        line.height = bottom - top
        offset_y = position_y - top
        line.margin_top = 0
        line.margin_bottom = 0

        offset_x = text_align(context, line, available_width,
                              last=resume_at is None or preserved_line_break)
        if offset_x != 0 or offset_y != 0:
            line.translate(offset_x, offset_y)

        if line.height <= candidate_height:
            break
        candidate_height = line.height

        new_excluded_shapes = context.excluded_shapes
        context.excluded_shapes = excluded_shapes
        position_x, position_y, available_width = avoid_collisions(
            context, line, containing_block, outer=False)
        if (position_x, position_y) == (
                linebox.position_x, linebox.position_y):
            context.excluded_shapes = new_excluded_shapes
            break

    absolute_boxes.extend(line_absolutes)
    fixed_boxes.extend(line_fixed)

    for placeholder in line_placeholders:
        if placeholder.style._weasy_specified_display.startswith('inline'):
            # Inline-level static position:
            placeholder.translate(0, position_y - placeholder.position_y)
        else:
            # Block-level static position: at the start of the next line
            placeholder.translate(
                line.position_x - placeholder.position_x,
                position_y + line.height - placeholder.position_y)

    float_children = []
    waiting_floats_y = line.position_y + line.height
    for waiting_float in waiting_floats:
        waiting_float.position_y = waiting_floats_y
        waiting_float = float_layout(
            context, waiting_float, containing_block, absolute_boxes,
            fixed_boxes)
        float_children.append(waiting_float)
    if float_children:
        line = line.copy_with_children(
            line.children + tuple(float_children))

    return line, resume_at


def skip_first_whitespace(box, skip_stack):
    """Return the ``skip_stack`` to start just after the remove spaces
    at the beginning of the line.

    See http://www.w3.org/TR/CSS21/text.html#white-space-model
    """
    if skip_stack is None:
        index = 0
        next_skip_stack = None
    else:
        index, next_skip_stack = skip_stack

    if isinstance(box, boxes.TextBox):
        assert next_skip_stack is None
        white_space = box.style.white_space
        length = len(box.text)
        if index == length:
            # Starting a the end of the TextBox, no text to see: Continue
            return 'continue'
        if white_space in ('normal', 'nowrap', 'pre-line'):
            while index < length and box.text[index] == ' ':
                index += 1
        return (index, None) if index else None

    if isinstance(box, (boxes.LineBox, boxes.InlineBox)):
        if index == 0 and not box.children:
            return None
        result = skip_first_whitespace(box.children[index], next_skip_stack)
        if result == 'continue':
            index += 1
            if index >= len(box.children):
                return 'continue'
            result = skip_first_whitespace(box.children[index], None)
        return (index, result) if (index or result) else None

    assert skip_stack is None, 'unexpected skip inside %s' % box
    return None


def remove_last_whitespace(context, box):
    """Remove in place space characters at the end of a line.

    This also reduces the width of the inline parents of the modified text.

    """
    ancestors = []
    while isinstance(box, (boxes.LineBox, boxes.InlineBox)):
        ancestors.append(box)
        if not box.children:
            return
        box = box.children[-1]
    if not (isinstance(box, boxes.TextBox) and
            box.style.white_space in ('normal', 'nowrap', 'pre-line')):
        return
    new_text = box.text.rstrip(' ')
    if new_text:
        if len(new_text) == len(box.text):
            return
        box.text = new_text
        new_box, resume, _ = split_text_box(context, box, None, None, 0)
        assert new_box is not None
        assert resume is None
        space_width = box.width - new_box.width
        box.width = new_box.width
    else:
        space_width = box.width
        box.width = 0
        box.text = ''

    for ancestor in ancestors:
        ancestor.width -= space_width

    # TODO: All tabs (U+0009) are rendered as a horizontal shift that
    # lines up the start edge of the next glyph with the next tab stop.
    # Tab stops occur at points that are multiples of 8 times the width
    # of a space (U+0020) rendered in the block's font from the block's
    # starting content edge.

    # TODO: If spaces (U+0020) or tabs (U+0009) at the end of a line have
    # 'white-space' set to 'pre-wrap', UAs may visually collapse them.


@handle_min_max_width
def replaced_box_width(box, device_size):
    """
    Compute and set the used width for replaced boxes (inline- or block-level)
    """
    # http://www.w3.org/TR/CSS21/visudet.html#inline-replaced-width
    intrinsic_width, intrinsic_height = box.replacement.get_intrinsic_size(
        box.style.image_resolution)
    # TODO: update this when we have replaced elements that do not
    # always have an intrinsic width. (See commented code below.)
    assert intrinsic_width is not None
    assert intrinsic_height is not None

    if box.width == 'auto':
        if box.height == 'auto':
            box.width = intrinsic_width
        else:
            intrinsic_ratio = intrinsic_width / intrinsic_height
            box.width = box.height * intrinsic_ratio


    # Untested code for when we do not always have an intrinsic width.
#    if box.height == 'auto' and box.width == 'auto':
#        if intrinsic_width is not None:
#            box.width = intrinsic_width
#        elif intrinsic_height is not None and intrinsic_ratio is not None:
#            box.width = intrinsic_ratio * intrinsic_height
#        elif box.height != 'auto' and intrinsic_ratio is not None:
#            box.width = intrinsic_ratio * box.height
#        elif intrinsic_ratio is not None:
#            pass
#            # TODO: Intrinsic ratio only: undefined in CSS 2.1.
#            # " It is suggested that, if the containing block's width does not
#            #   itself depend on the replaced element's width, then the used
#            #   value of 'width' is calculated from the constraint equation
#            #   used for block-level, non-replaced elements in normal flow. "

#    # Still no value
#    if box.width == 'auto':
#        if intrinsic_width is not None:
#            box.width = intrinsic_width
#        else:
#            # Then the used value of 'width' becomes 300px. If 300px is too
#            # wide to fit the device, UAs should use the width of the largest
#            # rectangle that has a 2:1 ratio and fits the device instead.
#            device_width, _device_height = device_size
#            box.width = min(300, device_width)


@handle_min_max_height
def replaced_box_height(box, device_size):
    """
    Compute and set the used height for replaced boxes (inline- or block-level)
    """
    # http://www.w3.org/TR/CSS21/visudet.html#inline-replaced-height
    intrinsic_width, intrinsic_height = box.replacement.get_intrinsic_size(
        box.style.image_resolution)
    # TODO: update this when we have replaced elements that do not
    # always have intrinsic dimensions. (See commented code below.)
    assert intrinsic_width is not None
    assert intrinsic_height is not None
    if intrinsic_height == 0:
        # Results in box.height == 0 if used, whatever the used width
        # or intrinsic width.
        intrinsic_ratio = float('inf')
    else:
        intrinsic_ratio = intrinsic_width / intrinsic_height

    # Test 'auto' on the computed width, not the used width
    if box.style.height == 'auto' and box.style.width == 'auto':
        box.height = intrinsic_height
    elif box.style.height == 'auto':
        box.height = box.width / intrinsic_ratio

    # Untested code for when we do not always have intrinsic dimensions.
#    if box.style.height == 'auto' and box.style.width == 'auto':
#        if intrinsic_height is not None:
#            box.height = intrinsic_height
#    elif intrinsic_ratio is not None and box.style.height == 'auto':
#        box.height = box.width / intrinsic_ratio
#    elif box.style.height == 'auto' and intrinsic_height is not None:
#        box.height = intrinsic_height
#    elif box.style.height == 'auto':
#        device_width, _device_height = device_size
#        box.height = min(150, device_width / 2)


def inline_replaced_box_layout(box, device_size):
    """Lay out an inline :class:`boxes.ReplacedBox` ``box``."""
    for side in ['top', 'right', 'bottom', 'left']:
        if getattr(box, 'margin_' + side) == 'auto':
            setattr(box, 'margin_' + side, 0)
    inline_replaced_box_width_height(box, device_size)


def inline_replaced_box_width_height(box, device_size):
    if box.style.width == 'auto' and box.style.height == 'auto':
        replaced_box_width.without_min_max(box, device_size)
        replaced_box_height.without_min_max(box, device_size)
        min_max_auto_replaced(box)
    else:
        replaced_box_width(box, device_size)
        replaced_box_height(box, device_size)


def min_max_auto_replaced(box):
    """Resolve {min,max}-{width,height} constraints on replaced elements
    that have 'auto' width and heights.
    """
    width = box.width
    height = box.height
    min_width = box.min_width
    min_height = box.min_height
    max_width = max(min_width, box.max_width)
    max_height = max(min_height, box.max_height)

    # (violation_width, violation_height)
    violations = (
        'min' if width < min_width else 'max' if width > max_width else '',
        'min' if height < min_height else 'max' if height > max_height else '')

    # Work around divisions by zero. These are pathological cases anyway.
    # TODO: is there a cleaner way?
    if width == 0:
        width = 1e-6
    if height == 0:
        height = 1e-6

    # ('', ''): nothing to do
    if violations == ('max', ''):
        box.width = max_width
        box.height = max(max_width * height / width, min_height)
    elif violations == ('min', ''):
        box.width = min_width
        box.height = min(min_width * height / width, max_height)
    elif violations == ('', 'max'):
        box.width = max(max_height * width / height, min_width)
        box.height = max_height
    elif violations == ('', 'min'):
        box.width = min(min_height * width / height, max_width)
        box.height = min_height
    elif violations == ('max', 'max'):
        if max_width / width <= max_height / height:
            box.width = max_width
            box.height = max(min_height, max_width * height / width)
        else:
            box.width = max(min_width, max_height * width / height)
            box.height = max_height
    elif violations == ('min', 'min'):
        if min_width / width <= min_height / height:
            box.width = min(max_width, min_height * width / height)
            box.height = min_height
        else:
            box.width = min_width
            box.height = min(max_height, min_width * height / width)
    elif violations == ('min', 'max'):
        box.width = min_width
        box.height = max_height
    elif violations == ('max', 'min'):
        box.width = max_width
        box.height = min_height


def atomic_box(context, box, position_x, skip_stack, containing_block,
               device_size, absolute_boxes, fixed_boxes):
    """Compute the width and the height of the atomic ``box``."""
    if isinstance(box, boxes.ReplacedBox):
        if getattr(box, 'is_list_marker', False):
            image_marker_layout(box)
        else:
            inline_replaced_box_layout(box, device_size)
        box.baseline = box.margin_height()
    elif isinstance(box, boxes.InlineBlockBox):
        if box.is_table_wrapper:
            table_wrapper_width(
                context, box,
                (containing_block.width, containing_block.height))
        box = inline_block_box_layout(
            context, box, position_x, skip_stack, containing_block,
            device_size, absolute_boxes, fixed_boxes)
    else:  # pragma: no cover
        raise TypeError('Layout for %s not handled yet' % type(box).__name__)
    return box


def inline_block_box_layout(context, box, position_x, skip_stack,
                            containing_block, device_size, absolute_boxes,
                            fixed_boxes):
    # Avoid a circular import
    from .blocks import block_container_layout

    resolve_percentages(box, containing_block)

    # http://www.w3.org/TR/CSS21/visudet.html#inlineblock-width
    if box.margin_left == 'auto':
        box.margin_left = 0
    if box.margin_right == 'auto':
        box.margin_right = 0

    inline_block_width(box, context, containing_block)

    box.position_x = position_x
    box.position_y = 0
    box, _, _, _, _ = block_container_layout(
        context, box, max_position_y=float('inf'), skip_stack=skip_stack,
        device_size=device_size, page_is_empty=True,
        absolute_boxes=absolute_boxes, fixed_boxes=fixed_boxes)
    box.baseline = inline_block_baseline(box)
    return box


def inline_block_baseline(box):
    """
    Return the y position of the baseline for an inline block
    from the top of its margin box.

    http://www.w3.org/TR/CSS21/visudet.html#propdef-vertical-align

    """
    if box.style.overflow == 'visible':
        result = find_in_flow_baseline(box, last=True)
        if result:
            return result
    return box.position_y + box.margin_height()


@handle_min_max_width
def inline_block_width(box, context, containing_block):
    if box.width == 'auto':
        box.width = shrink_to_fit(context, box, containing_block.width)


def split_inline_level(context, box, position_x, max_x, skip_stack,
                       containing_block, device_size, absolute_boxes,
                       fixed_boxes, line_placeholders, waiting_floats):
    """Fit as much content as possible from an inline-level box in a width.

    Return ``(new_box, resume_at)``. ``resume_at`` is ``None`` if all of the
    content fits. Otherwise it can be passed as a ``skip_stack`` parameter
    to resume where we left off.

    ``new_box`` is non-empty (unless the box is empty) and as big as possible
    while being narrower than ``available_width``, if possible (may overflow
    is no split is possible.)

    """
    resolve_percentages(box, containing_block)
    if isinstance(box, boxes.TextBox):
        box.position_x = position_x
        if skip_stack is None:
            skip = 0
        else:
            skip, skip_stack = skip_stack
            skip = skip or 0
            assert skip_stack is None

        new_box, skip, preserved_line_break = split_text_box(
            context, box, max_x - position_x, max_x, skip)

        if skip is None:
            resume_at = None
        else:
            resume_at = (skip, None)
    elif isinstance(box, boxes.InlineBox):
        if box.margin_left == 'auto':
            box.margin_left = 0
        if box.margin_right == 'auto':
            box.margin_right = 0
        new_box, resume_at, preserved_line_break = split_inline_box(
            context, box, position_x, max_x, skip_stack, containing_block,
            device_size, absolute_boxes, fixed_boxes, line_placeholders,
            waiting_floats)
    elif isinstance(box, boxes.AtomicInlineLevelBox):
        new_box = atomic_box(
            context, box, position_x, skip_stack, containing_block,
            device_size, absolute_boxes, fixed_boxes)
        new_box.position_x = position_x
        resume_at = None
        preserved_line_break = False
    #else: unexpected box type here
    return new_box, resume_at, preserved_line_break


def split_inline_box(context, box, position_x, max_x, skip_stack,
                     containing_block, device_size, absolute_boxes,
                     fixed_boxes, line_placeholders, waiting_floats):
    """Same behavior as split_inline_level."""
    is_start = skip_stack is None
    initial_position_x = position_x
    assert isinstance(box, (boxes.LineBox, boxes.InlineBox))
    left_spacing = (box.padding_left + box.margin_left +
                    box.border_left_width)
    right_spacing = (box.padding_right + box.margin_right +
                     box.border_right_width)
    if is_start:
        position_x += left_spacing
    content_box_left = position_x

    children = []
    preserved_line_break = False

    if box.style.position == 'relative':
        absolute_boxes = []

    if is_start:
        skip = 0
    else:
        skip, skip_stack = skip_stack

    for index, child in box.enumerate_skip(skip):
        child.position_y = box.position_y
        if child.is_absolutely_positioned():
            child.position_x = position_x
            placeholder = AbsolutePlaceholder(child)
            line_placeholders.append(placeholder)
            children.append(placeholder)
            if child.style.position == 'absolute':
                absolute_boxes.append(placeholder)
            else:
                fixed_boxes.append(placeholder)
            continue
        elif child.is_floated():
            child.position_x = position_x
            float_width = shrink_to_fit(
                context, child, containing_block.width)

            # To retrieve the real available space for floats, we must remove
            # the trailing whitespaces from the line
            non_floating_children = [
                child_ for child_ in children if not child_.is_floated()]
            if non_floating_children:
                float_width -= trailing_whitespace_size(
                    context, non_floating_children[-1])

            if float_width > max_x - position_x or waiting_floats:
                # TODO: the absolute and fixed boxes in the floats must be
                # added here, and not in iter_line_boxes
                waiting_floats.append(child)
            else:
                child = float_layout(
                    context, child, containing_block, absolute_boxes,
                    fixed_boxes)
                children.append(child)
                # TODO: use the main text direction of the line
                for old_child in children[:index]:
                    if not old_child.is_in_normal_flow():
                        continue
                    if child.style.float == 'left':  # and direction is ltr
                        old_child.translate(dx=child.margin_width())
                    # elif child.style.float == 'right' and direction is rtl:
                    #    old_child.translate(dx=-child.margin_width())
                if child.style.float == 'left':
                    position_x += child.margin_width()
                elif child.style.float == 'right':
                    max_x -= child.margin_width()
            continue

        new_child, resume_at, preserved = split_inline_level(
            context, child, position_x, max_x, skip_stack, containing_block,
            device_size, absolute_boxes, fixed_boxes, line_placeholders,
            waiting_floats)
        skip_stack = None
        if preserved:
            preserved_line_break = True

        # TODO: this is non-optimal when last_child is True and
        #   width <= remaining_width < width + right_spacing
        # with
        #   width = part1.margin_width()

        # TODO: on the last child, take care of right_spacing

        if new_child is None:
            # may be None where we would have an empty TextBox
            assert isinstance(child, boxes.TextBox)
        else:
            margin_width = new_child.margin_width()
            new_position_x = position_x + margin_width

            if (new_position_x > max_x and children):
                # too wide, and the inline is non-empty:
                # put child entirely on the next line.
                resume_at = (index, None)
                break
            else:
                position_x = new_position_x
                children.append(new_child)

        if resume_at is not None:
            resume_at = (index, resume_at)
            break
    else:
        resume_at = None

    new_box = box.copy_with_children(
        children, is_start=is_start, is_end=resume_at is None)
    if isinstance(box, boxes.LineBox):
        # Line boxes already have a position_x which may not be the same
        # as content_box_left when text-indent is non-zero.
        # This is important for justified text.
        new_box.width = position_x - new_box.position_x
    else:
        new_box.position_x = initial_position_x
        new_box.width = position_x - content_box_left

    line_height, new_box.baseline = strut_layout(
        box.style, context.enable_hinting)
    new_box.height = box.style.font_size
    half_leading = (line_height - new_box.height) / 2.
    # Set margins to the half leading but also compensate for borders and
    # paddings. We want margin_height() == line_height
    new_box.margin_top = (half_leading - new_box.border_top_width -
                          new_box.padding_top)
    new_box.margin_bottom = (half_leading - new_box.border_bottom_width -
                             new_box.padding_bottom)

    if new_box.style.position == 'relative':
        for absolute_box in absolute_boxes:
            absolute_layout(context, absolute_box, new_box, fixed_boxes)
    return new_box, resume_at, preserved_line_break


def split_text_box(context, box, available_width, line_width, skip):
    """Keep as much text as possible from a TextBox in a limitied width.
    Try not to overflow but always have some text in ``new_box``

    Return ``(new_box, skip)``. ``skip`` is the number of UTF-8 bytes
    to skip form the start of the TextBox for the next line, or ``None``
    if all of the text fits.

    Also break an preserved whitespace.

    """
    assert isinstance(box, boxes.TextBox)
    font_size = box.style.font_size
    text = box.text[skip:]
    if font_size == 0 or not text:
        return None, None, False
    # XXX ``resume_at`` is an index in UTF-8 bytes, not unicode codepoints.
    layout, length, resume_at, width, height, baseline = split_first_line(
        text, box.style, context.enable_hinting, available_width, line_width)

    # Convert ``length`` and ``resume_at`` from UTF-8 indexes in text
    # to Unicode indexes.
    # No need to encode what’s after resume_at (if set) or length (if
    # resume_at is not set). One code point is one or more byte, so
    # UTF-8 indexes are always bigger or equal to Unicode indexes.
    partial_text = text[:resume_at or length]
    utf8_text = partial_text.encode('utf8')
    new_text = utf8_text[:length].decode('utf8')
    new_length = len(new_text)
    if resume_at is not None:
        if length > resume_at:
            # Text has been hyphenated
            new_text += box.style.hyphenate_character
            between = ''
        else:
            between = utf8_text[length:resume_at].decode('utf8')
            if width > available_width:
                if between.strip(' ') not in ('', '\n', '\u2029'):
                    # Replace bad cutting value from Pango
                    between = utf8_text[length:new_length].decode('utf8')
            resume_at = new_length + len(between)
    length = new_length

    if length > 0:
        box = box.copy_with_text(new_text)
        box.width = width
        box.pango_layout = layout
        # "The height of the content area should be based on the font,
        #  but this specification does not specify how."
        # http://www.w3.org/TR/CSS21/visudet.html#inline-non-replaced
        # We trust Pango and use the height of the LayoutLine.
        box.height = height
        # "only the 'line-height' is used when calculating the height
        #  of the line box."
        # Set margins so that margin_height() == line_height
        line_height, _ = strut_layout(box.style, context.enable_hinting)
        half_leading = (line_height - height) / 2.
        box.margin_top = half_leading
        box.margin_bottom = half_leading
        # form the top of the content box
        box.baseline = baseline
        # form the top of the margin box
        box.baseline += box.margin_top
    else:
        box = None

    if resume_at is None:
        preserved_line_break = False
    else:
        preserved_line_break = (length != resume_at) and between.strip(' ')
        if preserved_line_break:
            # See http://unicode.org/reports/tr14/
            # TODO: are there others? Find Pango docs on this
            # The space is in this list, as it may have been removed by the
            # Step #2 of split_first_line
            assert between in (' ', '\n', '\u2029'), (
                'Got %r between two lines. '
                'Expected nothing or a preserved line break' % (between,))
        resume_at += skip

    return box, resume_at, preserved_line_break


def line_box_verticality(box):
    """Handle ``vertical-align`` within an :class:`LineBox` (or of a
    non-align sub-tree).

    Place all boxes vertically assuming that the baseline of ``box``
    is at `y = 0`.

    Return ``(max_y, min_y)``, the maximum and minimum vertical position
    of margin boxes.

    """
    top_bottom_subtrees = []
    subtrees_with_min_max = []
    max_y, min_y = aligned_subtree_verticality(
        box, top_bottom_subtrees, baseline_y=0)
    for subtree in top_bottom_subtrees:
        if subtree.is_floated():
            sub_min_y = None
            sub_max_y = None
        else:
            sub_max_y, sub_min_y = aligned_subtree_verticality(
                subtree, top_bottom_subtrees, baseline_y=0)
        subtrees_with_min_max.append(
            (subtree, sub_max_y, sub_min_y))

    if subtrees_with_min_max:
        sub_positions = [
            sub_max_y - sub_min_y
            for subtree, sub_max_y, sub_min_y in subtrees_with_min_max
            if not subtree.is_floated()]
        if sub_positions:
            highest_sub = max(sub_positions)
            max_y = max(max_y, min_y + highest_sub)

    for subtree, sub_max_y, sub_min_y in subtrees_with_min_max:
        if subtree.is_floated():
            dy = min_y - subtree.position_y
        elif subtree.style.vertical_align == 'top':
            dy = min_y - sub_min_y
        else:
            assert subtree.style.vertical_align == 'bottom'
            dy = max_y - sub_max_y
        translate_subtree(subtree, dy)
    return max_y, min_y


def translate_subtree(box, dy):
    if isinstance(box, boxes.InlineBox):
        box.position_y += dy
        if box.style.vertical_align in ('top', 'bottom'):
            for child in box.children:
                translate_subtree(child, dy)
    else:
        # Text or atomic boxes
        box.translate(dy=dy)


def aligned_subtree_verticality(box, top_bottom_subtrees, baseline_y):
    max_y, min_y = inline_box_verticality(box, top_bottom_subtrees, baseline_y)
    # Account for the line box itself:
    top = baseline_y - box.baseline
    bottom = top + box.margin_height()
    if min_y is None or top < min_y:
        min_y = top
    if max_y is None or bottom > max_y:
        max_y = bottom

    return max_y, min_y


def inline_box_verticality(box, top_bottom_subtrees, baseline_y):
    """Handle ``vertical-align`` within an :class:`InlineBox`.

    Place all boxes vertically assuming that the baseline of ``box``
    is at `y = baseline_y`.

    Return ``(max_y, min_y)``, the maximum and minimum vertical position
    of margin boxes.

    """
    max_y = None
    min_y = None
    if not isinstance(box, (boxes.LineBox, boxes.InlineBox)):
        return max_y, min_y

    for child in box.children:
        if not child.is_in_normal_flow():
            if child.is_floated():
                top_bottom_subtrees.append(child)
            continue
        vertical_align = child.style.vertical_align
        if vertical_align == 'baseline':
            child_baseline_y = baseline_y
        elif vertical_align == 'middle':
            one_ex = box.style.font_size * ex_ratio(box.style)
            top = baseline_y - (one_ex + child.margin_height()) / 2.
            child_baseline_y = top + child.baseline
        # TODO: actually implement vertical-align: top and bottom
        elif vertical_align == 'text-top':
            # align top with the top of the parent’s content area
            top = (baseline_y - box.baseline + box.margin_top +
                   box.border_top_width + box.padding_top)
            child_baseline_y = top + child.baseline
        elif vertical_align == 'text-bottom':
            # align bottom with the bottom of the parent’s content area
            bottom = (baseline_y - box.baseline + box.margin_top +
                      box.border_top_width + box.padding_top + box.height)
            child_baseline_y = bottom - child.margin_height() + child.baseline
        elif vertical_align in ('top', 'bottom'):
            # Later, we will assume for this subtree that its baseline
            # is at y=0.
            child_baseline_y = 0
        else:
            # Numeric value: The child’s baseline is `vertical_align` above
            # (lower y) the parent’s baseline.
            child_baseline_y = baseline_y - vertical_align

        # the child’s `top` is `child.baseline` above (lower y) its baseline.
        top = child_baseline_y - child.baseline
        if isinstance(child, boxes.InlineBlockBox):
            # This also includes table wrappers for inline tables.
            child.translate(dy=top - child.position_y)
        else:
            child.position_y = top
            # grand-children for inline boxes are handled below

        if vertical_align in ('top', 'bottom'):
            # top or bottom are special, they need to be handled in
            # a later pass.
            top_bottom_subtrees.append(child)
            continue

        bottom = top + child.margin_height()
        if min_y is None or top < min_y:
            min_y = top
        if max_y is None or bottom > max_y:
            max_y = bottom
        if isinstance(child, boxes.InlineBox):
            children_max_y, children_min_y = inline_box_verticality(
                child, top_bottom_subtrees, child_baseline_y)
            if children_min_y is not None and children_min_y < min_y:
                min_y = children_min_y
            if children_max_y is not None and children_max_y > max_y:
                max_y = children_max_y
    return max_y, min_y


def text_align(context, line, available_width, last):
    """Return how much the line should be moved horizontally according to
    the `text-align` property.

    """
    align = line.style.text_align
    if align in ('-weasy-start', '-weasy-end'):
        if (align == '-weasy-start') ^ (line.style.direction == 'rtl'):
            align = 'left'
        else:
            align = 'right'
    if align == 'justify' and last:
        align = 'right' if line.style.direction == 'rtl' else 'left'
    if align == 'left':
        return 0
    offset = available_width - line.width
    if align == 'justify':
        justify_line(context, line, offset)
        return 0
    if align == 'center':
        offset /= 2.
    else:
        assert align == 'right'
    return offset


def justify_line(context, line, extra_width):
    nb_spaces = count_spaces(line)
    if nb_spaces == 0:
        # TODO: what should we do with single-word lines?
        return
    add_word_spacing(context, line, extra_width / nb_spaces, 0)


def count_spaces(box):
    if isinstance(box, boxes.TextBox):
        # TODO: remove trailing spaces correctly
        return box.text.count(' ')
    elif isinstance(box, (boxes.LineBox, boxes.InlineBox)):
        return sum(count_spaces(child) for child in box.children)
    else:
        return 0


def add_word_spacing(context, box, extra_word_spacing, x_advance):
    if isinstance(box, boxes.TextBox):
        box.position_x += x_advance
        box.style.word_spacing += extra_word_spacing
        nb_spaces = count_spaces(box)
        if nb_spaces > 0:
            new_box, resume_at, _ = split_text_box(
                context, box, 1e10, None, 0)
            assert new_box is not None
            assert resume_at is None
            # XXX new_box.width - box.width is always 0???
            #x_advance +=  new_box.width - box.width
            x_advance += extra_word_spacing * nb_spaces
            box.width = new_box.width
            box.pango_layout = new_box.pango_layout
    elif isinstance(box, (boxes.LineBox, boxes.InlineBox)):
        box.position_x += x_advance
        previous_x_advance = x_advance
        for child in box.children:
            x_advance = add_word_spacing(
                context, child, extra_word_spacing, x_advance)
        box.width += x_advance - previous_x_advance
    else:
        # Atomic inline-level box
        box.translate(x_advance, 0)
    return x_advance


def is_phantom_linebox(linebox):
    """http://www.w3.org/TR/CSS21/visuren.html#phantom-line-box"""
    for child in linebox.children:
        if isinstance(child, boxes.InlineBox):
            if not is_phantom_linebox(child):
                return False
            for side in ('top', 'right', 'bottom', 'left'):
                if (getattr(child.style['margin_%s' % side], 'value', None) or
                        child.style['border_%s_width' % side] or
                        child.style['padding_%s' % side].value):
                    return False
        elif child.is_in_normal_flow():
            return False
    return True