WeasyPrint/weasyprint/css/computed_values.py

# coding: utf8
"""
    weasyprint.css.computed_values
    ------------------------------

    Convert *specified* property values (the result of the cascade and
    inhertance) into *computed* values (that are inherited).

    :copyright: Copyright 2011-2012 Simon Sapin and contributors, see AUTHORS.
    :license: BSD, see LICENSE for details.

"""

from __future__ import division, unicode_literals

import math

from .properties import INITIAL_VALUES, Dimension
from ..utils import get_url_attribute


ZERO_PIXELS = Dimension(0, 'px')


# How many CSS pixels is one <unit>?
# http://www.w3.org/TR/CSS21/syndata.html#length-units
LENGTHS_TO_PIXELS = {
    'px': 1,
    'pt': 1. / 0.75,
    'pc': 16.,  # LENGTHS_TO_PIXELS['pt'] * 12
    'in': 96.,  # LENGTHS_TO_PIXELS['pt'] * 72
    'cm': 96. / 2.54,  # LENGTHS_TO_PIXELS['in'] / 2.54
    'mm': 96. / 25.4,  # LENGTHS_TO_PIXELS['in'] / 25.4
}

# http://dev.w3.org/csswg/css3-values/#angles
# How many radians is one <unit>?
ANGLE_TO_RADIANS = {
    'rad': 1,
    'turn': 2 * math.pi,
    'deg': math.pi / 180,
    'grad': math.pi / 200,
}

# Value in pixels of font-size for <absolute-size> keywords: 12pt (16px) for
# medium, and scaling factors given in CSS3 for others:
# http://www.w3.org/TR/css3-fonts/#font-size-prop
# TODO: this will need to be ordered to implement 'smaller' and 'larger'
FONT_SIZE_KEYWORDS = dict(
    # medium is 16px, others are a ratio of medium
    (name, INITIAL_VALUES['font_size'] * a / b)
    for name, a, b in [
        ('xx-small', 3, 5),
        ('x-small', 3, 4),
        ('small', 8, 9),
        ('medium', 1, 1),
        ('large', 6, 5),
        ('x-large', 3, 2),
        ('xx-large', 2, 1),
    ]
)

# These are unspecified, other than 'thin' <='medium' <= 'thick'.
# Values are in pixels.
BORDER_WIDTH_KEYWORDS = {
    'thin': 1,
    'medium': 3,
    'thick': 5,
}
assert INITIAL_VALUES['border_top_width'] == BORDER_WIDTH_KEYWORDS['medium']

# http://www.w3.org/TR/CSS21/fonts.html#propdef-font-weight
FONT_WEIGHT_RELATIVE = dict(
    bolder={
        100: 400,
        200: 400,
        300: 400,
        400: 700,
        500: 700,
        600: 900,
        700: 900,
        800: 900,
        900: 900,
    },
    lighter={
        100: 100,
        200: 100,
        300: 100,
        400: 100,
        500: 100,
        600: 400,
        700: 400,
        800: 700,
        900: 700,
    },
)

# http://www.w3.org/TR/css3-page/#size
# name=(width in pixels, height in pixels)
PAGE_SIZES = dict(
    a5=(
        Dimension(148, 'mm'),
        Dimension(210, 'mm'),
    ),
    a4=(
        Dimension(210, 'mm'),
        Dimension(297, 'mm'),
    ),
    a3=(
        Dimension(297, 'mm'),
        Dimension(420, 'mm'),
    ),
    b5=(
        Dimension(176, 'mm'),
        Dimension(250, 'mm'),
    ),
    b4=(
        Dimension(250, 'mm'),
        Dimension(353, 'mm'),
    ),
    letter=(
        Dimension(8.5, 'in'),
        Dimension(11, 'in'),
    ),
    legal=(
        Dimension(8.5, 'in'),
        Dimension(14, 'in'),
    ),
    ledger=(
        Dimension(11, 'in'),
        Dimension(17, 'in'),
    ),
)
# In "portrait" orientation.
for w, h in PAGE_SIZES.values():
    assert w.value < h.value

INITIAL_PAGE_SIZE = PAGE_SIZES['a4']
INITIAL_VALUES['size'] = tuple(
    d.value * LENGTHS_TO_PIXELS[d.unit] for d in INITIAL_PAGE_SIZE)


def _computing_order():
    """Some computed values are required by others, so order matters."""
    first = ['font_size', 'line_height', 'color']
    order = sorted(INITIAL_VALUES)
    for name in first:
        order.remove(name)
    return tuple(first + order)
COMPUTING_ORDER = _computing_order()

# Maps property names to functions returning the computed values
COMPUTER_FUNCTIONS = {}


def register_computer(name):
    """Decorator registering a property ``name`` for a function."""
    name = name.replace('-', '_')
    def decorator(function):
        """Register the property ``name`` for ``function``."""
        COMPUTER_FUNCTIONS[name] = function
        return function
    return decorator


def compute(element, pseudo_type, specified, computed, parent_style):
    """
    Return a StyleDict of computed values.

    :param element: The HTML element these style apply to
    :param pseudo_type: The type of pseudo-element, eg 'before', None
    :param specified: a :class:`StyleDict` of specified values. Should contain
                      values for all properties.
    :param computed: a :class:`StyleDict` of already known computed values.
                     Only contains some properties (or none).
    :param parent_values: a :class:`StyleDict` of computed values of the parent
                          element (should contain values for all properties),
                          or ``None`` if ``element`` is the root element.
    """
    if parent_style is None:
        parent_style = INITIAL_VALUES

    computer = lambda: 0  # Dummy object that holds attributes
    computer.element = element
    computer.pseudo_type = pseudo_type
    computer.specified = specified
    computer.computed = computed
    computer.parent_style = parent_style

    getter = COMPUTER_FUNCTIONS.get

    for name in COMPUTING_ORDER:
        if name in computed:
            # Already computed
            continue

        value = specified[name]
        function = getter(name)
        if function is not None:
            value = function(computer, name, value)
        # else: same as specified

        assert value is not None
        computed[name] = value

    return computed


# Let's be consistent, always use ``name`` as an argument even when
# it is useless.
# pylint: disable=W0613

@register_computer('background-color')
@register_computer('border-top-color')
@register_computer('border-right-color')
@register_computer('border-bottom-color')
@register_computer('border-left-color')
def other_color(computer, name, value):
    """Compute the ``*-color`` properties."""
    if value == 'currentColor':
        return computer.computed.color
    else:
        # As specified
        return value


@register_computer('background-position')
@register_computer('transform-origin')
def length_or_percentage_tuple(computer, name, values):
    """Compute the lists of lengths that can be percentages."""
    return tuple(length(computer, name, value) for value in values)


@register_computer('border-spacing')
@register_computer('size')
@register_computer('clip')
def length_tuple(computer, name, values):
    """Compute the properties with a list of lengths."""
    return tuple(length(computer, name, value, pixels_only=True)
                 for value in values)


@register_computer('top')
@register_computer('right')
@register_computer('left')
@register_computer('bottom')
@register_computer('margin-top')
@register_computer('margin-right')
@register_computer('margin-bottom')
@register_computer('margin-left')
@register_computer('height')
@register_computer('width')
@register_computer('min-width')
@register_computer('min-height')
@register_computer('max-width')
@register_computer('max-height')
@register_computer('padding-top')
@register_computer('padding-right')
@register_computer('padding-bottom')
@register_computer('padding-left')
@register_computer('text-indent')
def length(computer, name, value, font_size=None, pixels_only=False):
    """Compute a length ``value``."""
    if value == 'auto':
        return value
    if value.value == 0:
        return 0 if pixels_only else ZERO_PIXELS

    unit = value.unit
    if unit == 'px':
        return value.value if pixels_only else value
    elif unit in LENGTHS_TO_PIXELS:
        # Convert absolute lengths to pixels
        factor = LENGTHS_TO_PIXELS[unit]
    elif unit in ('em', 'ex'):
        if font_size is None:
            factor = computer.computed.font_size
        else:
            factor = font_size
        if unit == 'ex':
            # TODO: find a better way to measure ex, see
            # http://www.w3.org/TR/CSS21/syndata.html#length-units
            factor *= 0.5
    else:
        # A percentage or 'auto': no conversion needed.
        return value

    result = value.value * factor
    return result if pixels_only else Dimension(result, 'px')


@register_computer('letter-spacing')
def pixel_length(computer, name, value):
    return length(computer, name, value, pixels_only=True)


@register_computer('background-size')
def background_size(computer, name, value):
    """Compute the ``background-size`` properties."""
    if value in ('contain', 'cover'):
        return value
    else:
        return length_or_percentage_tuple(computer, name, value)


@register_computer('border-top-width')
@register_computer('border-right-width')
@register_computer('border-left-width')
@register_computer('border-bottom-width')
def border_width(computer, name, value):
    """Compute the ``border-*-width`` properties."""
    style = computer.computed[name.replace('width', 'style')]
    if style in ('none', 'hidden'):
        return 0

    if value in BORDER_WIDTH_KEYWORDS:
        return BORDER_WIDTH_KEYWORDS[value]

    if isinstance(value, int):
        # The initial value can get here, but length() would fail as
        # it does not have a 'unit' attribute.
        return value

    return length(computer, name, value, pixels_only=True)


@register_computer('content')
def content(computer, name, values):
    """Compute the ``content`` property."""
    if values in ('normal', 'none'):
        return values
    else:
        return [('STRING', computer.element.get(value, ''))
                if type_ == 'attr' else (type_, value)
                for type_, value in values]


@register_computer('display')
def display(computer, name, value):
    """Compute the ``display`` property.

    See http://www.w3.org/TR/CSS21/visuren.html#dis-pos-flo

    """
    float_ = computer.specified.float
    position = computer.specified.position
    if position in ('absolute', 'fixed') or float_ != 'none' or \
            getattr(computer.element, 'getparent', lambda: None)() is None:
        if value == 'inline-table':
            return'table'
        elif value in ('inline', 'table-row-group', 'table-column',
                       'table-column-group', 'table-header-group',
                       'table-footer-group', 'table-row', 'table-cell',
                       'table-caption', 'inline-block'):
            return 'block'
    return value


@register_computer('float')
def compute_float(computer, name, value):
    """Compute the ``float`` property.

    See http://www.w3.org/TR/CSS21/visuren.html#dis-pos-flo

    """
    if computer.specified.position in ('absolute', 'fixed'):
        return 'none'
    else:
        return value


@register_computer('font-size')
def font_size(computer, name, value):
    """Compute the ``font-size`` property."""
    if value in FONT_SIZE_KEYWORDS:
        return FONT_SIZE_KEYWORDS[value]
    # TODO: support 'larger' and 'smaller'

    parent_font_size = computer.parent_style['font_size']
    if value.unit == '%':
        return value.value * parent_font_size / 100.
    else:
        return length(computer, name, value, pixels_only=True,
                      font_size=parent_font_size)


@register_computer('font-weight')
def font_weight(computer, name, value):
    """Compute the ``font-weight`` property."""
    if value == 'normal':
        return 400
    elif value == 'bold':
        return 700
    elif value in ('bolder', 'lighter'):
        parent_value = computer.parent_style['font_weight']
        # Use a string here as StyleDict.__setattr__ turns integers into pixel
        # lengths. This is a number without unit.
        return FONT_WEIGHT_RELATIVE[value][parent_value]
    else:
        return value


@register_computer('line-height')
def line_height(computer, name, value):
    """Compute the ``line-height`` property."""
    if value == 'normal':
        return value
    elif not value.unit:
        return ('NUMBER', value.value)
    elif value.unit == '%':
        factor = value.value / 100.
        font_size_value = computer.computed.font_size
        pixels = factor * font_size_value
    else:
        pixels = length(computer, name, value, pixels_only=True)
    return ('PIXELS', pixels)


@register_computer('link')
def link(computer, name, values):
    """Compute the ``link`` property."""
    if values == 'none':
        return None
    else:
        type_, value = values
        if type_ == 'attr':
            return get_url_attribute(computer.element, value)
        else:
            return value


@register_computer('transform')
def transform(computer, name, value):
    """Compute the ``transform`` property."""
    result = []
    for function, args in value:
        if function in ('rotate', 'skewx', 'skewy'):
            args = args.value * ANGLE_TO_RADIANS[args.unit]
        elif function == 'translate':
            args = length_or_percentage_tuple(computer, name, args)
        result.append((function, args))
    return result


@register_computer('vertical-align')
def vertical_align(computer, name, value):
    """Compute the ``vertical-align`` property."""
    # Use +/- half an em for super and sub, same as Pango.
    # (See the SUPERSUB_RISE constant in pango-markup.c)
    if value in ('baseline', 'middle', 'text-top', 'text-bottom',
                 'top', 'bottom'):
        return value
    elif value == 'super':
        return computer.computed.font_size * 0.5
    elif value == 'sub':
        return computer.computed.font_size * -0.5
    elif value.unit == '%':
        height = used_line_height({
            'line_height': computer.computed.line_height,
            'font_size': computer.computed.font_size
        })
        return height * value.value / 100.
    else:
        return length(computer, name, value, pixels_only=True)


@register_computer('word-spacing')
def word_spacing(computer, name, value):
    """Compute the ``word-spacing`` property."""
    if value == 'normal':
        return 0
    else:
        return length(computer, name, value, pixels_only=True)


def used_line_height(style):
    """Return the used value for the ``line-height`` property."""
    height = style['line_height']
    if height == 'normal':
        # a "reasonable" value
        # http://www.w3.org/TR/CSS21/visudet.html#line-height
        # TODO: use font metrics?
        height = ('NUMBER', 1.2)
    type_, value = height
    if type_ == 'NUMBER':
        return value * style['font_size']
    else:
        return value