servo/components/layout/model.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */

//! Borders, padding, and margins.

use std::cmp::{max, min};
use std::fmt;

use app_units::Au;
use euclid::SideOffsets2D;
use serde::Serialize;
use style::logical_geometry::{LogicalMargin, WritingMode};
use style::properties::ComputedValues;
use style::values::computed::{Inset, LengthPercentageOrAuto, Margin, MaxSize, Size};

use crate::fragment::Fragment;

/// A collapsible margin. See CSS 2.1 § 8.3.1.
#[derive(Clone, Copy, Debug)]
pub struct AdjoiningMargins {
    /// The value of the greatest positive margin.
    pub most_positive: Au,

    /// The actual value (not the absolute value) of the negative margin with the largest absolute
    /// value. Since this is not the absolute value, this is always zero or negative.
    pub most_negative: Au,
}

impl AdjoiningMargins {
    pub fn new() -> AdjoiningMargins {
        AdjoiningMargins {
            most_positive: Au(0),
            most_negative: Au(0),
        }
    }

    pub fn from_margin(margin_value: Au) -> AdjoiningMargins {
        if margin_value >= Au(0) {
            AdjoiningMargins {
                most_positive: margin_value,
                most_negative: Au(0),
            }
        } else {
            AdjoiningMargins {
                most_positive: Au(0),
                most_negative: margin_value,
            }
        }
    }

    pub fn union(&mut self, other: AdjoiningMargins) {
        self.most_positive = max(self.most_positive, other.most_positive);
        self.most_negative = min(self.most_negative, other.most_negative)
    }

    pub fn collapse(&self) -> Au {
        self.most_positive + self.most_negative
    }
}

impl Default for AdjoiningMargins {
    fn default() -> Self {
        Self::new()
    }
}

/// Represents the block-start and block-end margins of a flow with collapsible margins. See CSS 2.1 § 8.3.1.
#[derive(Clone, Copy, Debug)]
pub enum CollapsibleMargins {
    /// Margins may not collapse with this flow.
    None(Au, Au),

    /// Both the block-start and block-end margins (specified here in that order) may collapse, but the
    /// margins do not collapse through this flow.
    Collapse(AdjoiningMargins, AdjoiningMargins),

    /// Margins collapse *through* this flow. This means, essentially, that the flow doesn’t
    /// have any border, padding, or out-of-flow (floating or positioned) content
    CollapseThrough(AdjoiningMargins),
}

impl CollapsibleMargins {
    pub fn new() -> CollapsibleMargins {
        CollapsibleMargins::None(Au(0), Au(0))
    }

    /// Returns the amount of margin that should be applied in a noncollapsible context. This is
    /// currently used to apply block-start margin for hypothetical boxes, since we do not collapse
    /// margins of hypothetical boxes.
    pub fn block_start_margin_for_noncollapsible_context(&self) -> Au {
        match *self {
            CollapsibleMargins::None(block_start, _) => block_start,
            CollapsibleMargins::Collapse(ref block_start, _) |
            CollapsibleMargins::CollapseThrough(ref block_start) => block_start.collapse(),
        }
    }

    pub fn block_end_margin_for_noncollapsible_context(&self) -> Au {
        match *self {
            CollapsibleMargins::None(_, block_end) => block_end,
            CollapsibleMargins::Collapse(_, ref block_end) |
            CollapsibleMargins::CollapseThrough(ref block_end) => block_end.collapse(),
        }
    }
}

impl Default for CollapsibleMargins {
    fn default() -> Self {
        Self::new()
    }
}

enum FinalMarginState {
    MarginsCollapseThrough,
    BottomMarginCollapses,
}

pub struct MarginCollapseInfo {
    pub state: MarginCollapseState,
    pub block_start_margin: AdjoiningMargins,
    pub margin_in: AdjoiningMargins,
}

impl MarginCollapseInfo {
    pub fn initialize_block_start_margin(
        fragment: &Fragment,
        can_collapse_block_start_margin_with_kids: bool,
    ) -> MarginCollapseInfo {
        MarginCollapseInfo {
            state: if can_collapse_block_start_margin_with_kids {
                MarginCollapseState::AccumulatingCollapsibleTopMargin
            } else {
                MarginCollapseState::AccumulatingMarginIn
            },
            block_start_margin: AdjoiningMargins::from_margin(fragment.margin.block_start),
            margin_in: AdjoiningMargins::new(),
        }
    }

    pub fn finish_and_compute_collapsible_margins(
        mut self,
        fragment: &Fragment,
        containing_block_size: Option<Au>,
        can_collapse_block_end_margin_with_kids: bool,
        mut may_collapse_through: bool,
    ) -> (CollapsibleMargins, Au) {
        let state = match self.state {
            MarginCollapseState::AccumulatingCollapsibleTopMargin => {
                let content_block_size = fragment.style().content_block_size();
                may_collapse_through = may_collapse_through &&
                    content_block_size.is_definitely_zero() ||
                    content_block_size
                        .maybe_to_used_value(containing_block_size)
                        .is_none();

                if may_collapse_through {
                    if fragment.style.min_block_size().is_auto() ||
                        fragment.style().min_block_size().is_definitely_zero()
                    {
                        FinalMarginState::MarginsCollapseThrough
                    } else {
                        // If the fragment has non-zero min-block-size, margins may not
                        // collapse through it.
                        FinalMarginState::BottomMarginCollapses
                    }
                } else {
                    // If the fragment has an explicitly specified block-size, margins may not
                    // collapse through it.
                    FinalMarginState::BottomMarginCollapses
                }
            },
            MarginCollapseState::AccumulatingMarginIn => FinalMarginState::BottomMarginCollapses,
        };

        // Different logic is needed here depending on whether this flow can collapse its block-end
        // margin with its children.
        let block_end_margin = fragment.margin.block_end;
        if !can_collapse_block_end_margin_with_kids {
            match state {
                FinalMarginState::MarginsCollapseThrough => {
                    let advance = self.block_start_margin.collapse();
                    self.margin_in
                        .union(AdjoiningMargins::from_margin(block_end_margin));
                    (
                        CollapsibleMargins::Collapse(self.block_start_margin, self.margin_in),
                        advance,
                    )
                },
                FinalMarginState::BottomMarginCollapses => {
                    let advance = self.margin_in.collapse();
                    self.margin_in
                        .union(AdjoiningMargins::from_margin(block_end_margin));
                    (
                        CollapsibleMargins::Collapse(self.block_start_margin, self.margin_in),
                        advance,
                    )
                },
            }
        } else {
            match state {
                FinalMarginState::MarginsCollapseThrough => {
                    self.block_start_margin
                        .union(AdjoiningMargins::from_margin(block_end_margin));
                    (
                        CollapsibleMargins::CollapseThrough(self.block_start_margin),
                        Au(0),
                    )
                },
                FinalMarginState::BottomMarginCollapses => {
                    self.margin_in
                        .union(AdjoiningMargins::from_margin(block_end_margin));
                    (
                        CollapsibleMargins::Collapse(self.block_start_margin, self.margin_in),
                        Au(0),
                    )
                },
            }
        }
    }

    pub fn current_float_ceiling(&mut self) -> Au {
        match self.state {
            MarginCollapseState::AccumulatingCollapsibleTopMargin => {
                // We do not include the top margin in the float ceiling, because the float flow
                // needs to be positioned relative to our *border box*, not our margin box. See
                // `tests/ref/float_under_top_margin_a.html`.
                Au(0)
            },
            MarginCollapseState::AccumulatingMarginIn => self.margin_in.collapse(),
        }
    }

    /// Adds the child's potentially collapsible block-start margin to the current margin state and
    /// advances the Y offset by the appropriate amount to handle that margin. Returns the amount
    /// that should be added to the Y offset during block layout.
    pub fn advance_block_start_margin(
        &mut self,
        child_collapsible_margins: &CollapsibleMargins,
        can_collapse_block_start_margin: bool,
    ) -> Au {
        if !can_collapse_block_start_margin {
            self.state = MarginCollapseState::AccumulatingMarginIn
        }

        match (self.state, *child_collapsible_margins) {
            (
                MarginCollapseState::AccumulatingCollapsibleTopMargin,
                CollapsibleMargins::None(block_start, _),
            ) => {
                self.state = MarginCollapseState::AccumulatingMarginIn;
                block_start
            },
            (
                MarginCollapseState::AccumulatingCollapsibleTopMargin,
                CollapsibleMargins::Collapse(block_start, _),
            ) => {
                self.block_start_margin.union(block_start);
                self.state = MarginCollapseState::AccumulatingMarginIn;
                Au(0)
            },
            (
                MarginCollapseState::AccumulatingMarginIn,
                CollapsibleMargins::None(block_start, _),
            ) => {
                let previous_margin_value = self.margin_in.collapse();
                self.margin_in = AdjoiningMargins::new();
                previous_margin_value + block_start
            },
            (
                MarginCollapseState::AccumulatingMarginIn,
                CollapsibleMargins::Collapse(block_start, _),
            ) => {
                self.margin_in.union(block_start);
                let margin_value = self.margin_in.collapse();
                self.margin_in = AdjoiningMargins::new();
                margin_value
            },
            (_, CollapsibleMargins::CollapseThrough(_)) => {
                // For now, we ignore this; this will be handled by `advance_block_end_margin`
                // below.
                Au(0)
            },
        }
    }

    /// Adds the child's potentially collapsible block-end margin to the current margin state and
    /// advances the Y offset by the appropriate amount to handle that margin. Returns the amount
    /// that should be added to the Y offset during block layout.
    pub fn advance_block_end_margin(
        &mut self,
        child_collapsible_margins: &CollapsibleMargins,
    ) -> Au {
        match (self.state, *child_collapsible_margins) {
            (
                MarginCollapseState::AccumulatingCollapsibleTopMargin,
                CollapsibleMargins::None(..),
            ) |
            (
                MarginCollapseState::AccumulatingCollapsibleTopMargin,
                CollapsibleMargins::Collapse(..),
            ) => {
                // Can't happen because the state will have been replaced with
                // `MarginCollapseState::AccumulatingMarginIn` above.
                panic!("should not be accumulating collapsible block_start margins anymore!")
            },
            (
                MarginCollapseState::AccumulatingCollapsibleTopMargin,
                CollapsibleMargins::CollapseThrough(margin),
            ) => {
                self.block_start_margin.union(margin);
                Au(0)
            },
            (MarginCollapseState::AccumulatingMarginIn, CollapsibleMargins::None(_, block_end)) => {
                assert_eq!(self.margin_in.most_positive, Au(0));
                assert_eq!(self.margin_in.most_negative, Au(0));
                block_end
            },
            (
                MarginCollapseState::AccumulatingMarginIn,
                CollapsibleMargins::Collapse(_, block_end),
            ) |
            (
                MarginCollapseState::AccumulatingMarginIn,
                CollapsibleMargins::CollapseThrough(block_end),
            ) => {
                self.margin_in.union(block_end);
                Au(0)
            },
        }
    }
}

#[derive(Clone, Copy, Debug)]
pub enum MarginCollapseState {
    /// We are accumulating margin on the logical top of this flow.
    AccumulatingCollapsibleTopMargin,
    /// We are accumulating margin between two blocks.
    AccumulatingMarginIn,
}

/// Intrinsic inline-sizes, which consist of minimum and preferred.
#[derive(Clone, Copy, Serialize)]
pub struct IntrinsicISizes {
    /// The *minimum inline-size* of the content.
    pub minimum_inline_size: Au,
    /// The *preferred inline-size* of the content.
    pub preferred_inline_size: Au,
}

impl fmt::Debug for IntrinsicISizes {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(
            f,
            "min={:?}, pref={:?}",
            self.minimum_inline_size, self.preferred_inline_size
        )
    }
}

impl IntrinsicISizes {
    pub fn new() -> IntrinsicISizes {
        IntrinsicISizes {
            minimum_inline_size: Au(0),
            preferred_inline_size: Au(0),
        }
    }
}

impl Default for IntrinsicISizes {
    fn default() -> Self {
        Self::new()
    }
}

/// The temporary result of the computation of intrinsic inline-sizes.
#[derive(Debug)]
pub struct IntrinsicISizesContribution {
    /// Intrinsic sizes for the content only (not counting borders, padding, or margins).
    pub content_intrinsic_sizes: IntrinsicISizes,
    /// The inline size of borders and padding, as well as margins if appropriate.
    pub surrounding_size: Au,
}

impl IntrinsicISizesContribution {
    /// Creates and initializes an inline size computation with all sizes set to zero.
    pub fn new() -> IntrinsicISizesContribution {
        IntrinsicISizesContribution {
            content_intrinsic_sizes: IntrinsicISizes::new(),
            surrounding_size: Au(0),
        }
    }

    /// Adds the content intrinsic sizes and the surrounding size together to yield the final
    /// intrinsic size computation.
    pub fn finish(self) -> IntrinsicISizes {
        IntrinsicISizes {
            minimum_inline_size: self.content_intrinsic_sizes.minimum_inline_size +
                self.surrounding_size,
            preferred_inline_size: self.content_intrinsic_sizes.preferred_inline_size +
                self.surrounding_size,
        }
    }

    /// Updates the computation so that the minimum is the maximum of the current minimum and the
    /// given minimum and the preferred is the sum of the current preferred and the given
    /// preferred. This is used when laying out fragments in the inline direction.
    ///
    /// FIXME(pcwalton): This is incorrect when the inline fragment contains forced line breaks
    /// (e.g. `<br>` or `white-space: pre`).
    pub fn union_inline(&mut self, sizes: &IntrinsicISizes) {
        self.content_intrinsic_sizes.minimum_inline_size = max(
            self.content_intrinsic_sizes.minimum_inline_size,
            sizes.minimum_inline_size,
        );
        self.content_intrinsic_sizes.preferred_inline_size += sizes.preferred_inline_size
    }

    /// Updates the computation so that the minimum is the sum of the current minimum and the
    /// given minimum and the preferred is the sum of the current preferred and the given
    /// preferred. This is used when laying out fragments in the inline direction when
    /// `white-space` is `pre` or `nowrap`.
    pub fn union_nonbreaking_inline(&mut self, sizes: &IntrinsicISizes) {
        self.content_intrinsic_sizes.minimum_inline_size += sizes.minimum_inline_size;
        self.content_intrinsic_sizes.preferred_inline_size += sizes.preferred_inline_size
    }

    /// Updates the computation so that the minimum is the maximum of the current minimum and the
    /// given minimum and the preferred is the maximum of the current preferred and the given
    /// preferred. This can be useful when laying out fragments in the block direction (but note
    /// that it does not take floats into account, so `BlockFlow` does not use it).
    ///
    /// This is used when contributing the intrinsic sizes for individual fragments.
    pub fn union_block(&mut self, sizes: &IntrinsicISizes) {
        self.content_intrinsic_sizes.minimum_inline_size = max(
            self.content_intrinsic_sizes.minimum_inline_size,
            sizes.minimum_inline_size,
        );
        self.content_intrinsic_sizes.preferred_inline_size = max(
            self.content_intrinsic_sizes.preferred_inline_size,
            sizes.preferred_inline_size,
        )
    }
}

impl Default for IntrinsicISizesContribution {
    fn default() -> Self {
        Self::new()
    }
}

/// Useful helper data type when computing values for blocks and positioned elements.
#[derive(Clone, Copy, Debug, PartialEq)]
pub enum MaybeAuto {
    Auto,
    Specified(Au),
}

impl MaybeAuto {
    #[inline]
    pub fn from_style(length: &LengthPercentageOrAuto, containing_length: Au) -> MaybeAuto {
        match length {
            LengthPercentageOrAuto::Auto => MaybeAuto::Auto,
            LengthPercentageOrAuto::LengthPercentage(ref lp) => {
                MaybeAuto::Specified(lp.to_used_value(containing_length))
            },
        }
    }

    #[inline]
    pub fn from_inset(length: &Inset, containing_length: Au) -> MaybeAuto {
        match length {
            Inset::Auto => MaybeAuto::Auto,
            Inset::LengthPercentage(ref lp) => {
                MaybeAuto::Specified(lp.to_used_value(containing_length))
            },
            Inset::AnchorFunction(_) => unreachable!("anchor() should be disabled"),
            Inset::AnchorSizeFunction(_) => unreachable!("anchor-size() should be disabled"),
        }
    }

    #[inline]
    pub fn from_margin(length: &Margin, containing_length: Au) -> MaybeAuto {
        match length {
            Margin::Auto => MaybeAuto::Auto,
            Margin::LengthPercentage(ref lp) => {
                MaybeAuto::Specified(lp.to_used_value(containing_length))
            },
            Margin::AnchorSizeFunction(_) => unreachable!("anchor-size() should be disabled"),
        }
    }

    #[inline]
    pub fn from_option(au: Option<Au>) -> MaybeAuto {
        match au {
            Some(l) => MaybeAuto::Specified(l),
            _ => MaybeAuto::Auto,
        }
    }

    #[inline]
    pub fn as_option(&self) -> Option<Au> {
        match *self {
            MaybeAuto::Specified(value) => Some(value),
            MaybeAuto::Auto => None,
        }
    }

    #[inline]
    pub fn specified_or_default(&self, default: Au) -> Au {
        match *self {
            MaybeAuto::Auto => default,
            MaybeAuto::Specified(value) => value,
        }
    }

    #[inline]
    pub fn specified_or_zero(&self) -> Au {
        self.specified_or_default(Au::new(0))
    }

    #[inline]
    pub fn is_auto(&self) -> bool {
        match *self {
            MaybeAuto::Auto => true,
            MaybeAuto::Specified(..) => false,
        }
    }

    #[inline]
    pub fn map<F>(&self, mapper: F) -> MaybeAuto
    where
        F: FnOnce(Au) -> Au,
    {
        match *self {
            MaybeAuto::Auto => MaybeAuto::Auto,
            MaybeAuto::Specified(value) => MaybeAuto::Specified(mapper(value)),
        }
    }
}

/// Receive an optional container size and return used value for width or height.
///
/// `style_length`: content size as given in the CSS.
pub fn style_length(style_length: &Size, container_size: Option<Au>) -> MaybeAuto {
    MaybeAuto::from_option(style_length.maybe_to_used_value(container_size))
}

#[inline]
pub fn padding_from_style(
    style: &ComputedValues,
    containing_block_inline_size: Au,
    writing_mode: WritingMode,
) -> LogicalMargin<Au> {
    let padding_style = style.get_padding();
    LogicalMargin::from_physical(
        writing_mode,
        SideOffsets2D::new(
            padding_style
                .padding_top
                .to_used_value(containing_block_inline_size),
            padding_style
                .padding_right
                .to_used_value(containing_block_inline_size),
            padding_style
                .padding_bottom
                .to_used_value(containing_block_inline_size),
            padding_style
                .padding_left
                .to_used_value(containing_block_inline_size),
        ),
    )
}

/// Returns the explicitly-specified margin lengths from the given style. Percentage and auto
/// margins are returned as zero.
///
/// This is used when calculating intrinsic inline sizes.
#[inline]
pub fn specified_margin_from_style(
    style: &ComputedValues,
    writing_mode: WritingMode,
) -> LogicalMargin<Au> {
    let margin_style = style.get_margin();
    LogicalMargin::from_physical(
        writing_mode,
        SideOffsets2D::new(
            MaybeAuto::from_margin(&margin_style.margin_top, Au(0)).specified_or_zero(),
            MaybeAuto::from_margin(&margin_style.margin_right, Au(0)).specified_or_zero(),
            MaybeAuto::from_margin(&margin_style.margin_bottom, Au(0)).specified_or_zero(),
            MaybeAuto::from_margin(&margin_style.margin_left, Au(0)).specified_or_zero(),
        ),
    )
}

/// A min-size and max-size constraint. The constructor has a optional `border`
/// parameter, and when it is present the constraint will be subtracted. This is
/// used to adjust the constraint for `box-sizing: border-box`, and when you do so
/// make sure the size you want to clamp is intended to be used for content box.
#[derive(Clone, Copy, Debug, Serialize)]
pub struct SizeConstraint {
    min_size: Au,
    max_size: Option<Au>,
}

impl SizeConstraint {
    /// Create a `SizeConstraint` for an axis.
    pub fn new(
        container_size: Option<Au>,
        min_size: &Size,
        max_size: &MaxSize,
        border: Option<Au>,
    ) -> SizeConstraint {
        let mut min_size = min_size
            .maybe_to_used_value(container_size)
            .unwrap_or(Au(0));
        let mut max_size = max_size.maybe_to_used_value(container_size);

        // Make sure max size is not smaller than min size.
        max_size = max_size.map(|x| max(x, min_size));

        if let Some(border) = border {
            min_size = max(min_size - border, Au(0));
            max_size = max_size.map(|x| max(x - border, Au(0)));
        }

        SizeConstraint { min_size, max_size }
    }

    /// Clamp the given size by the given min size and max size constraint.
    pub fn clamp(&self, other: Au) -> Au {
        if other < self.min_size {
            self.min_size
        } else {
            match self.max_size {
                Some(max_size) if max_size < other => max_size,
                _ => other,
            }
        }
    }
}