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Add an implementation of the core float and clear placement logic in layout

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2020, not yet wired to the rest of layout.

This commit implements an object that handles the 10 rules in CSS 2.1:

https://www.w3.org/TR/CSS2/visuren.html#float-position

The implementation strategy is that of a persistent balanced binary search tree
of float bands. Binary search trees are commonly used for implementing float
positioning; e.g. by WebKit.  Persistence enables each object that interacts
with floats to efficiently contain a snapshot of the float list at the time
that object was laid out. That way, incremental layout can invalidate and start
reflow at any point in a containing block.

This commit features extensive use of
[QuickCheck](https://github.com/BurntSushi/quickcheck) to ensure that the rules
of the CSS specification are followed.

Because this is not yet connected to layout, floats will not actually be laid
out in Web pages yet.

Note that unit tests as set up in Servo currently require types that they
access to be public. Therefore, some internal layout 2020 types that were
previously private have been made public. This is somewhat unfortunate.

Part of #25167.
This commit is contained in:
Patrick Walton 2020-07-06 18:44:59 -07:00
parent 48bf169101
commit 5b36d211b4
10 changed files with 1354 additions and 20 deletions
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473
components/layout_2020/flow/float.rs
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@ -2,29 +2,494 @@
* 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/. */
//! Float layout.
//!
//! See CSS 2.1 § 9.5.1: https://www.w3.org/TR/CSS2/visuren.html#float-position
use crate::context::LayoutContext;
use crate::dom_traversal::{Contents, NodeAndStyleInfo, NodeExt};
use crate::formatting_contexts::IndependentFormattingContext;
use crate::geom::flow_relative::{Rect, Vec2};
use crate::style_ext::DisplayInside;
use euclid::num::Zero;
use servo_arc::Arc;
use std::f32;
use std::ops::Range;
use style::values::computed::Length;
use style::values::specified::text::TextDecorationLine;
/// A floating box.
#[derive(Debug, Serialize)]
pub(crate) struct FloatBox {
/// The formatting context that makes up the content of this box.
pub contents: IndependentFormattingContext,
}
/// Data kept during layout about the floats in a given block formatting context.
pub(crate) struct FloatContext {
// TODO
///
/// This is a persistent data structure. Each float has its own private copy of the float context,
/// although such copies may share portions of the `bands` tree.
#[derive(Clone, Debug)]
pub struct FloatContext {
/// A persistent AA tree of float bands.
///
/// This tree is immutable; modification operations return the new tree, which may share nodes
/// with previous versions of the tree.
pub bands: FloatBandTree,
/// The logical width of this context. No floats may extend outside the width of this context
/// unless they are as far (logically) left or right as possible.
pub inline_size: Length,
/// The current (logically) vertical position. No new floats may be placed (logically) above
/// this line.
pub ceiling: Length,
}
impl FloatContext {
pub fn new() -> Self {
FloatContext {}
/// Returns a new float context representing a containing block with the given content
/// inline-size.
pub fn new(inline_size: Length) -> Self {
let mut bands = FloatBandTree::new();
bands = bands.insert(FloatBand {
top: Length::zero(),
left: None,
right: None,
});
bands = bands.insert(FloatBand {
top: Length::new(f32::INFINITY),
left: None,
right: None,
});
FloatContext {
bands,
inline_size,
ceiling: Length::zero(),
}
}
/// Returns the current ceiling value. No new floats may be placed (logically) above this line.
pub fn ceiling(&self) -> Length {
self.ceiling
}
/// (Logically) lowers the ceiling to at least `new_ceiling` units.
///
/// If the ceiling is already logically lower (i.e. larger) than this, does nothing.
pub fn lower_ceiling(&mut self, new_ceiling: Length) {
self.ceiling = self.ceiling.max(new_ceiling);
}
/// Returns the highest block position that is both logically below the current ceiling and
/// clear of floats on the given side or sides.
pub fn clearance(&self, side: ClearSide) -> Length {
let mut band = self.bands.find(self.ceiling).unwrap();
while !band.is_clear(side) {
let next_band = self.bands.find_next(band.top).unwrap();
if next_band.top.px().is_infinite() {
break;
}
band = next_band;
}
band.top.max(self.ceiling)
}
/// Places a new float and adds it to the list. Returns the start corner of its margin box.
pub fn add_float(&mut self, new_float: FloatInfo) -> Vec2<Length> {
// Find the first band this float fits in.
let mut first_band = self.bands.find(self.ceiling).unwrap();
while !first_band.float_fits(&new_float, self.inline_size) {
let next_band = self.bands.find_next(first_band.top).unwrap();
if next_band.top.px().is_infinite() {
break;
}
first_band = next_band;
}
// The float fits perfectly here. Place it.
let (new_float_origin, new_float_extent);
match new_float.side {
FloatSide::Left => {
new_float_origin = Vec2 {
inline: first_band.left.unwrap_or(Length::zero()),
block: first_band.top.max(self.ceiling),
};
new_float_extent = new_float_origin.inline + new_float.size.inline;
},
FloatSide::Right => {
new_float_origin = Vec2 {
inline: first_band.right.unwrap_or(self.inline_size) - new_float.size.inline,
block: first_band.top.max(self.ceiling),
};
new_float_extent = new_float_origin.inline;
},
};
let new_float_rect = Rect {
start_corner: new_float_origin,
size: new_float.size,
};
// Split the first band if necessary.
first_band.top = new_float_rect.start_corner.block;
self.bands = self.bands.insert(first_band);
// Split the last band if necessary.
let mut last_band = self
.bands
.find(new_float_rect.max_block_position())
.unwrap();
last_band.top = new_float_rect.max_block_position();
self.bands = self.bands.insert(last_band);
// Update all bands that contain this float to reflect the new available size.
let block_range = new_float_rect.start_corner.block..new_float_rect.max_block_position();
self.bands = self
.bands
.set_range(&block_range, new_float.side, new_float_extent);
// CSS 2.1 § 9.5.1 rule 6: The outer top of a floating box may not be higher than the outer
// top of any block or floated box generated by an element earlier in the source document.
self.ceiling = self.ceiling.max(new_float_rect.start_corner.block);
new_float_rect.start_corner
}
}
/// Information needed to place a float.
#[derive(Clone, Debug)]
pub struct FloatInfo {
/// The *margin* box size of the float.
pub size: Vec2<Length>,
/// Whether the float is left or right.
pub side: FloatSide,
/// Which side or sides to clear floats on.
pub clear: ClearSide,
}
/// Whether the float is left or right.
///
/// See CSS 2.1 § 9.5.1: https://www.w3.org/TR/CSS2/visuren.html#float-position
#[derive(Clone, Copy, Debug, PartialEq)]
pub enum FloatSide {
Left,
Right,
}
/// Which side or sides to clear floats on.
///
/// See CSS 2.1 § 9.5.2: https://www.w3.org/TR/CSS2/visuren.html#flow-control
#[derive(Clone, Copy, Debug, PartialEq)]
pub enum ClearSide {
None = 0,
Left = 1,
Right = 2,
Both = 3,
}
/// Internal data structure that describes a nonoverlapping vertical region in which floats may be
/// placed. Floats must go between "left edge + `left`" and "right edge - `right`".
#[derive(Clone, Copy, Debug, PartialEq)]
pub struct FloatBand {
/// The logical vertical position of the top of this band.
pub top: Length,
/// The distance from the left edge to the first legal (logically) horizontal position where
/// floats may be placed. If `None`, there are no floats to the left; distinguishing between
/// the cases of "a zero-width float is present" and "no floats at all are present" is
/// necessary to, for example, clear past zero-width floats.
pub left: Option<Length>,
/// The distance from the right edge to the first legal (logically) horizontal position where
/// floats may be placed. If `None`, there are no floats to the right; distinguishing between
/// the cases of "a zero-width float is present" and "no floats at all are present" is
/// necessary to, for example, clear past zero-width floats.
pub right: Option<Length>,
}
impl FloatBand {
fn is_clear(&self, side: ClearSide) -> bool {
match (side, self.left, self.right) {
(ClearSide::Left, Some(_), _) |
(ClearSide::Right, _, Some(_)) |
(ClearSide::Both, Some(_), _) |
(ClearSide::Both, _, Some(_)) => false,
(ClearSide::None, _, _) |
(ClearSide::Left, None, _) |
(ClearSide::Right, _, None) |
(ClearSide::Both, None, None) => true,
}
}
fn float_fits(&self, new_float: &FloatInfo, container_inline_size: Length) -> bool {
let available_space =
self.right.unwrap_or(container_inline_size) - self.left.unwrap_or(Length::zero());
self.is_clear(new_float.clear) &&
(new_float.size.inline <= available_space ||
(self.left.is_none() && self.right.is_none()))
}
}
// Float band storage
/// A persistent AA tree for float band storage.
///
/// Bands here are nonoverlapping, and there is guaranteed to be a band at block-position 0 and
/// another band at block-position infinity.
///
/// AA trees were chosen for simplicity.
///
/// See: https://en.wikipedia.org/wiki/AA_tree
/// https://arxiv.org/pdf/1412.4882.pdf
#[derive(Clone, Debug)]
pub struct FloatBandTree {
pub root: FloatBandLink,
}
/// A single edge (or lack thereof) in the float band tree.
#[derive(Clone, Debug)]
pub struct FloatBandLink(pub Option<Arc<FloatBandNode>>);
/// A single node in the float band tree.
#[derive(Clone, Debug)]
pub struct FloatBandNode {
/// The actual band.
pub band: FloatBand,
/// The left child.
pub left: FloatBandLink,
/// The right child.
pub right: FloatBandLink,
/// The level, which increases as you go up the tree.
///
/// This value is needed for tree balancing.
pub level: i32,
}
impl FloatBandTree {
/// Creates a new float band tree.
pub fn new() -> FloatBandTree {
FloatBandTree {
root: FloatBandLink(None),
}
}
/// Returns the first band whose top is less than or equal to the given `block_position`.
pub fn find(&self, block_position: Length) -> Option<FloatBand> {
self.root.find(block_position)
}
/// Returns the first band whose top is strictly greater than to the given `block_position`.
pub fn find_next(&self, block_position: Length) -> Option<FloatBand> {
self.root.find_next(block_position)
}
/// Sets the side values of all bands within the given half-open range to be at least
/// `new_value`.
#[must_use]
pub fn set_range(
&self,
range: &Range<Length>,
side: FloatSide,
new_value: Length,
) -> FloatBandTree {
FloatBandTree {
root: FloatBandLink(
self.root
.0
.as_ref()
.map(|root| root.set_range(range, side, new_value)),
),
}
}
/// Inserts a new band into the tree. If the band has the same level as a pre-existing one,
/// replaces the existing band with the new one.
#[must_use]
pub fn insert(&self, band: FloatBand) -> FloatBandTree {
FloatBandTree {
root: self.root.insert(band),
}
}
}
impl FloatBandNode {
fn new(band: FloatBand) -> FloatBandNode {
FloatBandNode {
band,
left: FloatBandLink(None),
right: FloatBandLink(None),
level: 1,
}
}
/// Sets the side values of all bands within the given half-open range to be at least
/// `new_value`.
fn set_range(
&self,
range: &Range<Length>,
side: FloatSide,
new_value: Length,
) -> Arc<FloatBandNode> {
let mut new_band = self.band.clone();
if self.band.top >= range.start && self.band.top < range.end {
match side {
FloatSide::Left => match new_band.left {
None => new_band.left = Some(new_value),
Some(ref mut old_value) => *old_value = old_value.max(new_value),
},
FloatSide::Right => match new_band.right {
None => new_band.right = Some(new_value),
Some(ref mut old_value) => *old_value = old_value.min(new_value),
},
}
}
let new_left = match self.left.0 {
None => FloatBandLink(None),
Some(ref old_left) if range.start < new_band.top => {
FloatBandLink(Some(old_left.set_range(range, side, new_value)))
},
Some(ref old_left) => FloatBandLink(Some((*old_left).clone())),
};
let new_right = match self.right.0 {
None => FloatBandLink(None),
Some(ref old_right) if range.end > new_band.top => {
FloatBandLink(Some(old_right.set_range(range, side, new_value)))
},
Some(ref old_right) => FloatBandLink(Some((*old_right).clone())),
};
Arc::new(FloatBandNode {
band: new_band,
left: new_left,
right: new_right,
level: self.level,
})
}
}
impl FloatBandLink {
/// Returns the first band whose top is less than or equal to the given `block_position`.
fn find(&self, block_position: Length) -> Option<FloatBand> {
let this = match self.0 {
None => return None,
Some(ref node) => node,
};
if block_position < this.band.top {
return this.left.find(block_position);
}
// It's somewhere in this subtree, but we aren't sure whether it's here or in the right
// subtree.
if let Some(band) = this.right.find(block_position) {
return Some(band);
}
Some(this.band.clone())
}
/// Returns the first band whose top is strictly greater than the given `block_position`.
fn find_next(&self, block_position: Length) -> Option<FloatBand> {
let this = match self.0 {
None => return None,
Some(ref node) => node,
};
if block_position >= this.band.top {
return this.right.find_next(block_position);
}
// It's somewhere in this subtree, but we aren't sure whether it's here or in the left
// subtree.
if let Some(band) = this.left.find_next(block_position) {
return Some(band);
}
Some(this.band.clone())
}
// Inserts a new band into the tree. If the band has the same level as a pre-existing one,
// replaces the existing band with the new one.
fn insert(&self, band: FloatBand) -> FloatBandLink {
let mut this = match self.0 {
None => return FloatBandLink(Some(Arc::new(FloatBandNode::new(band)))),
Some(ref this) => (**this).clone(),
};
if band.top < this.band.top {
this.left = this.left.insert(band);
return FloatBandLink(Some(Arc::new(this))).skew().split();
}
if band.top > this.band.top {
this.right = this.right.insert(band);
return FloatBandLink(Some(Arc::new(this))).skew().split();
}
this.band = band;
FloatBandLink(Some(Arc::new(this)))
}
// Corrects tree balance:
//
// T L
// / \ / \
// L R → A T if level(T) = level(L)
// / \ / \
// A B B R
fn skew(&self) -> FloatBandLink {
if let Some(ref this) = self.0 {
if let Some(ref left) = this.left.0 {
if this.level == left.level {
return FloatBandLink(Some(Arc::new(FloatBandNode {
level: this.level,
left: left.left.clone(),
band: left.band.clone(),
right: FloatBandLink(Some(Arc::new(FloatBandNode {
level: this.level,
left: left.right.clone(),
band: this.band.clone(),
right: this.right.clone(),
}))),
})));
}
}
}
(*self).clone()
}
// Corrects tree balance:
//
// T R
// / \ / \
// A R → T X if level(T) = level(X)
// / \ / \
// B X A B
fn split(&self) -> FloatBandLink {
if let Some(ref this) = self.0 {
if let Some(ref right) = this.right.0 {
if let Some(ref right_right) = right.right.0 {
if this.level == right_right.level {
return FloatBandLink(Some(Arc::new(FloatBandNode {
level: this.level + 1,
left: FloatBandLink(Some(Arc::new(FloatBandNode {
level: this.level,
left: this.left.clone(),
band: this.band.clone(),
right: right.left.clone(),
}))),
band: right.band.clone(),
right: right.right.clone(),
})));
}
}
}
}
(*self).clone()
}
}
// Float boxes
impl FloatBox {
/// Creates a new float box.
pub fn construct<'dom>(
context: &LayoutContext,
info: &NodeAndStyleInfo<impl NodeExt<'dom>>,