path: root/src/components/layout/flow.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 http://mozilla.org/MPL/2.0/. */

//! Servo's experimental layout system builds a tree of `Flow` and `Fragment` objects and solves
//! layout constraints to obtain positions and display attributes of tree nodes. Positions are
//! computed in several tree traversals driven by the fundamental data dependencies required by
/// inline and block layout.
///
/// Flows are interior nodes in the layout tree and correspond closely to *flow contexts* in the
/// CSS specification. Flows are responsible for positioning their child flow contexts and fragments.
/// Flows have purpose-specific fields, such as auxiliary line structs, out-of-flow child
/// lists, and so on.
///
/// Currently, the important types of flows are:
///
/// * `BlockFlow`: A flow that establishes a block context. It has several child flows, each of
///   which are positioned according to block formatting context rules (CSS block boxes). Block
///   flows also contain a single box to represent their rendered borders, padding, etc.
///   The BlockFlow at the root of the tree has special behavior: it stretches to the boundaries of
///   the viewport.
///
/// * `InlineFlow`: A flow that establishes an inline context. It has a flat list of child
///   fragments/flows that are subject to inline layout and line breaking and structs to represent
///   line breaks and mapping to CSS boxes, for the purpose of handling `getClientRects()` and
///   similar methods.

use css::node_style::StyledNode;
use block::BlockFlow;
use context::LayoutContext;
use floats::Floats;
use flow_list::{FlowList, Link, FlowListIterator, MutFlowListIterator};
use flow_ref::FlowRef;
use fragment::{Fragment, TableRowFragment, TableCellFragment};
use incremental::RestyleDamage;
use inline::InlineFlow;
use model::{CollapsibleMargins, IntrinsicISizes, MarginCollapseInfo};
use parallel::FlowParallelInfo;
use table_wrapper::TableWrapperFlow;
use table::TableFlow;
use table_colgroup::TableColGroupFlow;
use table_rowgroup::TableRowGroupFlow;
use table_row::TableRowFlow;
use table_caption::TableCaptionFlow;
use table_cell::TableCellFlow;
use wrapper::ThreadSafeLayoutNode;

use collections::dlist::DList;
use geom::Point2D;
use gfx::display_list::DisplayList;
use gfx::render_task::RenderLayer;
use servo_msg::compositor_msg::LayerId;
use servo_util::geometry::Au;
use servo_util::logical_geometry::WritingMode;
use servo_util::logical_geometry::{LogicalRect, LogicalSize};
use std::mem;
use std::num::Zero;
use std::fmt;
use std::iter::Zip;
use std::raw;
use std::sync::atomics::{AtomicUint, Relaxed, SeqCst};
use std::slice::MutItems;
use style::computed_values::{clear, position, text_align};

/// Virtual methods that make up a float context.
///
/// Note that virtual methods have a cost; we should not overuse them in Servo. Consider adding
/// methods to `ImmutableFlowUtils` or `MutableFlowUtils` before adding more methods here.
pub trait Flow: fmt::Show + ToString + Share {
    // RTTI
    //
    // TODO(pcwalton): Use Rust's RTTI, once that works.

    /// Returns the class of flow that this is.
    fn class(&self) -> FlowClass;

    /// If this is a block flow, returns the underlying object. Fails otherwise.
    fn as_block<'a>(&'a mut self) -> &'a mut BlockFlow {
        debug!("called as_block() on a flow of type {}", self.class());
        fail!("called as_block() on a non-block flow")
    }

    /// If this is an inline flow, returns the underlying object, borrowed immutably. Fails
    /// otherwise.
    fn as_immutable_inline<'a>(&'a self) -> &'a InlineFlow {
        fail!("called as_immutable_inline() on a non-inline flow")
    }

    /// If this is an inline flow, returns the underlying object. Fails otherwise.
    fn as_inline<'a>(&'a mut self) -> &'a mut InlineFlow {
        fail!("called as_inline() on a non-inline flow")
    }

    /// If this is a table wrapper flow, returns the underlying object. Fails otherwise.
    fn as_table_wrapper<'a>(&'a mut self) -> &'a mut TableWrapperFlow {
        fail!("called as_table_wrapper() on a non-tablewrapper flow")
    }

    /// If this is a table flow, returns the underlying object. Fails otherwise.
    fn as_table<'a>(&'a mut self) -> &'a mut TableFlow {
        fail!("called as_table() on a non-table flow")
    }

    /// If this is a table colgroup flow, returns the underlying object. Fails otherwise.
    fn as_table_colgroup<'a>(&'a mut self) -> &'a mut TableColGroupFlow {
        fail!("called as_table_colgroup() on a non-tablecolgroup flow")
    }

    /// If this is a table rowgroup flow, returns the underlying object. Fails otherwise.
    fn as_table_rowgroup<'a>(&'a mut self) -> &'a mut TableRowGroupFlow {
        fail!("called as_table_rowgroup() on a non-tablerowgroup flow")
    }

    /// If this is a table row flow, returns the underlying object. Fails otherwise.
    fn as_table_row<'a>(&'a mut self) -> &'a mut TableRowFlow {
        fail!("called as_table_row() on a non-tablerow flow")
    }

    /// If this is a table cell flow, returns the underlying object. Fails otherwise.
    fn as_table_caption<'a>(&'a mut self) -> &'a mut TableCaptionFlow {
        fail!("called as_table_caption() on a non-tablecaption flow")
    }

    /// If this is a table cell flow, returns the underlying object. Fails otherwise.
    fn as_table_cell<'a>(&'a mut self) -> &'a mut TableCellFlow {
        fail!("called as_table_cell() on a non-tablecell flow")
    }

    /// If this is a table row or table rowgroup or table flow, returns column inline-sizes.
    /// Fails otherwise.
    fn col_inline_sizes<'a>(&'a mut self) -> &'a mut Vec<Au> {
        fail!("called col_inline_sizes() on an other flow than table-row/table-rowgroup/table")
    }

    /// If this is a table row flow or table rowgroup flow or table flow, returns column min inline-sizes.
    /// Fails otherwise.
    fn col_min_inline_sizes<'a>(&'a self) -> &'a Vec<Au> {
        fail!("called col_min_inline_sizes() on an other flow than table-row/table-rowgroup/table")
    }

    /// If this is a table row flow or table rowgroup flow or table flow, returns column min inline-sizes.
    /// Fails otherwise.
    fn col_pref_inline_sizes<'a>(&'a self) -> &'a Vec<Au> {
        fail!("called col_pref_inline_sizes() on an other flow than table-row/table-rowgroup/table")
    }

    // Main methods

    /// Pass 1 of reflow: computes minimum and preferred inline-sizes.
    ///
    /// Recursively (bottom-up) determine the flow's minimum and preferred inline-sizes. When called on
    /// this flow, all child flows have had their minimum and preferred inline-sizes set. This function
    /// must decide minimum/preferred inline-sizes based on its children's inline-sizes and the dimensions of
    /// any boxes it is responsible for flowing.
    fn bubble_inline_sizes(&mut self, _ctx: &LayoutContext) {
        fail!("bubble_inline_sizes not yet implemented")
    }

    /// Pass 2 of reflow: computes inline-size.
    fn assign_inline_sizes(&mut self, _ctx: &LayoutContext) {
        fail!("assign_inline_sizes not yet implemented")
    }

    /// Pass 3a of reflow: computes block-size.
    fn assign_block_size<'a>(&mut self, _ctx: &'a LayoutContext<'a>) {
        fail!("assign_block_size not yet implemented")
    }

    /// Assigns block-sizes in-order; or, if this is a float, places the float. The default
    /// implementation simply assigns block-sizes if this flow is impacted by floats. Returns true if
    /// this child was impacted by floats or false otherwise.
    fn assign_block_size_for_inorder_child_if_necessary<'a>(&mut self, layout_context: &'a LayoutContext<'a>)
                                                    -> bool {
        let impacted = base(&*self).flags.impacted_by_floats();
        if impacted {
            self.assign_block_size(layout_context);
        }
        impacted
    }

    /// Phase 4 of reflow: computes absolute positions.
    fn compute_absolute_position(&mut self) {
        // The default implementation is a no-op.
    }

    /// Returns the direction that this flow clears floats in, if any.
    fn float_clearance(&self) -> clear::T {
        clear::none
    }

    /// Returns true if this float is a block formatting context and false otherwise. The default
    /// implementation returns false.
    fn is_block_formatting_context(&self, _only_impactable_by_floats: bool) -> bool {
        false
    }

    fn compute_collapsible_block_start_margin(&mut self,
                                      _layout_context: &mut LayoutContext,
                                      _margin_collapse_info: &mut MarginCollapseInfo) {
        // The default implementation is a no-op.
    }

    /// Marks this flow as the root flow. The default implementation is a no-op.
    fn mark_as_root(&mut self) {}

    // Note that the following functions are mostly called using static method
    // dispatch, so it's ok to have them in this trait. Plus, they have
    // different behaviour for different types of Flow, so they can't go into
    // the Immutable / Mutable Flow Utils traits without additional casts.

    /// Return true if store overflow is delayed for this flow.
    ///
    /// Currently happens only for absolutely positioned flows.
    fn is_store_overflow_delayed(&mut self) -> bool {
        false
    }

    fn is_root(&self) -> bool {
        false
    }

    fn is_float(&self) -> bool {
        false
    }

    /// The 'position' property of this flow.
    fn positioning(&self) -> position::T {
        position::static_
    }

    /// Return true if this flow has position 'fixed'.
    fn is_fixed(&self) -> bool {
        self.positioning() == position::fixed
    }

    fn is_positioned(&self) -> bool {
        self.is_relatively_positioned() || self.is_absolutely_positioned()
    }

    fn is_relatively_positioned(&self) -> bool {
        self.positioning() == position::relative
    }

    fn is_absolutely_positioned(&self) -> bool {
        self.positioning() == position::absolute || self.is_fixed()
    }

    /// Return true if this is the root of an Absolute flow tree.
    fn is_root_of_absolute_flow_tree(&self) -> bool {
        false
    }

    /// Returns true if this is an absolute containing block.
    fn is_absolute_containing_block(&self) -> bool {
        false
    }

    /// Return the dimensions of the containing block generated by this flow for absolutely-
    /// positioned descendants. For block flows, this is the padding box.
    fn generated_containing_block_rect(&self) -> LogicalRect<Au> {
        fail!("generated_containing_block_position not yet implemented for this flow")
    }

    /// Returns a layer ID for the given fragment.
    fn layer_id(&self, fragment_id: uint) -> LayerId {
        unsafe {
            let pointer: uint = mem::transmute(self);
            LayerId(pointer, fragment_id)
        }
    }
}

// Base access

#[inline(always)]
pub fn base<'a>(this: &'a Flow) -> &'a BaseFlow {
    unsafe {
        let obj = mem::transmute::<&'a Flow, raw::TraitObject>(this);
        mem::transmute::<*mut (), &'a BaseFlow>(obj.data)
    }
}

/// Iterates over the children of this immutable flow.
pub fn imm_child_iter<'a>(flow: &'a Flow) -> FlowListIterator<'a> {
    base(flow).children.iter()
}

#[inline(always)]
pub fn mut_base<'a>(this: &'a mut Flow) -> &'a mut BaseFlow {
    unsafe {
        let obj = mem::transmute::<&'a mut Flow, raw::TraitObject>(this);
        mem::transmute::<*mut (), &'a mut BaseFlow>(obj.data)
    }
}

/// Iterates over the children of this flow.
pub fn child_iter<'a>(flow: &'a mut Flow) -> MutFlowListIterator<'a> {
    mut_base(flow).children.mut_iter()
}

pub trait ImmutableFlowUtils {
    // Convenience functions

    /// Returns true if this flow is a block or a float flow.
    fn is_block_like(self) -> bool;

    /// Returns true if this flow is a table flow.
    fn is_table(self) -> bool;

    /// Returns true if this flow is a table caption flow.
    fn is_table_caption(self) -> bool;

    /// Returns true if this flow is a proper table child.
    fn is_proper_table_child(self) -> bool;

    /// Returns true if this flow is a table row flow.
    fn is_table_row(self) -> bool;

    /// Returns true if this flow is a table cell flow.
    fn is_table_cell(self) -> bool;

    /// Returns true if this flow is a table colgroup flow.
    fn is_table_colgroup(self) -> bool;

    /// Returns true if this flow is a table rowgroup flow.
    fn is_table_rowgroup(self) -> bool;

    /// Returns true if this flow is one of table-related flows.
    fn is_table_kind(self) -> bool;

    /// Returns true if anonymous flow is needed between this flow and child flow.
    fn need_anonymous_flow(self, child: &Flow) -> bool;

    /// Generates missing child flow of this flow.
    fn generate_missing_child_flow(self, node: &ThreadSafeLayoutNode) -> FlowRef;

    /// Returns true if this flow has no children.
    fn is_leaf(self) -> bool;

    /// Returns the number of children that this flow possesses.
    fn child_count(self) -> uint;

    /// Return true if this flow is a Block Container.
    fn is_block_container(self) -> bool;

    /// Returns true if this flow is a block flow.
    fn is_block_flow(self) -> bool;

    /// Returns true if this flow is an inline flow.
    fn is_inline_flow(self) -> bool;

    /// Dumps the flow tree for debugging.
    fn dump(self);

    /// Dumps the flow tree for debugging, with a prefix to indicate that we're at the given level.
    fn dump_with_level(self, level: uint);
}

pub trait MutableFlowUtils {
    // Traversals

    /// Traverses the tree in preorder.
    fn traverse_preorder<T:PreorderFlowTraversal>(self, traversal: &mut T) -> bool;

    /// Traverses the tree in postorder.
    fn traverse_postorder<T:PostorderFlowTraversal>(self, traversal: &mut T) -> bool;

    // Mutators

    /// Computes the overflow region for this flow.
    fn store_overflow(self, _: &LayoutContext);

    /// Builds the display lists for this flow.
    fn build_display_list(self, layout_context: &LayoutContext);
}

pub trait MutableOwnedFlowUtils {
    /// Set absolute descendants for this flow.
    ///
    /// Set this flow as the Containing Block for all the absolute descendants.
    fn set_abs_descendants(&mut self, abs_descendants: AbsDescendants);
}

#[deriving(PartialEq, Show)]
pub enum FlowClass {
    BlockFlowClass,
    InlineFlowClass,
    TableWrapperFlowClass,
    TableFlowClass,
    TableColGroupFlowClass,
    TableRowGroupFlowClass,
    TableRowFlowClass,
    TableCaptionFlowClass,
    TableCellFlowClass,
}

/// A top-down traversal.
pub trait PreorderFlowTraversal {
    /// The operation to perform. Return true to continue or false to stop.
    fn process(&mut self, flow: &mut Flow) -> bool;

    /// Returns true if this node should be pruned. If this returns true, we skip the operation
    /// entirely and do not process any descendant nodes. This is called *before* child nodes are
    /// visited. The default implementation never prunes any nodes.
    fn should_prune(&mut self, _flow: &mut Flow) -> bool {
        false
    }
}

/// A bottom-up traversal, with a optional in-order pass.
pub trait PostorderFlowTraversal {
    /// The operation to perform. Return true to continue or false to stop.
    fn process(&mut self, flow: &mut Flow) -> bool;

    /// Returns false if this node must be processed in-order. If this returns false, we skip the
    /// operation for this node, but continue processing the ancestors. This is called *after*
    /// child nodes are visited.
    fn should_process(&mut self, _flow: &mut Flow) -> bool {
        true
    }

    /// Returns true if this node should be pruned. If this returns true, we skip the operation
    /// entirely and do not process any descendant nodes. This is called *before* child nodes are
    /// visited. The default implementation never prunes any nodes.
    fn should_prune(&mut self, _flow: &mut Flow) -> bool {
        false
    }
}

/// Flags used in flows, tightly packed to save space.
#[deriving(Clone)]
pub struct FlowFlags(pub u8);

/// The bitmask of flags that represent the `has_left_floated_descendants` and
/// `has_right_floated_descendants` fields.
///
/// NB: If you update this field, you must update the bitfields below.
static HAS_FLOATED_DESCENDANTS_BITMASK: u8 = 0b0000_0011;

// Whether this flow has descendants that float left in the same block formatting context.
bitfield!(FlowFlags, has_left_floated_descendants, set_has_left_floated_descendants, 0b0000_0001)

// Whether this flow has descendants that float right in the same block formatting context.
bitfield!(FlowFlags, has_right_floated_descendants, set_has_right_floated_descendants, 0b0000_0010)

// Whether this flow is impacted by floats to the left in the same block formatting context (i.e.
// its block-size depends on some prior flows with `float: left`).
bitfield!(FlowFlags, impacted_by_left_floats, set_impacted_by_left_floats, 0b0000_0100)

// Whether this flow is impacted by floats to the right in the same block formatting context (i.e.
// its block-size depends on some prior flows with `float: right`).
bitfield!(FlowFlags, impacted_by_right_floats, set_impacted_by_right_floats, 0b0000_1000)

/// The bitmask of flags that represent the text alignment field.
///
/// NB: If you update this field, you must update the bitfields below.
static TEXT_ALIGN_BITMASK: u8 = 0b0011_0000;

/// The number of bits we must shift off to handle the text alignment field.
///
/// NB: If you update this field, you must update the bitfields below.
static TEXT_ALIGN_SHIFT: u8 = 4;

// Whether this flow contains a flow that has its own layer within the same absolute containing
// block.
bitfield!(FlowFlags,
          layers_needed_for_descendants,
          set_layers_needed_for_descendants,
          0b0100_0000)

// Whether this flow must have its own layer. Even if this flag is not set, it might get its own
// layer if it's deemed to be likely to overlap flows with their own layer.
bitfield!(FlowFlags, needs_layer, set_needs_layer, 0b1000_0000)

impl FlowFlags {
    /// Creates a new set of flow flags.
    pub fn new() -> FlowFlags {
        FlowFlags(0)
    }

    /// Propagates text alignment flags from an appropriate parent flow per CSS 2.1.
    ///
    /// FIXME(#2265, pcwalton): It would be cleaner and faster to make this a derived CSS property
    /// `-servo-text-align-in-effect`.
    pub fn propagate_text_alignment_from_parent(&mut self, parent_flags: FlowFlags) {
        self.set_text_align_override(parent_flags);
    }

    #[inline]
    pub fn text_align(self) -> text_align::T {
        let FlowFlags(ff) = self;
        FromPrimitive::from_u8((ff & TEXT_ALIGN_BITMASK) >> TEXT_ALIGN_SHIFT as uint).unwrap()
    }

    #[inline]
    pub fn set_text_align(&mut self, value: text_align::T) {
        let FlowFlags(ff) = *self;
        *self = FlowFlags((ff & !TEXT_ALIGN_BITMASK) | ((value as u8) << TEXT_ALIGN_SHIFT as uint))
    }

    #[inline]
    pub fn set_text_align_override(&mut self, parent: FlowFlags) {
        let FlowFlags(ff) = *self;
        let FlowFlags(pff) = parent;
        *self = FlowFlags(ff | (pff & TEXT_ALIGN_BITMASK))
    }

    #[inline]
    pub fn union_floated_descendants_flags(&mut self, other: FlowFlags) {
        let FlowFlags(my_flags) = *self;
        let FlowFlags(other_flags) = other;
        *self = FlowFlags(my_flags | (other_flags & HAS_FLOATED_DESCENDANTS_BITMASK))
    }

    #[inline]
    pub fn impacted_by_floats(&self) -> bool {
        self.impacted_by_left_floats() || self.impacted_by_right_floats()
    }
}

/// The Descendants of a flow.
///
/// Also, details about their position wrt this flow.
pub struct Descendants {
    /// Links to every descendant. This must be private because it is unsafe to leak `FlowRef`s to
    /// layout.
    descendant_links: Vec<FlowRef>,

    /// Static y offsets of all descendants from the start of this flow box.
    pub static_b_offsets: Vec<Au>,
}

impl Descendants {
    pub fn new() -> Descendants {
        Descendants {
            descendant_links: Vec::new(),
            static_b_offsets: Vec::new(),
        }
    }

    pub fn len(&self) -> uint {
        self.descendant_links.len()
    }

    pub fn push(&mut self, given_descendant: FlowRef) {
        self.descendant_links.push(given_descendant);
    }

    /// Push the given descendants on to the existing descendants.
    ///
    /// Ignore any static y offsets, because they are None before layout.
    pub fn push_descendants(&mut self, given_descendants: Descendants) {
        for elem in given_descendants.descendant_links.move_iter() {
            self.descendant_links.push(elem);
        }
    }

    /// Return an iterator over the descendant flows.
    pub fn iter<'a>(&'a mut self) -> DescendantIter<'a> {
        DescendantIter {
            iter: self.descendant_links.mut_slice_from(0).mut_iter(),
        }
    }

    /// Return an iterator over (descendant, static y offset).
    pub fn iter_with_offset<'a>(&'a mut self) -> DescendantOffsetIter<'a> {
        let descendant_iter = DescendantIter {
            iter: self.descendant_links.mut_slice_from(0).mut_iter(),
        };
        descendant_iter.zip(self.static_b_offsets.mut_slice_from(0).mut_iter())
    }
}

pub type AbsDescendants = Descendants;

pub struct DescendantIter<'a> {
    iter: MutItems<'a, FlowRef>,
}

impl<'a> Iterator<&'a mut Flow> for DescendantIter<'a> {
    fn next(&mut self) -> Option<&'a mut Flow> {
        match self.iter.next() {
            None => None,
            Some(ref mut flow) => {
                unsafe {
                    let result: &'a mut Flow = mem::transmute(flow.get_mut());
                    Some(result)
                }
            }
        }
    }
}

pub type DescendantOffsetIter<'a> = Zip<DescendantIter<'a>, MutItems<'a, Au>>;

/// Information needed to compute absolute (i.e. viewport-relative) flow positions (not to be
/// confused with absolutely-positioned flows).
pub struct AbsolutePositionInfo {
    /// The size of the containing block for relatively-positioned descendants.
    pub relative_containing_block_size: LogicalSize<Au>,
    /// The position of the absolute containing block.
    pub absolute_containing_block_position: Point2D<Au>,
    /// Whether the absolute containing block forces positioned descendants to be layerized.
    ///
    /// FIXME(pcwalton): Move into `FlowFlags`.
    pub layers_needed_for_positioned_flows: bool,
}

impl AbsolutePositionInfo {
    pub fn new(writing_mode: WritingMode) -> AbsolutePositionInfo {
        // FIXME(pcwalton): The initial relative containing block-size should be equal to the size
        // of the root layer.
        AbsolutePositionInfo {
            relative_containing_block_size: LogicalSize::zero(writing_mode),
            absolute_containing_block_position: Zero::zero(),
            layers_needed_for_positioned_flows: false,
        }
    }
}

/// Data common to all flows.
pub struct BaseFlow {
    /// NB: Must be the first element.
    ///
    /// The necessity of this will disappear once we have dynamically-sized types.
    ref_count: AtomicUint,

    pub restyle_damage: RestyleDamage,

    /// The children of this flow.
    pub children: FlowList,
    pub next_sibling: Link,
    pub prev_sibling: Link,

    /* layout computations */
    // TODO: min/pref and position are used during disjoint phases of
    // layout; maybe combine into a single enum to save space.
    pub intrinsic_inline_sizes: IntrinsicISizes,

    /// The upper left corner of the box representing this flow, relative to the box representing
    /// its parent flow.
    ///
    /// For absolute flows, this represents the position with respect to its *containing block*.
    ///
    /// This does not include margins in the block flow direction, because those can collapse. So
    /// for the block direction (usually vertical), this represents the *border box*. For the
    /// inline direction (usually horizontal), this represents the *margin box*.
    pub position: LogicalRect<Au>,

    /// The amount of overflow of this flow, relative to the containing block. Must include all the
    /// pixels of all the display list items for correct invalidation.
    pub overflow: LogicalRect<Au>,

    /// Data used during parallel traversals.
    ///
    /// TODO(pcwalton): Group with other transient data to save space.
    pub parallel: FlowParallelInfo,

    /// The floats next to this flow.
    pub floats: Floats,

    /// The collapsible margins for this flow, if any.
    pub collapsible_margins: CollapsibleMargins,

    /// The position of this flow in page coordinates, computed during display list construction.
    pub abs_position: Point2D<Au>,

    /// Details about descendants with position 'absolute' or 'fixed' for which we are the
    /// containing block. This is in tree order. This includes any direct children.
    pub abs_descendants: AbsDescendants,

    /// Offset wrt the nearest positioned ancestor - aka the Containing Block
    /// for any absolutely positioned elements.
    pub absolute_static_i_offset: Au,

    /// Offset wrt the Initial Containing Block.
    pub fixed_static_i_offset: Au,

    /// Reference to the Containing Block, if this flow is absolutely positioned.
    pub absolute_cb: ContainingBlockLink,

    /// Information needed to compute absolute (i.e. viewport-relative) flow positions (not to be
    /// confused with absolutely-positioned flows).
    ///
    /// FIXME(pcwalton): Merge with `absolute_static_i_offset` and `fixed_static_i_offset` above?
    pub absolute_position_info: AbsolutePositionInfo,

    /// The unflattened display items for this flow.
    pub display_list: DisplayList,

    /// Any layers that we're bubbling up, in a linked list.
    pub layers: DList<RenderLayer>,

    /// Various flags for flows, tightly packed to save space.
    pub flags: FlowFlags,

    pub writing_mode: WritingMode,
}

#[unsafe_destructor]
impl Drop for BaseFlow {
    fn drop(&mut self) {
        if self.ref_count.load(SeqCst) != 0 {
            fail!("Flow destroyed before its ref count hit zero—this is unsafe!")
        }
    }
}

impl BaseFlow {
    #[inline]
    pub fn new(node: ThreadSafeLayoutNode) -> BaseFlow {
        let writing_mode = node.style().writing_mode;
        BaseFlow {
            ref_count: AtomicUint::new(1),

            restyle_damage: node.restyle_damage(),

            children: FlowList::new(),
            next_sibling: None,
            prev_sibling: None,

            intrinsic_inline_sizes: IntrinsicISizes::new(),
            position: LogicalRect::zero(writing_mode),
            overflow: LogicalRect::zero(writing_mode),

            parallel: FlowParallelInfo::new(),

            floats: Floats::new(writing_mode),
            collapsible_margins: CollapsibleMargins::new(),
            abs_position: Zero::zero(),
            abs_descendants: Descendants::new(),
            absolute_static_i_offset: Au::new(0),
            fixed_static_i_offset: Au::new(0),
            absolute_cb: ContainingBlockLink::new(),
            display_list: DisplayList::new(),
            layers: DList::new(),
            absolute_position_info: AbsolutePositionInfo::new(writing_mode),

            flags: FlowFlags::new(),
            writing_mode: writing_mode,
        }
    }

    pub fn child_iter<'a>(&'a mut self) -> MutFlowListIterator<'a> {
        self.children.mut_iter()
    }

    pub unsafe fn ref_count<'a>(&'a self) -> &'a AtomicUint {
        &self.ref_count
    }
}

impl<'a> ImmutableFlowUtils for &'a Flow {
    /// Returns true if this flow is a block or a float flow.
    fn is_block_like(self) -> bool {
        match self.class() {
            BlockFlowClass => true,
            _ => false,
        }
    }

    /// Returns true if this flow is a proper table child.
    /// 'Proper table child' is defined as table-row flow, table-rowgroup flow,
    /// table-column-group flow, or table-caption flow.
    fn is_proper_table_child(self) -> bool {
        match self.class() {
            TableRowFlowClass | TableRowGroupFlowClass |
                TableColGroupFlowClass | TableCaptionFlowClass => true,
            _ => false,
        }
    }

    /// Returns true if this flow is a table row flow.
    fn is_table_row(self) -> bool {
        match self.class() {
            TableRowFlowClass => true,
            _ => false,
        }
    }

    /// Returns true if this flow is a table cell flow.
    fn is_table_cell(self) -> bool {
        match self.class() {
            TableCellFlowClass => true,
            _ => false,
        }
    }

    /// Returns true if this flow is a table colgroup flow.
    fn is_table_colgroup(self) -> bool {
        match self.class() {
            TableColGroupFlowClass => true,
            _ => false,
        }
    }

    /// Returns true if this flow is a table flow.
    fn is_table(self) -> bool {
        match self.class() {
            TableFlowClass => true,
            _ => false,
        }
    }

    /// Returns true if this flow is a table caption flow.
    fn is_table_caption(self) -> bool {
        match self.class() {
            TableCaptionFlowClass => true,
            _ => false,
        }
    }

    /// Returns true if this flow is a table rowgroup flow.
    fn is_table_rowgroup(self) -> bool {
        match self.class() {
            TableRowGroupFlowClass => true,
            _ => false,
        }
    }

    /// Returns true if this flow is one of table-related flows.
    fn is_table_kind(self) -> bool {
        match self.class() {
            TableWrapperFlowClass | TableFlowClass |
                TableColGroupFlowClass | TableRowGroupFlowClass |
                TableRowFlowClass | TableCaptionFlowClass | TableCellFlowClass => true,
            _ => false,
        }
    }

    /// Returns true if anonymous flow is needed between this flow and child flow.
    /// Spec: http://www.w3.org/TR/CSS21/tables.html#anonymous-boxes
    fn need_anonymous_flow(self, child: &Flow) -> bool {
        match self.class() {
            TableFlowClass => !child.is_proper_table_child(),
            TableRowGroupFlowClass => !child.is_table_row(),
            TableRowFlowClass => !child.is_table_cell(),
            _ => false
        }
    }

    /// Generates missing child flow of this flow.
    fn generate_missing_child_flow(self, node: &ThreadSafeLayoutNode) -> FlowRef {
        let flow = match self.class() {
            TableFlowClass | TableRowGroupFlowClass => {
                let fragment = Fragment::new_anonymous_table_fragment(node, TableRowFragment);
                box TableRowFlow::from_node_and_fragment(node, fragment) as Box<Flow>
            },
            TableRowFlowClass => {
                let fragment = Fragment::new_anonymous_table_fragment(node, TableCellFragment);
                box TableCellFlow::from_node_and_fragment(node, fragment) as Box<Flow>
            },
            _ => {
                fail!("no need to generate a missing child")
            }
        };
        FlowRef::new(flow)
    }

    /// Returns true if this flow has no children.
    fn is_leaf(self) -> bool {
        base(self).children.len() == 0
    }

    /// Returns the number of children that this flow possesses.
    fn child_count(self) -> uint {
        base(self).children.len()
    }

    /// Return true if this flow is a Block Container.
    ///
    /// Except for table fragments and replaced elements, block-level fragments (`BlockFlow`) are
    /// also block container fragments.
    /// Non-replaced inline blocks and non-replaced table cells are also block
    /// containers.
    fn is_block_container(self) -> bool {
        match self.class() {
            // TODO: Change this when inline-blocks are supported.
            BlockFlowClass | TableCaptionFlowClass | TableCellFlowClass => {
                // FIXME: Actually check the type of the node
                self.child_count() != 0
            }
            _ => false,
        }
    }

    /// Returns true if this flow is a block flow.
    fn is_block_flow(self) -> bool {
        match self.class() {
            BlockFlowClass => true,
            _ => false,
        }
    }

    /// Returns true if this flow is an inline flow.
    fn is_inline_flow(self) -> bool {
        match self.class() {
            InlineFlowClass => true,
            _ => false,
        }
    }

    /// Dumps the flow tree for debugging.
    fn dump(self) {
        self.dump_with_level(0)
    }

    /// Dumps the flow tree for debugging, with a prefix to indicate that we're at the given level.
    fn dump_with_level(self, level: uint) {
        let mut indent = String::new();
        for _ in range(0, level) {
            indent.push_str("| ")
        }
        debug!("{}+ {}", indent, self.to_string());
        for kid in imm_child_iter(self) {
            kid.dump_with_level(level + 1)
        }
    }
}

impl<'a> MutableFlowUtils for &'a mut Flow {
    /// Traverses the tree in preorder.
    fn traverse_preorder<T:PreorderFlowTraversal>(self, traversal: &mut T) -> bool {
        if traversal.should_prune(self) {
            return true
        }

        if !traversal.process(self) {
            return false
        }

        for kid in child_iter(self) {
            if !kid.traverse_preorder(traversal) {
                return false
            }
        }

        true
    }

    /// Traverses the tree in postorder.
    fn traverse_postorder<T:PostorderFlowTraversal>(self, traversal: &mut T) -> bool {
        if traversal.should_prune(self) {
            return true
        }

        for kid in child_iter(self) {
            if !kid.traverse_postorder(traversal) {
                return false
            }
        }

        if !traversal.should_process(self) {
            return true
        }

        traversal.process(self)
    }

    /// Calculate and set overflow for current flow.
    ///
    /// CSS Section 11.1
    /// This is the union of rectangles of the flows for which we define the
    /// Containing Block.
    ///
    /// Assumption: This is called in a bottom-up traversal, so kids' overflows have
    /// already been set.
    /// Assumption: Absolute descendants have had their overflow calculated.
    fn store_overflow(self, _: &LayoutContext) {
        let my_position = mut_base(self).position;
        let mut overflow = my_position;

        if self.is_block_container() {
            for kid in child_iter(self) {
                if kid.is_store_overflow_delayed() {
                    // Absolute flows will be handled by their CB. If we are
                    // their CB, they will show up in `abs_descendants`.
                    continue;
                }
                let mut kid_overflow = base(kid).overflow;
                kid_overflow = kid_overflow.translate(&my_position.start);
                overflow = overflow.union(&kid_overflow)
            }

            // FIXME(#2004, pcwalton): This is wrong for `position: fixed`.
            for descendant_link in mut_base(self).abs_descendants.iter() {
                let mut kid_overflow = base(descendant_link).overflow;
                kid_overflow = kid_overflow.translate(&my_position.start);
                overflow = overflow.union(&kid_overflow)
            }
        }
        mut_base(self).overflow = overflow;
    }

    /// Push display items for current flow and its descendants onto the appropriate display lists
    /// of the given stacking context.
    ///
    /// Arguments:
    ///
    /// * `builder`: The display list builder, which contains information used during the entire
    ///   display list building pass.
    ///
    /// * `info`: Per-flow display list building information.
    fn build_display_list(self, layout_context: &LayoutContext) {
        debug!("Flow: building display list");
        match self.class() {
            BlockFlowClass => self.as_block().build_display_list_block(layout_context),
            InlineFlowClass => self.as_inline().build_display_list_inline(layout_context),
            TableWrapperFlowClass => {
                self.as_table_wrapper().build_display_list_table_wrapper(layout_context)
            }
            TableFlowClass => self.as_table().build_display_list_table(layout_context),
            TableRowGroupFlowClass => {
                self.as_table_rowgroup().build_display_list_table_rowgroup(layout_context)
            }
            TableRowFlowClass => self.as_table_row().build_display_list_table_row(layout_context),
            TableCaptionFlowClass => {
                self.as_table_caption().build_display_list_table_caption(layout_context)
            }
            TableCellFlowClass => {
                self.as_table_cell().build_display_list_table_cell(layout_context)
            }
            TableColGroupFlowClass => {
                // Nothing to do here, as column groups don't render.
            }
        }
    }
}

impl MutableOwnedFlowUtils for FlowRef {
    /// Set absolute descendants for this flow.
    ///
    /// Set yourself as the Containing Block for all the absolute descendants.
    ///
    /// This is called during flow construction, so nothing else can be accessing the descendant
    /// flows. This is enforced by the fact that we have a mutable `FlowRef`, which only flow
    /// construction is allowed to possess.
    fn set_abs_descendants(&mut self, abs_descendants: AbsDescendants) {
        let this = self.clone();

        let block = self.get_mut().as_block();
        block.base.abs_descendants = abs_descendants;
        block.base
             .parallel
             .children_and_absolute_descendant_count
             .fetch_add(block.base.abs_descendants.len() as int, Relaxed);

        for descendant_link in block.base.abs_descendants.iter() {
            let base = mut_base(descendant_link);
            base.absolute_cb.set(this.clone());
        }
    }
}

/// A link to a flow's containing block.
///
/// This cannot safely be a `Flow` pointer because this is a pointer *up* the tree, not *down* the
/// tree. A pointer up the tree is unsafe during layout because it can be used to access a node
/// with an immutable reference while that same node is being laid out, causing possible iterator
/// invalidation and use-after-free.
///
/// FIXME(pcwalton): I think this would be better with a borrow flag instead of `unsafe`.
pub struct ContainingBlockLink {
    /// The pointer up to the containing block.
    link: Option<FlowRef>,
}

impl ContainingBlockLink {
    fn new() -> ContainingBlockLink {
        ContainingBlockLink {
            link: None,
        }
    }

    fn set(&mut self, link: FlowRef) {
        self.link = Some(link)
    }

    pub unsafe fn get<'a>(&'a mut self) -> &'a mut Option<FlowRef> {
        &mut self.link
    }

    #[inline]
    pub fn generated_containing_block_rect(&mut self) -> LogicalRect<Au> {
        match self.link {
            None => fail!("haven't done it"),
            Some(ref mut link) => link.get_mut().generated_containing_block_rect(),
        }
    }
}