Move paragraph widow and orphan prevention into flow (#4767)

4 files changed, 48 insertions, 41 deletions

diff --git a/crates/typst/src/layout/flow.rs b/crates/typst/src/layout/flow.rs
index a1f1402e..e15159e1 100644
--- a/crates/typst/src/layout/flow.rs
+++ b/crates/typst/src/layout/flow.rs
@@ -14,10 +14,12 @@ use crate::foundations::{
 use crate::introspection::{Locator, SplitLocator, Tag, TagElem};
 use crate::layout::{
     Abs, AlignElem, Axes, BlockElem, ColbreakElem, FixedAlignment, FlushElem, Fr,
-    Fragment, Frame, FrameItem, PlaceElem, Point, Regions, Rel, Size, Spacing, VElem,
+    Fragment, Frame, FrameItem, PlaceElem, Point, Ratio, Regions, Rel, Size, Spacing,
+    VElem,
 };
 use crate::model::{FootnoteElem, FootnoteEntry, ParElem};
 use crate::realize::StyleVec;
+use crate::text::TextElem;
 use crate::utils::Numeric;
 
 /// Arranges spacing, paragraphs and block-level elements into a flow.
@@ -278,6 +280,7 @@ impl<'a, 'e> FlowLayouter<'a, 'e> {
         // Fetch properties.
         let align = AlignElem::alignment_in(styles).resolve(styles);
         let leading = ParElem::leading_in(styles);
+        let costs = TextElem::costs_in(styles);
 
         // Layout the paragraph into lines. This only depends on the base size,
         // not on the Y position.
@@ -305,12 +308,51 @@ impl<'a, 'e> FlowLayouter<'a, 'e> {
             }
         }
 
+        // Determine whether to prevent widow and orphans.
+        let len = lines.len();
+        let prevent_orphans =
+            costs.orphan() > Ratio::zero() && len >= 2 && !lines[1].is_empty();
+        let prevent_widows =
+            costs.widow() > Ratio::zero() && len >= 2 && !lines[len - 2].is_empty();
+        let prevent_all = len == 3 && prevent_orphans && prevent_widows;
+
+        // Store the heights of lines at the edges because we'll potentially
+        // need these later when `lines` is already moved.
+        let height_at = |i| lines.get(i).map(Frame::height).unwrap_or_default();
+        let front_1 = height_at(0);
+        let front_2 = height_at(1);
+        let back_2 = height_at(len.saturating_sub(2));
+        let back_1 = height_at(len.saturating_sub(1));
+
         // Layout the lines.
         for (i, mut frame) in lines.into_iter().enumerate() {
             if i > 0 {
                 self.handle_item(FlowItem::Absolute(leading, true))?;
             }
 
+            // To prevent widows and orphans, we require enough space for
+            // - all lines if it's just three
+            // - the first two lines if we're at the first line
+            // - the last two lines if we're at the second to last line
+            let needed = if prevent_all && i == 0 {
+                front_1 + leading + front_2 + leading + back_1
+            } else if prevent_orphans && i == 0 {
+                front_1 + leading + front_2
+            } else if prevent_widows && i >= 2 && i + 2 == len {
+                back_2 + leading + back_1
+            } else {
+                frame.height()
+            };
+
+            // If the line(s) don't fit into this region, but they do fit into
+            // the next, then advance.
+            if !self.regions.in_last()
+                && !self.regions.size.y.fits(needed)
+                && self.regions.iter().nth(1).is_some_and(|region| region.y.fits(needed))
+            {
+                self.finish_region(false)?;
+            }
+
             self.drain_tag(&mut frame);
             self.handle_item(FlowItem::Frame {
                 frame,
diff --git a/crates/typst/src/layout/inline/collect.rs b/crates/typst/src/layout/inline/collect.rs
index b6a847f5..5021dc55 100644
--- a/crates/typst/src/layout/inline/collect.rs
+++ b/crates/typst/src/layout/inline/collect.rs
@@ -128,11 +128,11 @@ pub fn collect<'a>(
     let mut iter = children.chain(styles).peekable();
     let mut locator = locator.split();
 
+    let outer_dir = TextElem::dir_in(*styles);
     let first_line_indent = ParElem::first_line_indent_in(*styles);
     if !first_line_indent.is_zero()
         && consecutive
-        && AlignElem::alignment_in(*styles).resolve(*styles).x
-            == TextElem::dir_in(*styles).start().into()
+        && AlignElem::alignment_in(*styles).resolve(*styles).x == outer_dir.start().into()
     {
         collector.push_item(Item::Absolute(first_line_indent.resolve(*styles), false));
         collector.spans.push(1, Span::detached());
@@ -144,8 +144,6 @@ pub fn collect<'a>(
         collector.spans.push(1, Span::detached());
     }
 
-    let outer_dir = TextElem::dir_in(*styles);
-
     while let Some((child, styles)) = iter.next() {
         let prev_len = collector.full.len();
 
diff --git a/crates/typst/src/layout/inline/finalize.rs b/crates/typst/src/layout/inline/finalize.rs
index c8ba4729..03493af5 100644
--- a/crates/typst/src/layout/inline/finalize.rs
+++ b/crates/typst/src/layout/inline/finalize.rs
@@ -1,5 +1,4 @@
 use super::*;
-use crate::layout::{Abs, Frame, Point};
 use crate::utils::Numeric;
 
 /// Turns the selected lines into frames.
@@ -26,38 +25,9 @@ pub fn finalize(
 
     // Stack the lines into one frame per region.
     let shrink = ParElem::shrink_in(styles);
-    let mut frames: Vec<Frame> = lines
+    lines
         .iter()
         .map(|line| commit(engine, p, line, width, region.y, shrink))
-        .collect::<SourceResult<_>>()?;
-
-    // Positive ratios enable prevention, while zero and negative ratios disable
-    // it.
-    if p.costs.orphan().get() > 0.0 {
-        // Prevent orphans.
-        if frames.len() >= 2 && !frames[1].is_empty() {
-            let second = frames.remove(1);
-            let first = &mut frames[0];
-            merge(first, second, p.leading);
-        }
-    }
-    if p.costs.widow().get() > 0.0 {
-        // Prevent widows.
-        let len = frames.len();
-        if len >= 2 && !frames[len - 2].is_empty() {
-            let second = frames.pop().unwrap();
-            let first = frames.last_mut().unwrap();
-            merge(first, second, p.leading);
-        }
-    }
-
-    Ok(Fragment::frames(frames))
-}
-
-/// Merge two line frames
-fn merge(first: &mut Frame, second: Frame, leading: Abs) {
-    let offset = first.height() + leading;
-    let total = offset + second.height();
-    first.push_frame(Point::with_y(offset), second);
-    first.size_mut().y = total;
+        .collect::<SourceResult<_>>()
+        .map(Fragment::frames)
 }
diff --git a/crates/typst/src/layout/inline/prepare.rs b/crates/typst/src/layout/inline/prepare.rs
index 59682b2c..9e73af66 100644
--- a/crates/typst/src/layout/inline/prepare.rs
+++ b/crates/typst/src/layout/inline/prepare.rs
@@ -43,8 +43,6 @@ pub struct Preparation<'a> {
     pub cjk_latin_spacing: bool,
     /// Whether font fallback is enabled for this paragraph.
     pub fallback: bool,
-    /// The leading of the paragraph.
-    pub leading: Abs,
     /// How to determine line breaks.
     pub linebreaks: Smart<Linebreaks>,
     /// The text size.
@@ -136,7 +134,6 @@ pub fn prepare<'a>(
         hang: ParElem::hanging_indent_in(styles),
         cjk_latin_spacing,
         fallback: TextElem::fallback_in(styles),
-        leading: ParElem::leading_in(styles),
         linebreaks: ParElem::linebreaks_in(styles),
         size: TextElem::size_in(styles),
     })