Implement sophisticated CJK punctuation adjustment (#954)

1 files changed, 58 insertions, 15 deletions

diff --git a/library/src/layout/par.rs b/library/src/layout/par.rs
index 2edbd93d..96d45f2f 100644
--- a/library/src/layout/par.rs
+++ b/library/src/layout/par.rs
@@ -8,7 +8,8 @@ use crate::layout::AlignElem;
 use crate::math::EquationElem;
 use crate::prelude::*;
 use crate::text::{
-    shape, LinebreakElem, Quoter, Quotes, ShapedText, SmartQuoteElem, SpaceElem, TextElem,
+    is_gb_style, shape, LinebreakElem, Quoter, Quotes, ShapedText, SmartQuoteElem,
+    SpaceElem, TextElem,
 };
 
 /// Arrange text, spacing and inline-level elements into a paragraph.
@@ -354,6 +355,13 @@ impl<'a> Item<'a> {
         }
     }
 
+    fn text_mut(&mut self) -> Option<&mut ShapedText<'a>> {
+        match self {
+            Self::Text(shaped) => Some(shaped),
+            _ => None,
+        }
+    }
+
     /// The text length of the item.
     fn len(&self) -> usize {
         match self {
@@ -715,9 +723,12 @@ fn shape_range<'a>(
     spans: &SpanMapper,
     styles: StyleChain<'a>,
 ) {
+    let lang = TextElem::lang_in(styles);
+    let region = TextElem::region_in(styles);
     let mut process = |range: Range, level: BidiLevel| {
         let dir = if level.is_ltr() { Dir::LTR } else { Dir::RTL };
-        let shaped = shape(vt, range.start, &bidi.text[range], spans, styles, dir);
+        let shaped =
+            shape(vt, range.start, &bidi.text[range], spans, styles, dir, lang, region);
         items.push(Item::Text(shaped));
     };
 
@@ -905,15 +916,11 @@ fn linebreak_optimized<'a>(vt: &Vt, p: &'a Preparation<'a>, width: Abs) -> Vec<L
                 // This often happens with monospace fonts and CJK texts.
                 ratio = 0.0;
             }
-            if ratio.is_infinite() {
-                // The line's not stretchable, we calculate the ratio in another way...
-                ratio = delta / (em / 2.0);
-                // ...and because it is underfull/overfull, make sure the ratio is at least 1.0.
-                if ratio > 0.0 {
-                    ratio += 1.0;
-                } else {
-                    ratio -= 1.0;
-                }
+            if ratio > 1.0 {
+                // We should stretch the line above its stretchability. Now calculate the extra amount.
+                let extra_stretch = (delta - adjust) / attempt.justifiables() as f64;
+                // Normalize the amount by half Em size.
+                ratio = 1.0 + extra_stretch / (em / 2.0);
             }
 
             // Determine the cost of the line.
@@ -1124,7 +1131,9 @@ fn line<'a>(
         let base = expanded.end - shaped.text.len();
         let start = range.start.max(base);
         let text = &p.bidi.text[start..range.end];
-        let trimmed = text.trim_end();
+        // U+200B ZERO WIDTH SPACE is used to provide a line break opportunity,
+        // we want to trim it too.
+        let trimmed = text.trim_end().trim_end_matches('\u{200B}');
         range.end = start + trimmed.len();
 
         // Deal with hyphens, dashes and justification.
@@ -1132,6 +1141,11 @@ fn line<'a>(
         dash = hyphen || shy || trimmed.ends_with(['-', '–', '—']);
         justify |= text.ends_with('\u{2028}');
 
+        // Deal with CJK punctuation at line ends.
+        let gb_style = is_gb_style(shaped.lang, shaped.region);
+        let end_cjk_punct = trimmed
+            .ends_with(['”', '’', '，', '。', '、', '：', '；', '》', '）', '』', '」']);
+
         // Usually, we don't want to shape an empty string because:
         // - We don't want the height of trimmed whitespace in a different
         //   font to be considered for the line height.
@@ -1141,12 +1155,21 @@ fn line<'a>(
         // need the shaped empty string to make the line the appropriate
         // height. That is the case exactly if the string is empty and there
         // are no other items in the line.
-        if hyphen || start + shaped.text.len() > range.end {
+        if hyphen || start + shaped.text.len() > range.end || end_cjk_punct {
             if hyphen || start < range.end || before.is_empty() {
                 let mut reshaped = shaped.reshape(vt, &p.spans, start..range.end);
                 if hyphen || shy {
                     reshaped.push_hyphen(vt);
                 }
+                let punct = reshaped.glyphs.last();
+                if let Some(punct) = punct {
+                    if punct.is_cjk_left_aligned_punctuation(gb_style) {
+                        let shrink_amount = punct.shrinkability().1;
+                        let punct = reshaped.glyphs.to_mut().last_mut().unwrap();
+                        punct.shrink_right(shrink_amount);
+                        reshaped.width -= shrink_amount.at(reshaped.size);
+                    }
+                }
                 width += reshaped.width;
                 last = Some(Item::Text(reshaped));
             }
@@ -1155,6 +1178,10 @@ fn line<'a>(
         }
     }
 
+    // Deal with CJK punctuation at line starts.
+    let text = &p.bidi.text[range.start..end];
+    let start_cjk_punct = text.starts_with(['“', '‘', '《', '（', '『', '「']);
+
     // Reshape the start item if it's split in half.
     let mut first = None;
     if let Some((Item::Text(shaped), after)) = inner.split_first() {
@@ -1163,8 +1190,8 @@ fn line<'a>(
         let end = range.end.min(base + shaped.text.len());
 
         // Reshape if necessary.
-        if range.start + shaped.text.len() > end {
-            if range.start < end {
+        if range.start + shaped.text.len() > end || start_cjk_punct {
+            if range.start < end || start_cjk_punct {
                 let reshaped = shaped.reshape(vt, &p.spans, range.start..end);
                 width += reshaped.width;
                 first = Some(Item::Text(reshaped));
@@ -1174,6 +1201,22 @@ fn line<'a>(
         }
     }
 
+    if start_cjk_punct {
+        let reshaped = first.as_mut().or(last.as_mut()).and_then(Item::text_mut);
+        if let Some(reshaped) = reshaped {
+            if let Some(punct) = reshaped.glyphs.first() {
+                if punct.is_cjk_right_aligned_punctuation() {
+                    let shrink_amount = punct.shrinkability().0;
+                    let punct = reshaped.glyphs.to_mut().first_mut().unwrap();
+                    punct.shrink_left(shrink_amount);
+                    let amount_abs = shrink_amount.at(reshaped.size);
+                    reshaped.width -= amount_abs;
+                    width -= amount_abs;
+                }
+            }
+        }
+    }
+
     // Measure the inner items.
     for item in inner {
         width += item.width();