From faf52e726fa54421dec713a6045f98b92bb266da Mon Sep 17 00:00:00 2001
From: bigfarts <bigfarts@punymail.com>
Date: Mon, 29 Aug 2022 19:23:52 -0700
Subject: [PATCH] Improve mixed CJK/Latin linebreaking.
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(actually the K part is a lie because I don't know much about Korean typesetting, but it should be easy to implement)

This changes the break on space rule to break on space, CJK, or pre-CJK, e.g.:

    aaaあああ
       ^ break is inserted here
---
 crates/epaint/src/text/text_layout.rs | 80 +++++++++++++++++++++------
 1 file changed, 62 insertions(+), 18 deletions(-)

diff --git a/crates/epaint/src/text/text_layout.rs b/crates/epaint/src/text/text_layout.rs
index 801ea3ec8dc..a690a1d0abc 100644
--- a/crates/epaint/src/text/text_layout.rs
+++ b/crates/epaint/src/text/text_layout.rs
@@ -719,11 +719,11 @@ struct RowBreakCandidates {
     /// is always the primary candidate.
     space: Option<usize>,
 
-    /// Logograms (single character representing a whole word) are good candidates for line break.
-    logogram: Option<usize>,
+    /// Logograms (single character representing a whole word) or kana (Japanese hiragana and katakana) are good candidates for line break.
+    cjk: Option<usize>,
 
-    /// Kana (Japanese hiragana and katakana) may be line broken unless before a gyōtō kinsoku character.
-    kana: Option<usize>,
+    /// Breaking anywhere before a CJK character is acceptable too.
+    pre_cjk: Option<usize>,
 
     /// Breaking at a dash is a super-
     /// good idea.
@@ -744,27 +744,30 @@ impl RowBreakCandidates {
         const NON_BREAKING_SPACE: char = '\u{A0}';
         if chr.is_whitespace() && chr != NON_BREAKING_SPACE {
             self.space = Some(index);
-        } else if is_cjk_ideograph(chr) {
-            self.logogram = Some(index);
+        } else if is_cjk(chr) && (glyphs.len() == 1 || is_cjk_break_allowed(glyphs[1].chr)) {
+            self.cjk = Some(index);
         } else if chr == '-' {
             self.dash = Some(index);
         } else if chr.is_ascii_punctuation() {
             self.punctuation = Some(index);
-        } else if is_kana(chr) && (glyphs.len() == 1 || !is_gyoto_kinsoku(glyphs[1].chr)) {
-            self.kana = Some(index);
+        } else if glyphs.len() > 1 && is_cjk(glyphs[1].chr) {
+            self.pre_cjk = Some(index);
         }
         self.any = Some(index);
     }
 
-    fn has_word_boundary(&self) -> bool {
-        self.space.is_some() || self.logogram.is_some()
+    fn word_boundary(&self) -> Option<usize> {
+        [self.space, self.cjk, self.pre_cjk]
+            .into_iter()
+            .max()
+            .flatten()
     }
 
     fn has_good_candidate(&self, break_anywhere: bool) -> bool {
         if break_anywhere {
             self.any.is_some()
         } else {
-            self.has_word_boundary()
+            self.word_boundary().is_some()
         }
     }
 
@@ -772,9 +775,7 @@ impl RowBreakCandidates {
         if break_anywhere {
             self.any
         } else {
-            self.space
-                .or(self.kana)
-                .or(self.logogram)
+            self.word_boundary()
                 .or(self.dash)
                 .or(self.punctuation)
                 .or(self.any)
@@ -796,10 +797,15 @@ fn is_kana(c: char) -> bool {
 }
 
 #[inline]
-fn is_gyoto_kinsoku(c: char) -> bool {
-    // Gyōtō (meaning "beginning of line") kinsoku characters in Japanese typesetting are characters that may not appear at the start of a line, according to kinsoku shori rules.
-    // The list of gyōtō kinsoku characters can be found at https://en.wikipedia.org/wiki/Line_breaking_rules_in_East_Asian_languages#Characters_not_permitted_on_the_start_of_a_line.
-    ")]｝〕〉》」』】〙〗〟'\"｠»ヽヾーァィゥェォッャュョヮヵヶぁぃぅぇぉっゃゅょゎゕゖㇰㇱㇲㇳㇴㇵㇶㇷㇸㇹㇺㇻㇼㇽㇾㇿ々〻‐゠–〜?!‼⁇⁈⁉・、:;,。.".contains(c)
+fn is_cjk(c: char) -> bool {
+    // TODO: Add support for Korean Hangul.
+    is_cjk_ideograph(c) || is_kana(c)
+}
+
+#[inline]
+fn is_cjk_break_allowed(c: char) -> bool {
+    // See: https://en.wikipedia.org/wiki/Line_breaking_rules_in_East_Asian_languages#Characters_not_permitted_on_the_start_of_a_line.
+    !")]｝〕〉》」』】〙〗〟'\"｠»ヽヾーァィゥェォッャュョヮヵヶぁぃぅぇぉっゃゅょゎゕゖㇰㇱㇲㇳㇴㇵㇶㇷㇸㇹㇺㇻㇼㇽㇾㇿ々〻‐゠–〜?!‼⁇⁈⁉・、:;,。.".contains(c)
 }
 
 // ----------------------------------------------------------------------------
@@ -812,3 +818,41 @@ fn test_zero_max_width() {
     let galley = super::layout(&mut fonts, layout_job.into());
     assert_eq!(galley.rows.len(), 1);
 }
+
+#[test]
+fn test_cjk() {
+    let mut fonts = FontsImpl::new(1.0, 1024, super::FontDefinitions::default());
+    let mut layout_job = LayoutJob::single_section(
+        "日本語とEnglishの混在した文章".into(),
+        super::TextFormat::default(),
+    );
+    layout_job.wrap.max_width = 90.0;
+    let galley = super::layout(&mut fonts, layout_job.into());
+    assert_eq!(
+        galley
+            .rows
+            .iter()
+            .map(|row| row.glyphs.iter().map(|g| g.chr).collect::<String>())
+            .collect::<Vec<_>>(),
+        vec!["日本語と", "Englishの混在", "した文章"]
+    );
+}
+
+#[test]
+fn test_pre_cjk() {
+    let mut fonts = FontsImpl::new(1.0, 1024, super::FontDefinitions::default());
+    let mut layout_job = LayoutJob::single_section(
+        "日本語とEnglishの混在した文章".into(),
+        super::TextFormat::default(),
+    );
+    layout_job.wrap.max_width = 100.0;
+    let galley = super::layout(&mut fonts, layout_job.into());
+    assert_eq!(
+        galley
+            .rows
+            .iter()
+            .map(|row| row.glyphs.iter().map(|g| g.chr).collect::<String>())
+            .collect::<Vec<_>>(),
+        vec!["日本語とEnglish", "の混在した文章"]
+    );
+}