Fix GraphemeCursor for GB11 case on a chunk boundary

src/grapheme.rs

@@ -8,9 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use core::cmp;
-
 use crate::tables::grapheme::GraphemeCat;
+use core::cmp;
 
 /// External iterator for grapheme clusters and byte offsets.
 ///
@@ -177,7 +176,11 @@ enum GraphemeState {
     Regional,
     /// The codepoint after is Extended_Pictographic,
     /// so whether it's a boundary depends on pre-context according to GB11.
-    Emoji,
+    Emoji {
+        /// Whether the ZWJ char has been seen already an only a "\p{Extended_Pictographic} Extend*"
+        /// part of GB11 has to be checked
+        seen_zwj: bool,
+    },
 }
 
 /// Cursor-based segmenter for grapheme clusters.
@@ -424,7 +427,7 @@ impl GraphemeCursor {
         match self.state {
             GraphemeState::InCbConsonant => self.handle_incb_consonant(chunk, chunk_start),
             GraphemeState::Regional => self.handle_regional(chunk, chunk_start),
-            GraphemeState::Emoji => self.handle_emoji(chunk, chunk_start),
+            GraphemeState::Emoji { seen_zwj } => self.handle_emoji(chunk, chunk_start, seen_zwj),
             _ => {
                 if self.cat_before.is_none() && self.offset == chunk.len() + chunk_start {
                     let ch = chunk.chars().next_back().unwrap();
@@ -532,13 +535,18 @@ impl GraphemeCursor {
     }
 
     #[inline]
-    fn handle_emoji(&mut self, chunk: &str, chunk_start: usize) {
+    fn handle_emoji(&mut self, chunk: &str, chunk_start: usize, mut seen_zwj: bool) {
+        // \p{Extended_Pictographic} Extend* ZWJ 	× 	\p{Extended_Pictographic}
         use crate::tables::grapheme as gr;
         let mut iter = chunk.chars().rev();
-        if let Some(ch) = iter.next() {
-            if self.grapheme_category(ch) != gr::GC_ZWJ {
-                self.decide(true);
-                return;
+        if !seen_zwj {
+            if let Some(ch) = iter.next() {
+                if self.grapheme_category(ch) != gr::GC_ZWJ {
+                    self.decide(true);
+                    return;
+                } else {
+                    seen_zwj = true;
+                }
             }
         }
         for ch in iter {
@@ -558,7 +566,7 @@ impl GraphemeCursor {
             self.decide(true);
         } else {
             self.pre_context_offset = Some(chunk_start);
-            self.state = GraphemeState::Emoji;
+            self.state = GraphemeState::Emoji { seen_zwj };
         }
     }
 
@@ -616,7 +624,9 @@ impl GraphemeCursor {
             match self.cat_after.unwrap() {
                 gr::GC_InCB_Consonant => self.state = GraphemeState::InCbConsonant,
                 gr::GC_Regional_Indicator => self.state = GraphemeState::Regional,
-                gr::GC_Extended_Pictographic => self.state = GraphemeState::Emoji,
+                gr::GC_Extended_Pictographic => {
+                    self.state = GraphemeState::Emoji { seen_zwj: false }
+                }
                 _ => need_pre_context = self.cat_before.is_none(),
             }
             if need_pre_context {
@@ -647,7 +657,7 @@ impl GraphemeCursor {
                 self.is_boundary_result()
             }
             PairResult::Emoji => {
-                self.handle_emoji(&chunk[..offset_in_chunk], chunk_start);
+                self.handle_emoji(&chunk[..offset_in_chunk], chunk_start, false);
                 self.is_boundary_result()
             }
         }
@@ -882,3 +892,105 @@ fn test_grapheme_cursor_prev_boundary_chunk_start() {
     );
     assert_eq!(c.prev_boundary(&s[..2], 0), Ok(Some(1)));
 }
+
+#[test]
+fn test_grapheme_cursor_boundary_with_zwj_on_chunk_start() {
+    use GraphemeIncomplete::*;
+
+    let chunk0 = "👩"; // 4 bytes
+    let chunk1 = "\u{200d}🔬"; // 3 bytes + 4 bytes
+
+    let full_len = chunk0.len() + chunk1.len();
+
+    let mut cur = GraphemeCursor::new(0, full_len, true);
+    assert_eq!(cur.next_boundary(chunk0, 0), Err(NextChunk));
+    match cur.next_boundary(chunk1, chunk0.len()) {
+        Ok(res) => assert_eq!(res, Some(11)),
+        Err(PreContext(_)) => {
+            cur.provide_context(chunk0, 0);
+            assert_eq!(cur.next_boundary(chunk1, chunk0.len()), Ok(Some(11)));
+        }
+        _ => unreachable!(),
+    }
+}
+
+#[test]
+fn test_grapheme_cursor_emoji_no_zwj() {
+    use GraphemeIncomplete::*;
+    let chunk0 = "🍒"; // 4 bytes
+    let chunk1 = "🥑"; // 4 bytes
+    let full_len = chunk0.len() + chunk1.len();
+
+    let mut c = GraphemeCursor::new(0, full_len, true);
+    assert_eq!(c.next_boundary(chunk0, 0), Err(NextChunk));
+    assert_eq!(
+        c.next_boundary(chunk1, chunk0.len()),
+        Err(PreContext(chunk0.len()))
+    );
+    c.provide_context(chunk0, 0);
+    assert_eq!(c.next_boundary(chunk1, chunk0.len()), Ok(Some(4)));
+    assert_eq!(c.next_boundary(chunk1, chunk0.len()), Ok(Some(8)));
+    assert_eq!(c.next_boundary(chunk1, chunk0.len()), Ok(None));
+}
+
+#[test]
+fn test_grapheme_cursor_emoji_chunk_boundary_before_zwj() {
+    use GraphemeIncomplete::*;
+    let chunk0 = "🍒"; // 4 bytes
+    let chunk1 = "\u{200d}🥑"; // 3 + 4 bytes
+    let full_len = chunk0.len() + chunk1.len(); // 11
+
+    let mut c = GraphemeCursor::new(0, full_len, true);
+    assert_eq!(c.next_boundary(chunk0, 0), Err(NextChunk));
+    assert_eq!(
+        c.next_boundary(chunk1, chunk0.len()),
+        Err(PreContext(chunk0.len()))
+    );
+    c.provide_context(chunk0, 0);
+    assert_eq!(c.next_boundary(chunk1, chunk0.len()), Ok(Some(11)));
+    assert_eq!(c.next_boundary(chunk1, chunk0.len()), Ok(None));
+}
+
+#[test]
+fn test_grapheme_cursor_emoji_chunk_boundary_after_zwj() {
+    use GraphemeIncomplete::*;
+    let chunk0 = "🍒\u{200d}"; // 4 + 3 bytes
+    let chunk1 = "🥑"; // 4 bytes
+    let full_len = chunk0.len() + chunk1.len(); // 11
+
+    let mut c = GraphemeCursor::new(0, full_len, true);
+    assert_eq!(c.next_boundary(chunk0, 0), Err(NextChunk));
+    assert_eq!(
+        c.next_boundary(chunk1, chunk0.len()),
+        Err(PreContext(chunk0.len()))
+    );
+    c.provide_context(chunk0, 0);
+    assert_eq!(c.next_boundary(chunk1, chunk0.len()), Ok(Some(11)));
+    assert_eq!(c.next_boundary(chunk1, chunk0.len()), Ok(None));
+}
+
+#[test]
+fn test_grapheme_cursor_emoji_zwj_across_chunks() {
+    use GraphemeIncomplete::*;
+    let chunk0 = "🍒"; // 4 bytes
+    let chunk1 = "\u{200d}"; // 3 bytes
+    let chunk2 = "🥑"; // 4 bytes
+    let full_len = chunk0.len() + chunk1.len() + chunk2.len(); // 11
+    let chunk2_start = chunk0.len() + chunk1.len();
+
+    let mut c = GraphemeCursor::new(0, full_len, true);
+    assert_eq!(c.next_boundary(chunk0, 0), Err(NextChunk));
+    assert_eq!(c.next_boundary(chunk1, chunk0.len()), Err(NextChunk));
+    assert_eq!(
+        c.next_boundary(chunk2, chunk2_start),
+        Err(PreContext(chunk2_start))
+    );
+    c.provide_context(chunk1, chunk0.len());
+    assert_eq!(
+        c.next_boundary(chunk2, chunk2_start),
+        Err(PreContext(chunk0.len()))
+    );
+    c.provide_context(chunk0, 0);
+    assert_eq!(c.next_boundary(chunk2, chunk2_start), Ok(Some(11)));
+    assert_eq!(c.next_boundary(chunk2, chunk2_start), Ok(None));
+}