stdx: Unify RopeSlice grapheme cluster iterators

This style for RopeGraphemes is identical to Ropey's Chars and Bytes
iterators. Being able to move the iterator types like cursors over the
bytes/chars/graphemes is useful in some cases. For example see
`helix_core::movement::<Chars as CharHelpers>::range_to_target`.

This change also adds `RopeSliceExt::graphemes_at` for flexibility.
`graphemes` and `graphemes_rev` are now implemented in terms of
`graphemes_at` and `RopeGraphemes::reversed`.

helix-stdx/src/rope.rs

@@ -135,7 +135,9 @@ pub trait RopeSliceExt<'a>: Sized {
     /// let graphemes: Vec<_> = text.graphemes().collect();
     /// assert_eq!(graphemes.as_slice(), &["😶‍🌫️", "🏴‍☠️", "🖼️"]);
     /// ```
-    fn graphemes(self) -> RopeGraphemes<'a>;
+    fn graphemes(self) -> RopeGraphemes<'a> {
+        self.graphemes_at(0)
+    }
     /// Returns an iterator over the grapheme clusters in the slice, reversed.
     ///
     /// The returned iterator starts at the end of the slice and ends at the beginning of the
@@ -150,7 +152,23 @@ pub trait RopeSliceExt<'a>: Sized {
     /// let graphemes: Vec<_> = text.graphemes_rev().collect();
     /// assert_eq!(graphemes.as_slice(), &["🖼️", "🏴‍☠️", "😶‍🌫️"]);
     /// ```
-    fn graphemes_rev(self) -> RevRopeGraphemes<'a>;
+    fn graphemes_rev(self) -> RopeGraphemes<'a>;
+    /// Returns an iterator over the grapheme clusters in the slice at the given byte index.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # use ropey::Rope;
+    /// # use helix_stdx::rope::RopeSliceExt;
+    /// let text = Rope::from_str("😶‍🌫️🏴‍☠️🖼️");
+    /// // 14 is the byte index of the pirate flag's starting cluster boundary.
+    /// let graphemes: Vec<_> = text.slice(..).graphemes_at(14).collect();
+    /// assert_eq!(graphemes.as_slice(), &["🏴‍☠️", "🖼️"]);
+    /// // 27 is the byte index of the pirate flag's ending cluster boundary.
+    /// let graphemes: Vec<_> = text.slice(..).graphemes_at(27).reversed().collect();
+    /// assert_eq!(graphemes.as_slice(), &["🏴‍☠️", "😶‍🌫️"]);
+    /// ```
+    fn graphemes_at(self, byte_idx: usize) -> RopeGraphemes<'a>;
     /// Finds the byte index of the next grapheme boundary after `byte_idx`.
     ///
     /// If the byte index lies on the last grapheme cluster in the slice then this function
@@ -401,29 +419,21 @@ impl<'a> RopeSliceExt<'a> for RopeSlice<'a> {
         }
     }
 
-    fn graphemes(self) -> RopeGraphemes<'a> {
+    fn graphemes_rev(self) -> RopeGraphemes<'a> {
-        let mut chunks = self.chunks();
+        self.graphemes_at(self.len_bytes()).reversed()
-        let first_chunk = chunks.next().unwrap_or("");
+    }
+
+    fn graphemes_at(self, byte_idx: usize) -> RopeGraphemes<'a> {
+        let (mut chunks, chunk_byte_idx, _, _) = self.chunks_at_byte(byte_idx);
+        let current_chunk = chunks.next().unwrap_or("");
+
         RopeGraphemes {
             text: self,
             chunks,
-            cur_chunk: first_chunk,
+            current_chunk,
-            cur_chunk_start: 0,
+            chunk_byte_idx,
-            cursor: GraphemeCursor::new(0, self.len_bytes(), true),
+            cursor: GraphemeCursor::new(byte_idx, self.len_bytes(), true),
-        }
+            is_reversed: false,
-    }
-
-    fn graphemes_rev(self) -> RevRopeGraphemes<'a> {
-        let (mut chunks, mut cur_chunk_start, _, _) = self.chunks_at_byte(self.len_bytes());
-        chunks.reverse();
-        let first_chunk = chunks.next().unwrap_or("");
-        cur_chunk_start -= first_chunk.len();
-        RevRopeGraphemes {
-            text: self,
-            chunks,
-            cur_chunk: first_chunk,
-            cur_chunk_start,
-            cursor: GraphemeCursor::new(self.len_bytes(), self.len_bytes(), true),
         }
     }
 
@@ -511,13 +521,18 @@ const fn is_utf8_char_boundary(b: u8) -> bool {
 }
 
 /// An iterator over the graphemes of a `RopeSlice`.
+///
+/// This iterator is cursor-like: rather than implementing DoubleEndedIterator it can be reversed
+/// like a cursor.
 #[derive(Clone)]
 pub struct RopeGraphemes<'a> {
     text: RopeSlice<'a>,
     chunks: Chunks<'a>,
-    cur_chunk: &'a str,
+    current_chunk: &'a str,
-    cur_chunk_start: usize,
+    /// Byte index of the start of the current chunk.
+    chunk_byte_idx: usize,
     cursor: GraphemeCursor,
+    is_reversed: bool,
 }
 
 impl fmt::Debug for RopeGraphemes<'_> {
@@ -525,112 +540,116 @@ impl fmt::Debug for RopeGraphemes<'_> {
         f.debug_struct("RopeGraphemes")
             .field("text", &self.text)
             .field("chunks", &self.chunks)
-            .field("cur_chunk", &self.cur_chunk)
+            .field("current_chunk", &self.current_chunk)
-            .field("cur_chunk_start", &self.cur_chunk_start)
+            .field("chunk_byte_idx", &self.chunk_byte_idx)
             // .field("cursor", &self.cursor)
+            .field("is_reversed", &self.is_reversed)
             .finish()
     }
 }
 
+impl<'a> RopeGraphemes<'a> {
+    #[allow(clippy::should_implement_trait)]
+    pub fn next(&mut self) -> Option<RopeSlice<'a>> {
+        if self.is_reversed {
+            self.prev_impl()
+        } else {
+            self.next_impl()
+        }
+    }
+
+    pub fn prev(&mut self) -> Option<RopeSlice<'a>> {
+        if self.is_reversed {
+            self.next_impl()
+        } else {
+            self.prev_impl()
+        }
+    }
+
+    pub fn reverse(&mut self) {
+        self.is_reversed = !self.is_reversed;
+    }
+
+    #[must_use]
+    pub fn reversed(mut self) -> Self {
+        self.reverse();
+        self
+    }
+
+    fn next_impl(&mut self) -> Option<RopeSlice<'a>> {
+        let a = self.cursor.cur_cursor();
+        let b;
+        loop {
+            match self
+                .cursor
+                .next_boundary(self.current_chunk, self.chunk_byte_idx)
+            {
+                Ok(None) => return None,
+                Ok(Some(n)) => {
+                    b = n;
+                    break;
+                }
+                Err(GraphemeIncomplete::NextChunk) => {
+                    self.chunk_byte_idx += self.current_chunk.len();
+                    self.current_chunk = self.chunks.next().unwrap_or("");
+                }
+                Err(GraphemeIncomplete::PreContext(idx)) => {
+                    let (chunk, byte_idx, _, _) = self.text.chunk_at_byte(idx.saturating_sub(1));
+                    self.cursor.provide_context(chunk, byte_idx);
+                }
+                _ => unreachable!(),
+            }
+        }
+
+        if a < self.chunk_byte_idx {
+            Some(self.text.byte_slice(a..b))
+        } else {
+            let a2 = a - self.chunk_byte_idx;
+            let b2 = b - self.chunk_byte_idx;
+            Some((&self.current_chunk[a2..b2]).into())
+        }
+    }
+
+    fn prev_impl(&mut self) -> Option<RopeSlice<'a>> {
+        let a = self.cursor.cur_cursor();
+        let b;
+        loop {
+            match self
+                .cursor
+                .prev_boundary(self.current_chunk, self.chunk_byte_idx)
+            {
+                Ok(None) => return None,
+                Ok(Some(n)) => {
+                    b = n;
+                    break;
+                }
+                Err(GraphemeIncomplete::PrevChunk) => {
+                    self.current_chunk = self.chunks.prev().unwrap_or("");
+                    self.chunk_byte_idx -= self.current_chunk.len();
+                }
+                Err(GraphemeIncomplete::PreContext(idx)) => {
+                    let (chunk, byte_idx, _, _) = self.text.chunk_at_byte(idx.saturating_sub(1));
+                    self.cursor.provide_context(chunk, byte_idx);
+                }
+                _ => unreachable!(),
+            }
+        }
+
+        if a >= self.chunk_byte_idx + self.current_chunk.len() {
+            Some(self.text.byte_slice(b..a))
+        } else {
+            let a2 = a - self.chunk_byte_idx;
+            let b2 = b - self.chunk_byte_idx;
+            Some((&self.current_chunk[b2..a2]).into())
+        }
+    }
+}
+
 impl<'a> Iterator for RopeGraphemes<'a> {
     type Item = RopeSlice<'a>;
 
     fn next(&mut self) -> Option<Self::Item> {
-        let a = self.cursor.cur_cursor();
+        RopeGraphemes::next(self)
-        let b;
-        loop {
-            match self
-                .cursor
-                .next_boundary(self.cur_chunk, self.cur_chunk_start)
-            {
-                Ok(None) => {
-                    return None;
-                }
-                Ok(Some(n)) => {
-                    b = n;
-                    break;
-                }
-                Err(GraphemeIncomplete::NextChunk) => {
-                    self.cur_chunk_start += self.cur_chunk.len();
-                    self.cur_chunk = self.chunks.next().unwrap_or("");
-                }
-                Err(GraphemeIncomplete::PreContext(idx)) => {
-                    let (chunk, byte_idx, _, _) = self.text.chunk_at_byte(idx.saturating_sub(1));
-                    self.cursor.provide_context(chunk, byte_idx);
-                }
-                _ => unreachable!(),
-            }
-        }
-
-        if a < self.cur_chunk_start {
-            Some(self.text.byte_slice(a..b))
-        } else {
-            let a2 = a - self.cur_chunk_start;
-            let b2 = b - self.cur_chunk_start;
-            Some((&self.cur_chunk[a2..b2]).into())
-        }
-    }
-}
-
-/// An iterator over the graphemes of a `RopeSlice` in reverse.
-#[derive(Clone)]
-pub struct RevRopeGraphemes<'a> {
-    text: RopeSlice<'a>,
-    chunks: Chunks<'a>,
-    cur_chunk: &'a str,
-    cur_chunk_start: usize,
-    cursor: GraphemeCursor,
-}
-
-impl fmt::Debug for RevRopeGraphemes<'_> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.debug_struct("RevRopeGraphemes")
-            .field("text", &self.text)
-            .field("chunks", &self.chunks)
-            .field("cur_chunk", &self.cur_chunk)
-            .field("cur_chunk_start", &self.cur_chunk_start)
-            // .field("cursor", &self.cursor)
-            .finish()
-    }
-}
-
-impl<'a> Iterator for RevRopeGraphemes<'a> {
-    type Item = RopeSlice<'a>;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        let a = self.cursor.cur_cursor();
-        let b;
-        loop {
-            match self
-                .cursor
-                .prev_boundary(self.cur_chunk, self.cur_chunk_start)
-            {
-                Ok(None) => {
-                    return None;
-                }
-                Ok(Some(n)) => {
-                    b = n;
-                    break;
-                }
-                Err(GraphemeIncomplete::PrevChunk) => {
-                    self.cur_chunk = self.chunks.next().unwrap_or("");
-                    self.cur_chunk_start -= self.cur_chunk.len();
-                }
-                Err(GraphemeIncomplete::PreContext(idx)) => {
-                    let (chunk, byte_idx, _, _) = self.text.chunk_at_byte(idx.saturating_sub(1));
-                    self.cursor.provide_context(chunk, byte_idx);
-                }
-                _ => unreachable!(),
-            }
-        }
-
-        if a >= self.cur_chunk_start + self.cur_chunk.len() {
-            Some(self.text.byte_slice(b..a))
-        } else {
-            let a2 = a - self.cur_chunk_start;
-            let b2 = b - self.cur_chunk_start;
-            Some((&self.cur_chunk[b2..a2]).into())
-        }
     }
 }