graphemes: Optimize nth_next/nth_prev operation.

It's used a lot more than it used to in position calculation. Instead of
throwing away state between boundary calculation, reuse it.

helix-core/src/graphemes.rs

@@ -27,23 +27,11 @@ pub fn grapheme_width(g: &str) -> usize {
 }
 
 pub fn nth_prev_grapheme_boundary(slice: RopeSlice, char_idx: usize, n: usize) -> usize {
-    // TODO: implement this more efficiently.  This has to do a lot of
-    // re-scanning of rope chunks.  Probably move the main implementation here,
-    // and have prev_grapheme_boundary call this instead.
-    let mut char_idx = char_idx;
-    for _ in 0..n {
-        char_idx = prev_grapheme_boundary(slice, char_idx);
-    }
-    char_idx
-}
-
-/// Finds the previous grapheme boundary before the given char position.
-pub fn prev_grapheme_boundary(slice: RopeSlice, char_idx: usize) -> usize {
     // Bounds check
     debug_assert!(char_idx <= slice.len_chars());
 
     // We work with bytes for this, so convert.
-    let byte_idx = slice.char_to_byte(char_idx);
+    let mut byte_idx = slice.char_to_byte(char_idx);
 
     // Get the chunk with our byte index in it.
     let (mut chunk, mut chunk_byte_idx, mut chunk_char_idx, _) = slice.chunk_at_byte(byte_idx);
@@ -52,12 +40,13 @@ pub fn prev_grapheme_boundary(slice: RopeSlice, char_idx: usize) -> usize {
     let mut gc = GraphemeCursor::new(byte_idx, slice.len_bytes(), true);
 
     // Find the previous grapheme cluster boundary.
+    for _ in 0..n {
         loop {
             match gc.prev_boundary(chunk, chunk_byte_idx) {
                 Ok(None) => return 0,
                 Ok(Some(n)) => {
-                let tmp = byte_to_char_idx(chunk, n - chunk_byte_idx);
-                return chunk_char_idx + tmp;
+                    byte_idx = n;
+                    break;
                 }
                 Err(GraphemeIncomplete::PrevChunk) => {
                     let (a, b, c, _) = slice.chunk_at_byte(chunk_byte_idx - 1);
@@ -73,25 +62,21 @@ pub fn prev_grapheme_boundary(slice: RopeSlice, char_idx: usize) -> usize {
             }
         }
     }
-
-pub fn nth_next_grapheme_boundary(slice: RopeSlice, char_idx: usize, n: usize) -> usize {
-    // TODO: implement this more efficiently.  This has to do a lot of
-    // re-scanning of rope chunks.  Probably move the main implementation here,
-    // and have next_grapheme_boundary call this instead.
-    let mut char_idx = char_idx;
-    for _ in 0..n {
-        char_idx = next_grapheme_boundary(slice, char_idx);
+    let tmp = byte_to_char_idx(chunk, byte_idx + chunk_byte_idx);
+    chunk_char_idx + tmp
 }
-    char_idx
+
+/// Finds the previous grapheme boundary before the given char position.
+pub fn prev_grapheme_boundary(slice: RopeSlice, char_idx: usize) -> usize {
+    nth_prev_grapheme_boundary(slice, char_idx, 1)
 }
 
-/// Finds the next grapheme boundary after the given char position.
-pub fn next_grapheme_boundary(slice: RopeSlice, char_idx: usize) -> usize {
+pub fn nth_next_grapheme_boundary(slice: RopeSlice, char_idx: usize, n: usize) -> usize {
     // Bounds check
     debug_assert!(char_idx <= slice.len_chars());
 
     // We work with bytes for this, so convert.
-    let byte_idx = slice.char_to_byte(char_idx);
+    let mut byte_idx = slice.char_to_byte(char_idx);
 
     // Get the chunk with our byte index in it.
     let (mut chunk, mut chunk_byte_idx, mut chunk_char_idx, _) = slice.chunk_at_byte(byte_idx);
@@ -99,13 +84,14 @@ pub fn next_grapheme_boundary(slice: RopeSlice, char_idx: usize) -> usize {
     // Set up the grapheme cursor.
     let mut gc = GraphemeCursor::new(byte_idx, slice.len_bytes(), true);
 
-    // Find the next grapheme cluster boundary.
+    // Find the nth next grapheme cluster boundary.
+    for _ in 0..n {
         loop {
             match gc.next_boundary(chunk, chunk_byte_idx) {
                 Ok(None) => return slice.len_chars(),
                 Ok(Some(n)) => {
-                let tmp = byte_to_char_idx(chunk, n - chunk_byte_idx);
-                return chunk_char_idx + tmp;
+                    byte_idx = n;
+                    break;
                 }
                 Err(GraphemeIncomplete::NextChunk) => {
                     chunk_byte_idx += chunk.len();
@@ -121,6 +107,14 @@ pub fn next_grapheme_boundary(slice: RopeSlice, char_idx: usize) -> usize {
             }
         }
     }
+    let tmp = byte_to_char_idx(chunk, byte_idx - chunk_byte_idx);
+    chunk_char_idx + tmp
+}
+
+/// Finds the next grapheme boundary after the given char position.
+pub fn next_grapheme_boundary(slice: RopeSlice, char_idx: usize) -> usize {
+    nth_next_grapheme_boundary(slice, char_idx, 1)
+}
 
 /// Returns whether the given char position is a grapheme boundary.
 pub fn is_grapheme_boundary(slice: RopeSlice, char_idx: usize) -> bool {