Make char count bidirectional

Author: dtolnay

src/fallback.rs

@@ -382,28 +382,45 @@ impl FileInfo {
         let char_index = (ch - self.span.lo) as usize;
 
         // Look up offset of the largest already-computed char index that is
-        // less than or equal to the current requested one. We resume counting
-        // chars from that point.
-        let (&last_char_index, &last_byte_offset) = self
+        // less than or equal to the current requested one.
+        let (&previous_char_index, &previous_byte_offset) = self
             .char_index_to_byte_offset
             .range(..=char_index)
             .next_back()
             .unwrap_or((&0, &0));
 
-        if last_char_index == char_index {
-            last_byte_offset
-        } else {
-            let total_byte_offset = match self.source_text[last_byte_offset..]
-                .char_indices()
-                .nth(char_index - last_char_index)
-            {
-                Some((additional_offset, _ch)) => last_byte_offset + additional_offset,
-                None => self.source_text.len(),
-            };
-            self.char_index_to_byte_offset
-                .insert(char_index, total_byte_offset);
-            total_byte_offset
+        if previous_char_index == char_index {
+            return previous_byte_offset;
         }
+
+        // Look up next char index that is greater than the requested one. We
+        // resume counting chars from whichever point is closer.
+        let byte_offset = match self.char_index_to_byte_offset.range(char_index..).next() {
+            Some((&next_char_index, &next_byte_offset))
+                if next_char_index - char_index < char_index - previous_char_index =>
+            {
+                self.source_text[..next_byte_offset]
+                    .char_indices()
+                    .nth_back(next_char_index - char_index - 1)
+                    .unwrap()
+                    .0
+            }
+            _ => {
+                match self.source_text[previous_byte_offset..]
+                    .char_indices()
+                    .nth(char_index - previous_char_index)
+                {
+                    Some((byte_offset_from_previous, _ch)) => {
+                        previous_byte_offset + byte_offset_from_previous
+                    }
+                    None => self.source_text.len(),
+                }
+            }
+        };
+
+        self.char_index_to_byte_offset
+            .insert(char_index, byte_offset);
+        byte_offset
     }
 
     fn source_text(&mut self, span: Span) -> String {