Fix bug in TextBufferWriter with multibyte chars

Fixed a UTF-8 decoding error that would occur if the `buf` argument to
`TextBufferWriter` ended in the middle of a multibyte character.

core/src/text_buffer/writer.rs

@@ -1,25 +1,50 @@
 use super::{Point, TextBuffer};
-
 pub struct TextBufferWriter<'a> {
     text_buffer: &'a mut TextBuffer,
+    /// Stores any partial multi-byte characters that are left over from the
+    /// last call to `write()`.
+    overflow: Vec<u8>,
 }
 
 impl<'a> TextBufferWriter<'a> {
     pub fn new(text_buffer: &'a mut TextBuffer) -> Self {
-        Self { text_buffer }
+        Self {
+            text_buffer,
+            overflow: vec![],
+        }
     }
 }
 
 impl<'a> std::io::Write for TextBufferWriter<'a> {
     fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
-        self.text_buffer.insert_text(
-            str::from_utf8(buf).map_err(std::io::Error::other)?,
-            Point::End,
-        );
+        // If we get a UTF-8 decoding error, try backing off up to three bytes.
+        // We might be in the middle of a multipart character and in that case
+        // we should store the partial character in `overflow`. Not the most
+        // efficient way to do this, ideally I should write some way to decode
+        // text one character at a time.
+        let bytes = if !self.overflow.is_empty() {
+            self.overflow.extend_from_slice(buf);
+            &self.overflow
+        } else {
+            buf
+        };
+        let text = str::from_utf8(bytes)
+            .or_else(|_| str::from_utf8(&bytes[0..bytes.len() - 1]))
+            .or_else(|_| str::from_utf8(&bytes[0..bytes.len() - 2]))
+            .or_else(|_| str::from_utf8(&bytes[0..bytes.len() - 3]))
+            .map_err(std::io::Error::other)?;
+        self.text_buffer.insert_text(text, Point::End);
+        self.overflow = bytes[text.len()..bytes.len()].to_vec();
         Ok(buf.len())
     }
 
     fn flush(&mut self) -> std::io::Result<()> {
+        if !self.overflow.is_empty() {
+            self.text_buffer.insert_text(
+                str::from_utf8(&self.overflow).map_err(std::io::Error::other)?,
+                Point::End,
+            );
+        }
         Ok(())
     }
 }