fix: keep multibyte characters split across output chunks

src/input/line-buffer.ts

@@ -1,20 +1,24 @@
+import {StringDecoder} from 'node:string_decoder';
+
 export class LineBuffer {
+  readonly #decoder = new StringDecoder('utf8');
   #remainder = '';
 
   push(chunk: string | Buffer): string[] {
-    const text = this.#remainder + chunk.toString('utf8');
+    const decoded = typeof chunk === 'string' ? chunk : this.#decoder.write(chunk);
+    const text = this.#remainder + decoded;
     const parts = text.split(/\r?\n/);
     this.#remainder = parts.pop() ?? '';
     return parts;
   }
 
   flush(): string[] {
-    if (this.#remainder.length === 0) {
+    const finalLine = this.#remainder + this.#decoder.end();
+    this.#remainder = '';
+    if (finalLine.length === 0) {
       return [];
     }
 
-    const finalLine = this.#remainder;
-    this.#remainder = '';
     return [finalLine];
   }
 }

tests/input.test.ts

@@ -9,6 +9,14 @@ describe('LineBuffer', () => {
     expect(buffer.flush()).toEqual(['three']);
   });
 
+  it('keeps multibyte characters that are split across chunks', () => {
+    const buffer = new LineBuffer();
+    const bytes = Buffer.from('café ✓\n', 'utf8');
+    expect(buffer.push(bytes.subarray(0, 4))).toEqual([]);
+    expect(buffer.push(bytes.subarray(4, 7))).toEqual([]);
+    expect(buffer.push(bytes.subarray(7))).toEqual(['café ✓']);
+  });
+
   it('does not invent a line when the buffer is empty', () => {
     expect(new LineBuffer().flush()).toEqual([]);
   });