fix AI comments

Author: hanabi1224

src/db/car/forest.rs

@@ -148,7 +148,12 @@ impl<ReaderT: super::RandomAccessFileReader> ForestCar<ReaderT> {
                 "not recognizable as a `{FOREST_CAR_FILE_EXTENSION}` file"
             ))
         })?;
-        let index_start_pos = footer.index - ZSTD_SKIP_FRAME_LEN;
+        let index_start_pos = footer.index.checked_sub(ZSTD_SKIP_FRAME_LEN).ok_or_else(||
+            invalid_data(format!(
+                "unexpected error: footer.index({}) < ZSTD_SKIP_FRAME_LEN({ZSTD_SKIP_FRAME_LEN})",
+                footer.index
+            )),
+        )?;
 
         let cursor = Cursor::new_pos(&reader, 0);
         let mut header_zstd_frame = decode_zstd_single_frame(cursor)?.into();

src/db/car/forest/index/mod.rs

@@ -476,7 +476,7 @@ impl Writer {
     }
     pub async fn write_zstd_skip_frames_into(self, writer: impl AsyncWrite) -> io::Result<()> {
         // write every 128MiB slots to a skip frame
-        const CHUNK_FRAME_DATA_MAX_BYTES: usize = 128 * 1024;
+        const CHUNK_FRAME_DATA_MAX_BYTES: usize = 128 * 1024 * 1024;
         let written_len = self.written_len();
         self.write_zstd_skip_frames_into_inner(
             writer,