Generate test strings with multi-byte characters

crates/bpe/src/byte_pair_encoding.rs

@@ -567,20 +567,47 @@ fn is_char_boundary(b: u8) -> bool {
 #[cfg(feature = "rand")]
 pub fn create_test_string(bpe: &BytePairEncoding, min_bytes: usize) -> String {
     use rand::{thread_rng, Rng};
-    let mut text = String::new();
-    while text.len() < min_bytes {
-        loop {
+    // the bytes we accumulated thus far
+    let mut bytes = Vec::new();
+    // the tokens we added so we can backtrack
+    let mut tokens = Vec::new();
+    // the number of valid UTF-8 bytes
+    let mut valid_bytes = 0;
+    'keep: while valid_bytes < min_bytes {
+        // try a few times to find a suitable token
+        for _ in 0..8 {
+            // pick a random token and provisionally add it
             let i = thread_rng().gen_range(0..bpe.num_tokens());
-            let s = bpe.token_bytes(i as u32);
-            if s.iter().all(|b| is_char_boundary(*b)) {
-                if let Ok(s) = std::str::from_utf8(s) {
-                    text.push_str(s);
-                    break;
-                }
+            bytes.extend(bpe.token_bytes(i as u32));
+            // test if the additional bytes are valid utf-8
+            // the last character is not included, because it may be incomplete
+            let last = bytes
+                .iter()
+                .rev()
+                .find_position(|b| is_char_boundary(**b))
+                .map_or(0, |(offset, _)| bytes.len() - (offset + 1));
+            assert!(last >= valid_bytes);
+            if std::str::from_utf8(&bytes[valid_bytes..last]).is_ok() {
+                tokens.push(i);
+                valid_bytes = last;
+                continue 'keep;
+            } else {
+                bytes.truncate(bytes.len() - bpe.token_len(i as u32));
             }
         }
+        // we didn't find anything after a few tries, backtrack
+        if let Some(i) = tokens.pop() {
+            bytes.truncate(bytes.len() - bpe.token_len(i as u32));
+            valid_bytes = bytes
+                .iter()
+                .rev()
+                .find_position(|b| is_char_boundary(**b))
+                .map_or(0, |(offset, _)| bytes.len() - (offset + 1));
+        }
     }
-    text
+    // truncate to the know valid bytes
+    bytes.truncate(valid_bytes);
+    String::from_utf8(bytes).expect("should be valid here")
 }
 
 #[cfg(feature = "rand")]