add count_till_limit function

crates/bpe/src/byte_pair_encoding.rs

@@ -335,12 +335,35 @@ impl BytePairEncoding {
         last_token
     }
 
+    /// Counts the number tokens produced when encoding the text.
     pub fn count(&self, text: &[u8]) -> usize {
         let mut enc = BacktrackEncoder::new(self, text);
         while enc.step().is_some() {}
         enc.count()
     }
 
+    /// Returns the token count iff the total token count stays below the specified `token_limit`.
+    /// Otherwise, it returns false.
+    /// This function can be faster than `count` when the token_limit is much smaller than the provided text.
+    pub fn count_till_limit(&self, text: &[u8], token_limit: usize) -> Option<usize> {
+        let mut enc = BacktrackEncoder::new(self, text);
+        // When the text has exactly the desired number of tokens, then it could in theory happen that
+        // the token_limit is exceeded before the end of the text is reached (and a different encoding is tested).
+        // To be on the "safe" side, we add a little buffer for such cases.
+        // TODO: Determine exactly how large this buffer must be in the worst case.
+        let limit_with_buffer = token_limit.saturating_add(10);
+        while enc.step().is_some() {
+            if enc.count() > limit_with_buffer {
+                return None;
+            }
+        }
+        if enc.count() <= token_limit {
+            Some(enc.count())
+        } else {
+            None
+        }
+    }
+
     pub fn encode_via_table(&self, text: &[u8]) -> Vec<u32> {
         let last_token = self.encode_all_prefixes(text);
         let mut encoded = Vec::with_capacity(text.len() / 3);