Avoid calling byte_pair_encode for existing tokens

This was byte_pair_encode can be optimized further, assuming we'll always have at least 2 tokens

src/lib.rs

@@ -162,10 +162,10 @@ fn hash_current_thread() -> usize {
     // that works great for our use case of avoiding collisions in our array. Unfortunately,
     // it's private. However, there are only so many ways you can layout a u64, so just transmute
     // https://github.com/rust-lang/rust/issues/67939
-    const _: [u8; 8] = [0; std::mem::size_of::<std::thread::ThreadId>()];
+    const _: [u8; 8] = [0; std::mem::size_of::<thread::ThreadId>()];
     const _: [u8; 8] = [0; std::mem::size_of::<FakeThreadId>()];
     let x = unsafe {
-        std::mem::transmute::<std::thread::ThreadId, FakeThreadId>(thread::current().id()).0
+        std::mem::transmute::<thread::ThreadId, FakeThreadId>(thread::current().id()).0
     };
     u64::from(x) as usize
 }
@@ -214,11 +214,10 @@ impl CoreBPE {
         let mut ret = vec![];
         for mat in regex.find_iter(text) {
             let piece = mat.unwrap().as_str().as_bytes();
-            if let Some(token) = self.encoder.get(piece) {
-                ret.push(*token);
-                continue;
+            match self.encoder.get(piece) {
+                Some(token) => ret.push(*token),
+                None => ret.extend(&byte_pair_encode(piece, &self.encoder)),
             }
-            ret.extend(&byte_pair_encode(piece, &self.encoder));
         }
         ret
     }
@@ -516,7 +515,10 @@ impl CoreBPE {
                         unstable_bytes.extend_from_slice(&bytes[e.valid_up_to()..]);
 
                         tokens.truncate(tokens.len() - last_piece_token_len);
-                        tokens.extend(byte_pair_encode(&unstable_bytes, &self.encoder));
+                        match self.encoder.get(&unstable_bytes) {
+                            Some(token) => tokens.push(*token),
+                            None => tokens.extend(&byte_pair_encode(&unstable_bytes, &self.encoder)),
+                        }
                     }
                     tokens
                 }
@@ -597,15 +599,26 @@ fn _tiktoken(_py: Python, m: &PyModule) -> PyResult<()> {
 mod tests {
     use rustc_hash::FxHashMap as HashMap;
 
-    use crate::byte_pair_split;
+    use crate::{byte_pair_split, Rank};
 
-    #[test]
-    fn very_simple_test() {
-        let mut ranks = HashMap::default();
-        ranks.insert(b"ab".to_vec(), 1);
-        ranks.insert(b"cd".to_vec(), 2);
+    fn setup_ranks() -> HashMap<Vec<u8>, Rank> {
+        HashMap::from_iter([
+            (b"ab".to_vec(), 0),
+            (b"cd".to_vec(), 1),
+        ])
+    }
 
+    #[test]
+    fn test_simple_characters() {
+        let ranks = setup_ranks();
         let res = byte_pair_split(b"abcd", &ranks);
         assert_eq!(res, vec![b"ab", b"cd"]);
     }
+
+    #[test]
+    fn test_repeated_characters() {
+        let ranks = setup_ranks();
+        let res = byte_pair_split(b"abab", &ranks);
+        assert_eq!(res, vec![b"ab", b"ab"]);
+    }
 }