Fix decoding manually added tokens (#1164)

* Fix decoding manually added tokens

* fix + test

* nit

* nit

* no lag bpe

---------

Co-authored-by: Awni Hannun <awni@apple.com>

llms/mlx_lm/tokenizer_utils.py

@@ -127,23 +127,23 @@ class SPMStreamingDetokenizer(StreamingDetokenizer):
         self.text = ""
         self.tokens = []
 
-    def _flush(self):
+    def _try_flush(self, force=False):
         text = self._unflushed.replace(self._sep, b" ").decode("utf-8", "replace")
+        if not force and text.endswith("\ufffd"):
+            return
         if not self.text and self.trim_space and text and text[0] == " ":
             text = text[1:]
         self.text += text
+        self._unflushed = b""
 
     def add_token(self, token):
         self.tokens.append(token)
         v = self.tokenmap[token]
-        if v.startswith(self._sep):
-            self._flush()
-            self._unflushed = v
-        else:
         self._unflushed += v
+        self._try_flush()
 
     def finalize(self):
-        self._flush()
+        self._try_flush(force=True)
         self._unflushed = b""
 
 
@@ -158,7 +158,6 @@ class BPEStreamingDetokenizer(StreamingDetokenizer):
     _space_matches = (".", "?", "!", ",", "n't", "'m", "'s", "'ve", "'re")
 
     def __init__(self, tokenizer):
-
         self.clean_spaces = tokenizer.clean_up_tokenization_spaces
 
         # Extract the tokens in a list from id to text
@@ -172,14 +171,22 @@ class BPEStreamingDetokenizer(StreamingDetokenizer):
         # https://github.com/openai/gpt-2/blob/master/src/encoder.py
         self.make_byte_decoder()
 
-        self._added_ids = set(tokenizer.added_tokens_decoder.keys())
-
     def reset(self):
         self.offset = 0
         self._unflushed = ""
         self.text = ""
         self.tokens = []
 
+    def _decode_bytes(self, seq):
+        barr = bytearray()
+        for c in seq:
+            res = self._byte_decoder.get(c, False)
+            if res:
+                barr.append(res)
+            else:
+                barr.extend(bytes(c, "utf-8"))
+        return barr.decode("utf-8", "replace")
+
     def _maybe_trim_space(self, current_text):
         if len(current_text) == 0:
             return current_text
@@ -194,15 +201,14 @@ class BPEStreamingDetokenizer(StreamingDetokenizer):
     def add_token(self, token):
         self.tokens.append(token)
         v = self.tokenmap[token]
-        is_added = token in self._added_ids
-        if not is_added:
         self._unflushed += v
-        text = bytearray(self._byte_decoder[c] for c in self._unflushed).decode(
-            "utf-8", "replace"
-        )
-        if is_added:
-            text += v
-        if not text.endswith("\ufffd"):
+        text = self._decode_bytes(self._unflushed)
+
+        # For multi-byte utf-8 wait until they are complete
+        # For single spaces wait until the next token to clean it if needed
+        if not text.endswith("\ufffd") and not (
+            len(v) == 1 and self._byte_decoder[v[0]] == 32
+        ):
             self.text += self._maybe_trim_space(text)
             self._unflushed = ""
 

llms/tests/test_tokenizers.py

@@ -58,6 +58,9 @@ class TestTokenizers(unittest.TestCase):
         tokens = tokenizer.encode("import 'package:flutter/material.dart';")
         check(tokens)
 
+        tokens = tokenizer.encode("hello\nworld")
+        check(tokens)
+
     def test_tokenizers(self):
         tokenizer_repos = [
             ("mlx-community/Qwen1.5-0.5B-Chat-4bit", BPEStreamingDetokenizer),
@@ -65,6 +68,7 @@ class TestTokenizers(unittest.TestCase):
             ("mlx-community/Phi-3.5-mini-instruct-4bit", SPMStreamingDetokenizer),
             ("mlx-community/Mistral-7B-Instruct-v0.3", SPMStreamingDetokenizer),
             ("mlx-community/Llama-3.2-1B-Instruct-4bit", BPEStreamingDetokenizer),
+            ("mlx-community/Falcon3-7B-Instruct-4bit", BPEStreamingDetokenizer),
         ]
         for tokenizer_repo, expected_detokenizer in tokenizer_repos:
             with self.subTest(tokenizer=tokenizer_repo):