diff --git a/minbpe/base.py b/minbpe/base.py
index 65cc45cf..077d836e 100644
--- a/minbpe/base.py
+++ b/minbpe/base.py
@@ -6,6 +6,7 @@
 some concessions are made for simplicity.
 """
 import unicodedata
+from typing import Optional
 
 # -----------------------------------------------------------------------------
 # a few helper functions useful for both BasicTokenizer and RegexTokenizer
@@ -22,7 +23,7 @@ def get_stats(ids, counts=None):
     return counts
 
 
-def merge(ids, pair, idx):
+def merge(ids, pair, idx, stats: Optional[dict] = None):
     """
     In the list of integers (ids), replace all consecutive occurrences
     of pair with the new integer token idx
@@ -30,13 +31,29 @@ def merge(ids, pair, idx):
     """
     newids = []
     i = 0
+    curr_new_idx = -1
     while i < len(ids):
         # if not at the very last position AND the pair matches, replace it
         if ids[i] == pair[0] and i < len(ids) - 1 and ids[i+1] == pair[1]:
             newids.append(idx)
+            curr_new_idx += 1
+            if stats is not None:
+                if (curr_new_idx > 0): # there is a token before this pair. Updating stat
+                    old_pair = (newids[curr_new_idx - 1], ids[i])
+                    new_pair = (newids[curr_new_idx - 1], idx)
+                    assert old_pair in stats
+                    stats[old_pair] -= 1
+                    stats[new_pair] = stats.get(new_pair, 0) + 1
+                if (i < (len(ids) - 2)): # there is a token after this pair. Updating stat
+                    old_pair = (ids[i + 1], ids[i + 2])
+                    new_pair = (idx, ids[i + 2])
+                    assert old_pair in stats
+                    stats[old_pair] -= 1
+                    stats[new_pair] = stats.get(new_pair, 0) + 1
             i += 2
         else:
             newids.append(ids[i])
+            curr_new_idx += 1
             i += 1
     return newids
 
diff --git a/minbpe/basic.py b/minbpe/basic.py
index 9bc5ab76..56bb431d 100644
--- a/minbpe/basic.py
+++ b/minbpe/basic.py
@@ -28,21 +28,22 @@ def train(self, text, vocab_size, verbose=False):
         # iteratively merge the most common pairs to create new tokens
         merges = {} # (int, int) -> int
         vocab = {idx: bytes([idx]) for idx in range(256)} # int -> bytes
+        # count up the number of times every consecutive pair appears
+        stats = get_stats(ids)
         for i in range(num_merges):
-            # count up the number of times every consecutive pair appears
-            stats = get_stats(ids)
-            # find the pair with the highest count
-            pair = max(stats, key=stats.get)
             # mint a new token: assign it the next available id
             idx = 256 + i
+            # find the pair with the highest count
+            pair = max(stats, key=stats.get)
             # replace all occurrences of pair in ids with idx
-            ids = merge(ids, pair, idx)
+            ids = merge(ids, pair, idx, stats)
             # save the merge
             merges[pair] = idx
             vocab[idx] = vocab[pair[0]] + vocab[pair[1]]
             # prints
             if verbose:
                 print(f"merge {i+1}/{num_merges}: {pair} -> {idx} ({vocab[idx]}) had {stats[pair]} occurrences")
+            stats[pair] = 0
 
         # save class variables
         self.merges = merges # used in encode()
diff --git a/minbpe/regex.py b/minbpe/regex.py
index 9ed78e43..8e45767d 100644
--- a/minbpe/regex.py
+++ b/minbpe/regex.py
@@ -47,16 +47,14 @@ def train(self, text, vocab_size, verbose=False):
         merges = {} # (int, int) -> int
         vocab = {idx: bytes([idx]) for idx in range(256)} # idx -> bytes
         for i in range(num_merges):
-            # count the number of times every consecutive pair appears
             stats = {}
             for chunk_ids in ids:
-                # passing in stats will update it in place, adding up counts
                 get_stats(chunk_ids, stats)
             # find the pair with the highest count
             pair = max(stats, key=stats.get)
             # mint a new token: assign it the next available id
             idx = 256 + i
-            # replace all occurrences of pair in ids with idx
+            # replace all occurrences of pair in ids with idx and update pair-count simultaneously
             ids = [merge(chunk_ids, pair, idx) for chunk_ids in ids]
             # save the merge
             merges[pair] = idx
@@ -64,6 +62,7 @@ def train(self, text, vocab_size, verbose=False):
             # prints
             if verbose:
                 print(f"merge {i+1}/{num_merges}: {pair} -> {idx} ({vocab[idx]}) had {stats[pair]} occurrences")
+            stats[pair] = 0
 
         # save class variables
         self.merges = merges # used in encode()