diff --git a/bpe.py b/bpe.py
new file mode 100644
index 0000000..c9901ca
--- /dev/null
+++ b/bpe.py
@@ -0,0 +1,159 @@
+import os
+import re
+from collections import Counter
+from typing import Iterable, Iterator
+
+# Regular expression for GPT-2 tokenization (approximation without regex module)
+TOKEN_PATTERN = re.compile(
+    r"'s|'t|'re|'ve|'m|'ll|'d| ?[^\\W\\d_]+| ?\\d+| ?[^\\s\\w]+|\\s+(?!\\S)|\\s+",
+    re.UNICODE,
+)
+
+
+def _tokenize(text: str, special_tokens: list[str] | None) -> list[str]:
+    """Split text into tokens, keeping special tokens intact."""
+    if not special_tokens:
+        return TOKEN_PATTERN.findall(text)
+    tokens: list[str] = []
+    i = 0
+    last = 0
+    specials = set(special_tokens)
+    length = len(text)
+    while i < length:
+        match = None
+        for s in specials:
+            if text.startswith(s, i):
+                match = s
+                break
+        if match:
+            if last < i:
+                tokens.extend(TOKEN_PATTERN.findall(text[last:i]))
+            tokens.append(match)
+            i += len(match)
+            last = i
+        else:
+            i += 1
+    if last < length:
+        tokens.extend(TOKEN_PATTERN.findall(text[last:]))
+    return tokens
+
+
+def train_bpe(
+    input_path: str | os.PathLike[str],
+    vocab_size: int,
+    special_tokens: list[str] | None = None,
+) -> tuple[dict[int, bytes], list[tuple[bytes, bytes]]]:
+    """Train a simple byte pair encoder on the given corpus."""
+    with open(input_path, "r", encoding="utf-8") as f:
+        text = f.read()
+
+    token_strings = _tokenize(text, special_tokens or [])
+
+    token_freq = Counter(token_strings)
+
+    # Represent each token as a tuple of byte strings
+    word_freq: dict[tuple[bytes, ...], int] = {}
+    specials = set(special_tokens or [])
+    for token, freq in token_freq.items():
+        if token in specials:
+            word = (token.encode("utf-8"),)
+        else:
+            word = tuple(bytes([b]) for b in token.encode("utf-8"))
+        word_freq[word] = freq
+
+    vocab: dict[int, bytes] = {i: bytes([i]) for i in range(256)}
+    merges: list[tuple[bytes, bytes]] = []
+
+    while len(vocab) + len(specials) < vocab_size:
+        # Count adjacent pairs
+        pair_counts: Counter[tuple[bytes, bytes]] = Counter()
+        for word, freq in word_freq.items():
+            for i in range(len(word) - 1):
+                pair_counts[(word[i], word[i + 1])] += freq
+        if not pair_counts:
+            break
+        best_pair = max(pair_counts.items(), key=lambda x: (x[1], x[0]))[0]
+        merges.append(best_pair)
+        new_symbol = best_pair[0] + best_pair[1]
+        vocab[len(vocab)] = new_symbol
+
+        # Replace occurrences of best_pair in all words
+        new_word_freq: dict[tuple[bytes, ...], int] = {}
+        first, second = best_pair
+        for word, freq in word_freq.items():
+            i = 0
+            new_word: list[bytes] = []
+            while i < len(word):
+                if i < len(word) - 1 and word[i] == first and word[i + 1] == second:
+                    new_word.append(new_symbol)
+                    i += 2
+                else:
+                    new_word.append(word[i])
+                    i += 1
+            new_word_freq[tuple(new_word)] = freq
+        word_freq = new_word_freq
+
+    # Append special tokens to vocab
+    for token in specials:
+        vocab[len(vocab)] = token.encode("utf-8")
+
+    return vocab, merges
+
+
+class Tokenizer:
+    def __init__(
+        self,
+        vocab: dict[int, bytes],
+        merges: list[tuple[bytes, bytes]],
+        special_tokens: list[str] | None = None,
+    ) -> None:
+        self.vocab = vocab
+        self.byte_to_id = {v: k for k, v in vocab.items()}
+        self.special_tokens = special_tokens or []
+        self.bpe_ranks = {pair: i for i, pair in enumerate(merges)}
+
+    def _bpe(self, token_bytes: bytes) -> list[bytes]:
+        word = [bytes([b]) for b in token_bytes]
+        if not word:
+            return []
+        while True:
+            pairs = [(word[i], word[i + 1]) for i in range(len(word) - 1)]
+            if not pairs:
+                break
+            ranked = [self.bpe_ranks.get(p, float("inf")) for p in pairs]
+            min_rank = min(ranked)
+            if min_rank == float("inf"):
+                break
+            idx = ranked.index(min_rank)
+            first, second = word[idx], word[idx + 1]
+            word = (
+                word[:idx]
+                + [first + second]
+                + word[idx + 2 :]
+            )
+        return word
+
+    def _encode_generator(self, text: str) -> Iterator[int]:
+        tokens = _tokenize(text, self.special_tokens)
+        specials = {s: self.byte_to_id[s.encode("utf-8")] for s in self.special_tokens}
+        for token in tokens:
+            if token in specials:
+                yield specials[token]
+            else:
+                token_bytes = token.encode("utf-8")
+                for piece in self._bpe(token_bytes):
+                    yield self.byte_to_id[piece]
+
+    def encode(self, text: str) -> list[int]:
+        return list(self._encode_generator(text))
+
+    def encode_iterable(self, iterable: Iterable[str]) -> Iterator[int]:
+        for chunk in iterable:
+            yield from self._encode_generator(chunk)
+
+    def decode(self, ids: Iterable[int]) -> str:
+        output = b"".join(self.vocab[i] for i in ids)
+        return output.decode("utf-8", errors="replace")
+
+
+__all__ = ["train_bpe", "Tokenizer"]
diff --git a/tests/adapters.py b/tests/adapters.py
index 11a817c..2e408ce 100644
--- a/tests/adapters.py
+++ b/tests/adapters.py
@@ -9,6 +9,8 @@
 import torch
 from torch import Tensor
 
+from bpe import Tokenizer, train_bpe
+
 
 def run_linear(
     d_in: int,
@@ -559,7 +561,7 @@ def get_tokenizer(
     Returns:
         A BPE tokenizer that uses the provided vocab, merges, and special tokens.
     """
-    raise NotImplementedError
+    return Tokenizer(vocab, merges, special_tokens)
 
 
 def run_train_bpe(
@@ -589,4 +591,4 @@ def run_train_bpe(
                 representing that <token1> was merged with <token2>.
                 Merges are ordered by order of creation.
     """
-    raise NotImplementedError
+    return train_bpe(input_path, vocab_size, special_tokens)