# Copyright 2022 Cerebras Systems.
#
# Licensed under the Apache License, Version 2.0 (the "License");
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#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
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"""Byte pair encoding/decoding utilities
Modified from the GPT-2 codebase: https://github.com/openai/gpt-2
"""
import json
from functools import lru_cache
import regex as re
[docs]@lru_cache()
def bytes_to_unicode():
"""
Returns list of utf-8 byte and a corresponding list of unicode strings.
The reversible bpe codes work on unicode strings.
This means you need a large # of unicode characters in your vocab if you want to avoid UNKs.
When you're at something like a 10B token dataset you end up needing around 5K for decent coverage.
This is a signficant percentage of your normal, say, 32K bpe vocab.
To avoid that, we want lookup tables between utf-8 bytes and unicode strings.
And avoids mapping to whitespace/control characters the bpe code barfs on.
"""
bs = (
list(range(ord("!"), ord("~") + 1))
+ list(range(ord("¡"), ord("¬") + 1))
+ list(range(ord("®"), ord("ÿ") + 1))
)
cs = bs[:]
n = 0
for b in range(2 ** 8):
if b not in bs:
bs.append(b)
cs.append(2 ** 8 + n)
n += 1
cs = [chr(n) for n in cs]
return dict(zip(bs, cs))
[docs]def get_pairs(word):
"""Return set of symbol pairs in a word.
Word is represented as tuple of symbols (symbols being variable-length strings).
"""
pairs = set()
prev_char = word[0]
for char in word[1:]:
pairs.add((prev_char, char))
prev_char = char
return pairs
[docs]class BPETokenizer:
[docs] def __init__(
self, vocab_file, encoder_file, errors='replace', special_tokens=None
):
with open(vocab_file, 'r', encoding="utf-8") as f:
bpe_data = f.read()
bpe_merges = [
tuple(merge_str.split()) for merge_str in bpe_data.split('\n')[1:-1]
]
with open(encoder_file, 'r') as f:
self.encoder = json.load(f)
self.decoder = {v: k for k, v in self.encoder.items()}
# Assert encoder file is 1-1
assert len(self.encoder) == len(self.decoder), (
f"BPETokenizer: Length mismatch."
f" This can happen when multiple words in the encoder"
f" are mapped to the same id."
)
if special_tokens:
for t in special_tokens:
self.add_token(t)
self.errors = errors # how to handle errors in decoding
self.byte_encoder = bytes_to_unicode()
self.byte_decoder = {v: k for k, v in self.byte_encoder.items()}
self.bpe_ranks = dict(zip(bpe_merges, range(len(bpe_merges))))
self.cache = {}
# Should haved added re.IGNORECASE so BPE merges can happen for
# capitalized versions of contractions
self.pat = re.compile(
r"""'s|'t|'re|'ve|'m|'ll|'d| ?\p{L}+| ?\p{N}+| ?[^\s\p{L}\p{N}]+|\s+(?!\S)|\s+"""
)
[docs] def bpe(self, token):
if token in self.cache:
return self.cache[token]
word = tuple(token)
pairs = get_pairs(word)
if not pairs:
return token
while True:
bigram = min(
pairs, key=lambda pair: self.bpe_ranks.get(pair, float('inf'))
)
if bigram not in self.bpe_ranks:
break
first, second = bigram
new_word = []
i = 0
while i < len(word):
try:
j = word.index(first, i)
new_word.extend(word[i:j])
i = j
except:
new_word.extend(word[i:])
break
if (
word[i] == first
and i < len(word) - 1
and word[i + 1] == second
):
new_word.append(first + second)
i += 2
else:
new_word.append(word[i])
i += 1
new_word = tuple(new_word)
word = new_word
if len(word) == 1:
break
else:
pairs = get_pairs(word)
word = ' '.join(word)
self.cache[token] = word
return word
[docs] def encode(self, text):
bpe_tokens = []
for token in re.findall(self.pat, text):
token = ''.join(self.byte_encoder[b] for b in token.encode('utf-8'))
bpe_tokens.extend(
self.encoder[bpe_token]
for bpe_token in self.bpe(token).split(' ')
)
return bpe_tokens
[docs] def decode(self, tokens):
text = ''.join([self.decoder[token] for token in tokens])
text = bytearray([self.byte_decoder[c] for c in text]).decode(
'utf-8', errors=self.errors
)
return text
[docs] def get_token_id(self, token):
if token in self.encoder:
return self.encoder[token]
[docs] def add_token(self, token):
if token in self.encoder:
print(
f"BPETokenizer: {token} already exists in tokenizer"
f" with id {self.encoder[token]}."
)
else:
token_id = max(self.decoder.keys()) + 1
self.encoder[token] = token_id
self.decoder[token_id] = token
print(f"BPETokenizer: {token} added with token_id {token_id}")