simple_tokenizer.py

import gzip
import html
import os
from functools import lru_cache

import ftfy
import regex as re


@lru_cache
def default_bpe():
    """Returns the file path to the default BPE vocabulary file 'bpe_simple_vocab_16e6.txt.gz'."""
    return os.path.join(os.path.dirname(os.path.abspath(__file__)), "bpe_simple_vocab_16e6.txt.gz")


@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 significant 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))


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


def basic_clean(text):
    """Clean text by fixing encoding issues and unescaping HTML entities, then stripping extraneous whitespace."""
    text = ftfy.fix_text(text)
    text = html.unescape(html.unescape(text))
    return text.strip()


def whitespace_clean(text):
    """Clean text by collapsing multiple whitespace characters into a single space and trimming leading/trailing
    whitespace.
    """
    text = re.sub(r"\s+", " ", text)
    text = text.strip()
    return text


class SimpleTokenizer:
    """Tokenizes text using byte pair encoding (BPE) and predefined tokenization rules for efficient text processing."""

    def __init__(self, bpe_path: str = default_bpe()):
        """Initialize the SimpleTokenizer object with byte pair encoding (BPE) paths and set up encoders, decoders, and
        patterns.
        """
        self.byte_encoder = bytes_to_unicode()
        self.byte_decoder = {v: k for k, v in self.byte_encoder.items()}
        merges = gzip.open(bpe_path).read().decode("utf-8").split("\n")
        merges = merges[1 : 49152 - 256 - 2 + 1]
        merges = [tuple(merge.split()) for merge in merges]
        vocab = list(bytes_to_unicode().values())
        vocab += [f"{v}</w>" for v in vocab]
        vocab.extend("".join(merge) for merge in merges)
        vocab.extend(["<|startoftext|>", "<|endoftext|>"])
        self.encoder = dict(zip(vocab, range(len(vocab))))
        self.decoder = {v: k for k, v in self.encoder.items()}
        self.bpe_ranks = dict(zip(merges, range(len(merges))))
        self.cache = {"<|startoftext|>": "<|startoftext|>", "<|endoftext|>": "<|endoftext|>"}
        self.pat = re.compile(
            r"""<\|startoftext\|>|<\|endoftext\|>|'s|'t|'re|'ve|'m|'ll|'d|[\p{L}]+|[\p{N}]|[^\s\p{L}\p{N}]+""",
            re.IGNORECASE,
        )

    def bpe(self, token):
        """Apply byte pair encoding (BPE) to a given token and cache the result."""
        if token in self.cache:
            return self.cache[token]
        word = tuple(token[:-1]) + (f"{token[-1]}</w>",)
        pairs = get_pairs(word)

        if not pairs:
            return f"{token}</w>"

        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 Exception:
                    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

    def encode(self, text):
        """Converts input text to BPE tokens using byte-pair encoding and pre-defined tokenization rules."""
        bpe_tokens = []
        text = whitespace_clean(basic_clean(text)).lower()
        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

    def decode(self, tokens):
        """Decodes a list of BPE tokens into a UTF-8 string, replacing '</w>' with a space."""
        text = "".join([self.decoder[token] for token in tokens])
        return bytearray([self.byte_decoder[c] for c in text]).decode("utf-8", errors="replace").replace("</w>", " ")