tokenizers

This repository contains per-character, sub-word and word level tokenizers.

Per-Character

PerCharTokenizer() in perChar directory contains a character level tokenizer. It's very simple to understand and use. Each unique character present in the train_data builds the vocab for the tokenizer. Not very reliable for big projects, only good for training small models for experimentation.

# this is a basic character level tokeinzer

chars = sorted(list(set(text)))
vocab_size = len(chars)

# encoder - decoder
stoi = { ch:i for i,ch in enumerate(chars) }
itos = { i:ch for i,ch in enumerate(chars) }
encode = lambda s: [stoi[c] for c in s] # encoder: takes a string, returns a list of integers
decode = lambda l: ''.join([itos[i] for i in l]) # decoder: takes a list of integers, returns a string

How to use

tokenizer = PerCharTokenizer()
tokenizer.train(train_text=train_data)
tokenizer.save_model(prefix='perChar')  # saves the model
tokenizer.load(model_path='../path_to_model')  # loads the model

text = "My name is Alan"
print(tokenizer.encode(text))
print(tokenizer.decode(tokenizer.encode(text)))

Sub-word

A byte-pair encoding tokenizer, with a little different architecture. Instead of using 'utf-8' encodings for the initial vocab of size 256, it uses the all the unique characters present in a data set for the initial vocab which means vocab_size can be larger or smaller than 256 at first. Then it adds rest of the pairs and merges vocab like usual byte-pair encoder.

# basic code

class BasicTokenizer:
  def __init__(self, train_text):
    super().__init__()
    self.chars = sorted(list(set(train_text)))
    self.train_data = train_text
    self.vocab_size = len(self.chars)
    self.string_to_index = { ch:i for i,ch in enumerate(self.chars) }
    self.index_to_string = { i:ch for i,ch in enumerate(self.chars) }

  def _build_vocab(self, merges):
    vocab = {i: ids for i, ids in enumerate(self.chars)}
    for (p0, p1), idx in merges.items():
      vocab[idx] = vocab[p0] + vocab[p1]
    return vocab

  def train(self, target_vocab):
    tokens = list(self._encode(self.train_data))
    ids = list(tokens)
    n_merges = target_vocab - self.vocab_size
    merges = {}
    for i in tqdm(range(n_merges), desc='Training the tokenizer\t'):
      stats = self._get_stats(ids)
      pair = max(stats, key=stats.get)
      idx = self.vocab_size + i
      ids = self._merge(ids, pair, idx)
      merges[pair] = idx
    self.vocab = self._build_vocab(merges)
    self.merges = merges
   return self.vocab, self.merges

# ... continued

This one is better preferred for big applications like training a LLM model. Language models don't see text like you and I, instead they see a sequence of numbers (known as tokens). Byte pair encoding (BPE) is a way of converting text into tokens. It has a couple desirable properties:

1. It's reversible and lossless, so you can convert tokens back into the original text
2. It works on arbitrary text, even text that is not in the tokenizer's training data
3. It compresses the text: the token sequence is shorter than the bytes corresponding to the original text
4. It attempts to let the model see common sub-words. For instance, "ing" is a common sub-word in English, so BPE encodings will often split "encoding" into tokens like "encod" and "ing" (instead of e.g. "enc" and "oding"). Because the model will then see the "ing" token again and again in different contexts, it helps models generalize and better understand grammar.

How to use:

from miniBPE import BasicTokenizer
name = '../models/basicCharMap'
tokenizer = BasicTokenizer(train_text)
tokenizer.train(target_vocab=4000)
tokenizer.save_model(name)  # saves the model
tokenizer.load('../path_to_model')  # loads the model

text = "My name is Alan"
print(tokenizer.encode(text))  # encoder
print(tokenizer.decode(tokenizer.encode(text)))  # decoder

Word level

still to build

DNA tokenizer (k-mers)

Let's say we have a long sequence of DNA. This tokenizer splits that sequence into sections of consecutively occurring bases, and each section has length of value equal to k_mer which is by default set to 4. This way, the vocab formed will be equal to {k_mers^(no. of unique characters)}

How to use:

from tokenizer import KMerTokenizer

tokenizer = KMerTokenizer(k_mers=5)
tokenizer.build_vocab([train_data])
tokenizer.save_model('../tokenizer/trained models')

encoded_tokens = tokenizer.encode(test_data)
decoded_tokens = tokenizer.decode(encoded_tokens)

One more feature is there, if you want to train it in many iterations like transformers, you can use continue_train() function, to keep increasing the size of vocab with your needs.

from subDNA import DNAtokenizer

token = DNAtokenizer()
token.load_model(model_path='path_to_model')
token.continue_train(train_data=data, n_merges=200)
token.save_model(model_prefix='path_to_model')

Contributing

Pull requests are welcome. For major changes, please open an issue first to discuss what you would like to change. Please make sure to update tests as appropriate.

License

none for now!