A new tutorial documents the step-by-step process of building and training a byte-level BPE tokenizer from first principles. The guide covers key implementation details such as Unicode-aware pretokenization, incremental pair-frequency updates, and using a max heap with lazy deletion.

  • Training starts from 256 raw byte tokens and learns merges on multilingual text.
  • A pair-to-pretoken inverted index is implemented to optimize the training process.
  • The model was trained on 100 MiB of mC4 data across eight languages.
  • It results in an 8,192-token vocabulary with 7,936 learned merges.
  • Training took roughly one hour on a 12-core M2 Pro with 32 GB of memory.

The author provides the full source code and explanations via a GitHub repository and includes a live demo for users to interact with the custom tokenizer.