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.