GAT-K

Overview

This repository contains a Grapheme-Aware Tokenizer (GAT) specifically trained for Kannada, designed to handle the unique orthographic and phonological structure of the language. Unlike traditional subword tokenizers such as BPE, SentencePiece, or WordPiece, this tokenizer operates at the grapheme level, improving representation fidelity and reducing subword fragmentation.

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Available Vocabulary Sizes

This repository includes three tokenizer variants:

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Why Grapheme-Aware Preprocessing?

Kannada is an Abugida script where a single grapheme (akshara) may be composed of:

• multiple consonants
• a halant (virama)
• vowel diacritics (matra)

For example:

ಕ್ರಿ

is one grapheme, but composed of multiple Unicode codepoints.

ಕ್ + ರ್ + ಿ → 3–4 fragments

Problem with BPE / SentencePiece / WordPiece

These tokenizers operate at the byte or character level:

This results in:

• stable semantic units
• better compression
• more efficient tokenization

GAT Solution

GAT applies a custom grapheme parser that merges the components into one atomic unit:

GAT uses a rule-based finite-state parser that correctly handles:

• consonants
• vowels
• halants
• vowel signs
• anusvara & visarga

<p align="center"> <img src="./GAT-algo.png" width="650"/> </p>

After grapheme segmentation, Byte Pair Encoding (BPE) is applied to learn higher-level merges.

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Training Data

Tokenizer training uses a composite 4.5M-sentence Kannada corpus:

1. Samanantar Dataset (AI4Bharat)
2. Kannada-Instruct Dataset (Cognitive Lab)

This provides broad coverage of conversational, literary, and instruction-following Kannada.

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Tokenizer Metrics

These metrics evaluate tokenizer quality independent of any downstream NLP model.

Compression Ratio (CR)

Higher = better (larger text compressed into fewer bytes)

Fertility Score (FS)

Lower = better (#tokens produced per grapheme/character)

Results for CR and FS

GAT consistently showed better compression ratio and fertility score across vocab sizes .

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CR : 3.5 -> 3.9 -> 4.8

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FS : 2.1 -> 1.8 -> 1.6

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💻 Usage Example

Load the 32k tokenizer

from transformers import PreTrainedTokenizerFast

tokenizer = PreTrainedTokenizerFast.from_pretrained(
    "varuni/GAT-K",
    tokenizer_file="GAT_Kannada_32k.json"
)

text = "ನಿಮ್ಮ ಹೆಸರು ಏನು?"
print(tokenizer.encode(text))

Related work :

• M. Velayuthan and K. Sarveswaran, “Egalitarian Language Representation in Language Models: It All Begins with Tokenizers,” COLING 2025.
  arXiv:2409.11501 [cs.CL]. DOI: https://doi.org/10.48550/arXiv.2409.11501

• Unicode Normalization and Grapheme Parsing of Indic Languages, 2023. [Online]. Available: https://arxiv.org/abs/2306.01743

• M. K. H. and A. Giri, “Orthographic Syllable Pair Encoding for Language Modelling Tasks in Indic Languages,” in 2023 IEEE MIT Undergraduate Research Technology Conference (URTC), Cambridge, MA, USA, pp. 1–6, 2023.
  DOI: https://doi.org/10.1109/URTC60662.2023.10534970

• R. Sennrich, B. Haddow, and A. Birch, “Neural machine translation of rare words with subword units,” in Proceedings of the 54th Annual Meeting of the Association for Computational Linguistics (ACL), 2016. [Online]. Available: https://arxiv.org/abs/1508.07909