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# SillokBert-Scratch 프로젝트 3단계: 사전학습 (Pre-training)
# -----------------------------------------------------------------
# 1, 2단계 결과물을 사용하여 Masked Language Modeling(MLM)으로
# 사전학습을 진행합니다.
# -----------------------------------------------------------------
import os
from pathlib import Path
from transformers import (
    BertConfig,
    BertForMaskedLM,
    PreTrainedTokenizerFast,
    DataCollatorForLanguageModeling,
    Trainer,
    TrainingArguments,
)
from datasets import load_dataset
from itertools import chain

def pretrain_sillok_bert():
    """SillokBert-Scratch 모델의 사전학습을 수행합니다."""
    # --- 경로 설정 ---
    project_dir = Path("/home/work/baro/sillok/sillok_scratch_20250626")
    tokenizer_dir = project_dir / "sillok_tokenizer_bpe_preprocessed"
    tokenizer_file = tokenizer_dir / "tokenizer.json"
    dataset_dir = "/home/work/baro/sillok25060103/preprocessed_corpus/"
    output_dir = project_dir / "sillokbert_scratch_pretraining_output"

    print("--- 3. SillokBert-Scratch Pre-training ---")

    # --- 토크나이저 로드 ---
    tokenizer = PreTrainedTokenizerFast(tokenizer_file=str(tokenizer_file))
    if tokenizer.pad_token is None: tokenizer.pad_token = '[PAD]'
    if tokenizer.mask_token is None: tokenizer.mask_token = '[MASK]'

    # --- 모델 아키텍처 로드 ---
    block_size = 512
    config = BertConfig(
        vocab_size=tokenizer.vocab_size, hidden_size=768, num_hidden_layers=12,
        num_attention_heads=12, intermediate_size=3072,
        max_position_embeddings=block_size, pad_token_id=tokenizer.pad_token_id,
    )
    model = BertForMaskedLM(config=config)

    # --- 데이터셋 준비 ---
    dataset = load_dataset('text', data_files={
        'train': os.path.join(dataset_dir, 'train.txt'),
        'validation': os.path.join(dataset_dir, 'validation.txt'),
    })
    
    def tokenize_function(examples):
        return tokenizer(examples['text'], add_special_tokens=False, return_special_tokens_mask=False)

    tokenized_datasets = dataset.map(tokenize_function, batched=True, num_proc=4, remove_columns=['text'])
    
    def group_texts(examples):
        concatenated_examples = {k: list(chain(*examples[k])) for k in examples.keys()}
        total_length = len(concatenated_examples[list(examples.keys())[0]])
        total_length = (total_length // block_size) * block_size
        result = {k: [t[i : i + block_size] for i in range(0, total_length, block_size)] for k, t in concatenated_examples.items()}
        return result

    lm_datasets = tokenized_datasets.map(group_texts, batched=True, num_proc=4)

    # --- Trainer 설정 및 훈련 ---
    data_collator = DataCollatorForLanguageModeling(tokenizer=tokenizer, mlm=True, mlm_probability=0.15)
    
    training_args = TrainingArguments(
        output_dir=output_dir, overwrite_output_dir=True, num_train_epochs=10,
        per_device_train_batch_size=4, per_device_eval_batch_size=8,
        gradient_accumulation_steps=4, gradient_checkpointing=True,
        learning_rate=5e-5, warmup_steps=1000, weight_decay=0.01,
        logging_dir=output_dir / 'logs', logging_steps=500, save_steps=2000,
        eval_strategy="steps", eval_steps=2000, load_best_model_at_end=True, fp16=True,
    )

    trainer = Trainer(
        model=model, args=training_args, data_collator=data_collator,
        train_dataset=lm_datasets['train'], eval_dataset=lm_datasets['validation'],
    )
    
    print("사전학습을 시작합니다...")
    trainer.train()

    # --- 최종 모델 저장 ---
    final_model_path = output_dir / "final_model"
    trainer.save_model(final_model_path)
    tokenizer.save_pretrained(final_model_path)
    print(f"\n🎉 사전학습 완료. 최종 모델 저장 경로: {final_model_path}")

if __name__ == "__main__":
    pretrain_sillok_bert()