import os
import torch
from datasets import load_dataset
from transformers import (
    AutoTokenizer,
    AutoModelForCausalLM,
    TrainingArguments,
    BitsAndBytesConfig,
)
from peft import (
    LoraConfig,
    get_peft_model,
    prepare_model_for_kbit_training,
)
from trl import SFTTrainer

# ----------------------------
# Environment safety (Windows)
# ----------------------------
os.environ["TORCHDYNAMO_DISABLE"] = "1"

# ----------------------------
# Global configuration
# ----------------------------
MODEL_NAME = "Qwen/Qwen2.5-7B-Instruct"
OUTPUT_DIR = "./qwen2.5-7b-uk-fr-lora"
DATA_FILE = "paires_clean.json"
MAX_SEQ_LENGTH = 1024

print(f"\n=== Starting fine-tuning script for {MODEL_NAME} ===\n")

# ----------------------------
# [1/7] Tokenizer
# ----------------------------
print(f"{80 * '_'}\n[1/7] Loading tokenizer...")
tokenizer = AutoTokenizer.from_pretrained(
    MODEL_NAME,
    trust_remote_code=True
)

tokenizer.pad_token = tokenizer.eos_token
tokenizer.model_max_length = MAX_SEQ_LENGTH

print("Tokenizer loaded.")
print(f"Pad token id: {tokenizer.pad_token_id}")
print(f"Max sequence length: {tokenizer.model_max_length}")

# ----------------------------
# [2/7] Quantization config (QLoRA)
# ----------------------------
print(f"{80 * '_'}\n[2/7] Configuring 4-bit quantization (BitsAndBytes)...")
bnb_config = BitsAndBytesConfig(
    load_in_4bit=True,
    bnb_4bit_quant_type="nf4",
    bnb_4bit_compute_dtype=torch.float16,
    bnb_4bit_use_double_quant=True,
)

print("4-bit NF4 quantization configured.")

print("Loading model...")
model = AutoModelForCausalLM.from_pretrained(
    MODEL_NAME,
    device_map="auto",
    quantization_config=bnb_config,
    dtype=torch.float16,
    trust_remote_code=True,
)
print("Model loaded successfully.")

# ----------------------------
# [3/7] Prepare model for k-bit training
# ----------------------------
print(f"{80 * '_'}\n[3/7] Preparing model for k-bit training...")
model = prepare_model_for_kbit_training(model)

model.gradient_checkpointing_enable(
    gradient_checkpointing_kwargs={"use_reentrant": False}
)

print("Model prepared for k-bit training.")
print("Gradient checkpointing enabled (non-reentrant).")

# ----------------------------
# [4/7] LoRA configuration
# ----------------------------
print(f"{80 * '_'}\n[4/7] Configuring LoRA adapters...")
lora_config = LoraConfig(
    r=32,
    lora_alpha=64,
    lora_dropout=0.02,
    bias="none",
    task_type="CAUSAL_LM",
    target_modules=[
        "q_proj",
        "k_proj",
        "v_proj",
        "o_proj",
        "gate_proj",
        "up_proj",
        "down_proj",
    ],
)

model = get_peft_model(model, lora_config)
model.print_trainable_parameters()

print("LoRA adapters successfully attached.")

# ----------------------------
# [5/7] Dataset loading & formatting
# ----------------------------
print(f"{80 * '_'}\n[5/7] Loading dataset from JSON file...")
dataset = load_dataset("json", data_files=DATA_FILE)

print(f"Dataset loaded with {len(dataset['train'])} samples.")

print("Formatting dataset for Ukrainian → French translation...")

def format_prompt(example):
    return {
        "text": (
            "<|user|>\n"
            "Translate the following Ukrainian text into French.\n"
            f"Ukrainian: {example['text']}\n"
            "<|assistant|>\n"
            f"{example['translation']}"
        )
    }

dataset = dataset.map(
    format_prompt,
    remove_columns=dataset["train"].column_names
)

print("Dataset formatting completed.")
print("Example prompt:\n")
print(dataset["train"][0]["text"])

# ----------------------------
# [6/7] Training arguments
# ----------------------------
print(f"{80 * '_'}\n[6/7] Initializing training arguments...")
training_args = TrainingArguments(
    output_dir=OUTPUT_DIR,
    per_device_train_batch_size=1,
    gradient_accumulation_steps=8,
    learning_rate=1e-4,
    num_train_epochs=3,
    fp16=False,
    bf16=False,
    optim="paged_adamw_32bit",
    logging_steps=10,
    save_steps=500,
    save_total_limit=2,
    report_to="none",
)

print("Training arguments ready.")
print(f"Output directory: {OUTPUT_DIR}")
print(f"Epochs: {training_args.num_train_epochs}")
print(
    f"Effective batch size: "
    f"{training_args.per_device_train_batch_size * training_args.gradient_accumulation_steps}"
)

# ----------------------------
# Trainer
# ----------------------------
print("Initializing SFTTrainer...")
trainer = SFTTrainer(
    model=model,
    train_dataset=dataset["train"],
    tokenizer=tokenizer,
    args=training_args,
)
print("Trainer initialized.")

# ----------------------------
# [7/7] Training
# ----------------------------
print(f"{80 * '_'}\n[7/7] Starting training...")
try:
    trainer.train(resume_from_checkpoint=True)
except Exception as e:
    print("No checkpoint found or resume failed, starting fresh training.")
    print(f"Reason: {e}")
    trainer.train()

print("Training completed successfully.")

# ----------------------------
# Save LoRA adapter
# ----------------------------
print(f"{80 * '_'}\nSaving LoRA adapter and tokenizer...")
trainer.model.save_pretrained(OUTPUT_DIR)
tokenizer.save_pretrained(OUTPUT_DIR)

print("\n=== Fine-tuning finished ===")
print(f"LoRA adapter saved in: {OUTPUT_DIR}")
