app/main.py

import os
import torch
from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse
from transformers import AutoModelForCausalLM, AutoTokenizer
from peft import PeftModel
from starlette.middleware.cors import CORSMiddleware

# === Setup FastAPI ===
app = FastAPI()

# === CORS (optional for frontend access) ===
app.add_middleware(
    CORSMiddleware,
    allow_origins=["*"],
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

# === Load API Key from Hugging Face Secrets ===
API_KEY = os.getenv("API_KEY", "undefined")  # Add API_KEY in your HF Space Secrets

# === Model Settings ===
BASE_MODEL = "Qwen/Qwen2-0.5B-Instruct"
ADAPTER_PATH = "adapter"

print("🔧 Loading tokenizer...")
tokenizer = AutoTokenizer.from_pretrained(BASE_MODEL, trust_remote_code=True)

print("🧠 Loading base model on CPU...")
base_model = AutoModelForCausalLM.from_pretrained(
    BASE_MODEL,
    trust_remote_code=True,
    torch_dtype=torch.float32
).cpu()

print("🔗 Applying LoRA adapter...")
model = PeftModel.from_pretrained(base_model, ADAPTER_PATH).cpu()
model.eval()

print("✅ Model and adapter loaded successfully.")

# === Root Route ===
@app.get("/")
def root():
    return {"message": "🧠 Qwen2.5-0.5B-Instruct API is running on CPU!"}

# === Chat Completion API ===
@app.post("/v1/chat/completions")
async def chat(request: Request):
    # ✅ API Key Authorization
    auth_header = request.headers.get("Authorization", "")
    if not auth_header.startswith("Bearer "):
        return JSONResponse(status_code=401, content={"error": "Missing Bearer token in Authorization header."})
    
    token = auth_header.replace("Bearer ", "").strip()
    if token != API_KEY:
        return JSONResponse(status_code=401, content={"error": "Invalid API key."})

    # ✅ Parse Request
    try:
        body = await request.json()
        messages = body.get("messages", [])
        if not messages or not isinstance(messages, list):
            raise ValueError("Invalid or missing 'messages' field.")
        
        # ✅ FIXED: Process full conversation history, not just last message
        temperature = body.get("temperature", 0.7)
        max_tokens = body.get("max_tokens", 512)
        
    except Exception as e:
        return JSONResponse(status_code=400, content={"error": f"Bad request: {str(e)}"})

    # ✅ FIXED: Build full conversation prompt with history
    formatted_prompt = ""
    
    for message in messages:
        role = message.get("role", "")
        content = message.get("content", "")
        
        if role == "system":
            formatted_prompt += f"<|im_start|>system\n{content}<|im_end|>\n"
        elif role == "user":
            formatted_prompt += f"<|im_start|>user\n{content}<|im_end|>\n"
        elif role == "assistant":
            formatted_prompt += f"<|im_start|>assistant\n{content}<|im_end|>\n"
    
    # Add the assistant start token for generation
    formatted_prompt += "<|im_start|>assistant\n"
    
    print(f"🤖 Processing conversation with {len(messages)} messages")
    print(f"📝 Full prompt preview: {formatted_prompt[:200]}...")

    inputs = tokenizer(formatted_prompt, return_tensors="pt").to("cpu")

    # ✅ Generate Response
    with torch.no_grad():
        outputs = model.generate(
            **inputs,
            max_new_tokens=max_tokens,
            temperature=temperature,
            top_p=0.9,
            do_sample=True,
            pad_token_id=tokenizer.eos_token_id
        )

    decoded = tokenizer.decode(outputs[0], skip_special_tokens=True)
    
    # ✅ FIXED: Clean extraction of only the new assistant response
    final_answer = decoded.split("<|im_start|>assistant\n")[-1].strip()
    
    # Remove any potential end tokens or artifacts
    if "<|im_end|>" in final_answer:
        final_answer = final_answer.split("<|im_end|>")[0].strip()
    
    print(f"✅ Generated response: {final_answer[:100]}...")

    # ✅ OpenAI-style Response
    return {
        "id": "chatcmpl-local-001",
        "object": "chat.completion",
        "model": "Qwen2.5-0.5B-Instruct-LoRA",
        "choices": [
            {
                "index": 0,
                "message": {
                    "role": "assistant",
                    "content": final_answer
                },
                "finish_reason": "stop"
            }
        ]
    }