app.py

import gradio as gr

# -----------------------------
# LlamaIndex (LLM Layer)
# -----------------------------
from llama_index.llms.huggingface import HuggingFaceLLM

llm = HuggingFaceLLM(
    model_name="google/flan-t5-base",
    tokenizer_name="google/flan-t5-base",
    context_window=512,
    max_new_tokens=256,
    generate_kwargs={
        "temperature": 0.3,
        "do_sample": False
    }
)

# -----------------------------
# LangChain (Prompt + Chain)
# -----------------------------
from langchain_core.prompts import PromptTemplate
from langchain.chains import LLMChain
from langchain.llms.base import LLM

# -----------------------------
# Adapter: LlamaIndex → LangChain
# -----------------------------
class LlamaIndexLLMAdapter(LLM):
    """Adapter to use LlamaIndex LLM inside LangChain"""

    @property
    def _llm_type(self):
        return "llamaindex-huggingface"

    def _call(self, prompt: str, stop=None):
        response = llm.complete(prompt)
        return response.text

# Create LangChain-compatible LLM
langchain_llm = LlamaIndexLLMAdapter()

# Prompt Template
prompt = PromptTemplate(
    input_variables=["question"],
    template="""
You are a helpful AI assistant.
Answer clearly and concisely.

Question: {question}
Answer:
"""
)

# LangChain Chain
chain = LLMChain(
    llm=langchain_llm,
    prompt=prompt
)

# -----------------------------
# Chat Function
# -----------------------------
def chat(user_input):
    if not user_input.strip():
        return "Please enter a message."
    return chain.run(user_input)

# -----------------------------
# Gradio UI
# -----------------------------
demo = gr.Interface(
    fn=chat,
    inputs=gr.Textbox(lines=2, placeholder="Ask something..."),
    outputs="text",
    title="LangChain + LlamaIndex Chatbot",
    description="Integrated chatbot (No RAG, No Vector DB)"
)

demo.launch()