app.py

import gradio as gr
import os
from transformers import (
    GPT2LMHeadModel, GPT2Tokenizer,
    T5ForConditionalGeneration, T5Tokenizer,
    AutoTokenizer, AutoModelForCausalLM
)
import torch

# Configuration for multiple models, can add more by extending MODEL_CONFIGS dict
MODEL_CONFIGS = {
    "gpt2": {
        "type": "causal",
        "model_class": GPT2LMHeadModel,
        "tokenizer_class": GPT2Tokenizer,
        "description": "Original GPT-2, good for creative writing",
        "size": "117M"
    },
    "distilgpt2": {
        "type": "causal",
        "model_class": AutoModelForCausalLM,
        "tokenizer_class": AutoTokenizer,
        "description": "Smaller, faster GPT-2",
        "size": "82M"
    },
    "google/flan-t5-small": {
        "type": "seq2seq",
        "model_class": T5ForConditionalGeneration,
        "tokenizer_class": T5Tokenizer,
        "description": "Instruction-following T5 model",
        "size": "80M"
    },
    "microsoft/DialoGPT-small": {
        "type": "causal",
        "model_class": AutoModelForCausalLM,
        "tokenizer_class": AutoTokenizer,
        "description": "Conversational AI model",
        "size": "117M"
    }
}

# Environment variables for optional authentication and private model access
HF_TOKEN = os.getenv("HF_TOKEN")
API_KEY = os.getenv("API_KEY")
ADMIN_PASSWORD = os.getenv("ADMIN_PASSWORD")

# Global state for caching loaded model and tokenizer
loaded_model_name = None
model = None
tokenizer = None

def load_model_and_tokenizer(model_name):
    global loaded_model_name, model, tokenizer
    if model_name == loaded_model_name and model is not None and tokenizer is not None:
        return model, tokenizer
    
    config = MODEL_CONFIGS[model_name]
    if HF_TOKEN:
        tokenizer = config["tokenizer_class"].from_pretrained(model_name, use_auth_token=HF_TOKEN)
        model = config["model_class"].from_pretrained(model_name, use_auth_token=HF_TOKEN)
    else:
        tokenizer = config["tokenizer_class"].from_pretrained(model_name)
        model = config["model_class"].from_pretrained(model_name)

    # Set pad token for causal models if missing (important for generation padding)
    if config["type"] == "causal" and tokenizer.pad_token is None:
        tokenizer.pad_token = tokenizer.eos_token

    loaded_model_name = model_name
    return model, tokenizer

def authenticate_api_key(key):
    if API_KEY and key != API_KEY:
        return False
    return True

def generate_text(prompt, model_name, max_length, temperature, top_p, top_k, api_key=""):
    if API_KEY and not authenticate_api_key(api_key):
        return "Error: Invalid API key"

    try:
        config = MODEL_CONFIGS[model_name]
        model, tokenizer = load_model_and_tokenizer(model_name)

        if config["type"] == "causal":
            inputs = tokenizer.encode(prompt, return_tensors="pt", max_length=512, truncation=True)
            with torch.no_grad():
                outputs = model.generate(
                    inputs,
                    max_length=min(max_length + inputs.shape[1], 512),
                    temperature=temperature,
                    top_p=top_p,
                    top_k=top_k,
                    do_sample=True,
                    pad_token_id=tokenizer.pad_token_id,
                    num_return_sequences=1
                )
            generated_text = tokenizer.decode(outputs[0], skip_special_tokens=True)
            # Return generated continuation (remove original prompt)
            return generated_text[len(prompt):].strip()

        elif config["type"] == "seq2seq":
            # Add task prefix for certain seq2seq models like flan-t5
            task_prompt = f"Complete this text: {prompt}" if "flan-t5" in model_name.lower() else prompt
            inputs = tokenizer(task_prompt, return_tensors="pt", max_length=512, truncation=True)
            with torch.no_grad():
                outputs = model.generate(
                    **inputs,
                    max_length=max_length,
                    temperature=temperature,
                    top_p=top_p,
                    top_k=top_k,
                    do_sample=True,
                    num_return_sequences=1
                )
            generated_text = tokenizer.decode(outputs[0], skip_special_tokens=True)
            return generated_text.strip()

    except Exception as e:
        return f"Error generating text: {str(e)}"

with gr.Blocks(title="Multi-Model Text Generation Server") as demo:
    gr.Markdown("# Multi-Model Text Generation Server")
    gr.Markdown("Choose a model from the dropdown, enter a text prompt, and generate text.")

    with gr.Row():
        with gr.Column():
            model_selector = gr.Dropdown(
                label="Model",
                choices=list(MODEL_CONFIGS.keys()),
                value="gpt2",
                interactive=True
            )
            prompt_input = gr.Textbox(
                label="Text Prompt",
                placeholder="Enter the text prompt here...",
                lines=4
            )
            max_length_slider = gr.Slider(
                10, 200, 100, 10,
                label="Max Generation Length"
            )
            temperature_slider = gr.Slider(
                0.1, 2.0, 0.7, 0.1,
                label="Temperature"
            )
            top_p_slider = gr.Slider(
                0.1, 1.0, 0.9, 0.05,
                label="Top-p (nucleus sampling)"
            )
            top_k_slider = gr.Slider(
                1, 100, 50, 1,
                label="Top-k sampling"
            )
            if API_KEY:
                api_key_input = gr.Textbox(
                    label="API Key",
                    type="password",
                    placeholder="Enter API Key"
                )
            else:
                api_key_input = gr.Textbox(value="", visible=False)

            generate_btn = gr.Button("Generate Text", variant="primary")

        with gr.Column():
            output_textbox = gr.Textbox(
                label="Generated Text",
                lines=10,
                placeholder="Generated text will appear here..."
            )

    generate_btn.click(
        fn=generate_text,
        inputs=[prompt_input, model_selector, max_length_slider, temperature_slider, top_p_slider, top_k_slider, api_key_input],
        outputs=output_textbox
    )

    gr.Examples(
        examples=[
            ["Once upon a time in a distant galaxy,"],
            ["The future of artificial intelligence is"],
            ["In the heart of the ancient forest,"],
            ["The detective walked into the room and noticed"],
        ],
        inputs=prompt_input
    )

auth_config = ("admin", ADMIN_PASSWORD) if ADMIN_PASSWORD else None

if __name__ == "__main__":
    demo.launch(
        auth=auth_config,
        share=True,        # Required for Spaces if localhost isn't accessible
        server_name="0.0.0.0",
        server_port=7860,
        ssr_mode=False     # Optional: disable server-side rendering to avoid Svelte i18n error
    )