app.py

import gradio as gr
import numpy as np
import matplotlib.pyplot as plt
# Correct the import name clash
from scipy.stats import beta as beta_distribution # <--- RENAMED IMPORT
import random
import io
from PIL import Image # Requires Pillow: pip install Pillow

# --- Configuration ---
priors = {
    "Uniform Prior (Beta(1,1))": (1, 1),
    "Prior Biased Toward Heads (Beta(5,1))": (5, 1),
    "Prior Biased Toward Fair (Beta(2,2))": (2, 2)
}
prior_names = list(priors.keys())
x_pdf = np.linspace(0, 1, 300)

# --- Helper Function to Generate Plot Image (NumPy Array) ---
def generate_plot_image(alpha, beta, title_prefix=""): # Function arguments 'alpha', 'beta' are the numbers
    """Generates a Matplotlib plot and returns it as a NumPy array."""
    fig, ax = plt.subplots(figsize=(6, 4))
    plot_title = f"{title_prefix}Beta({alpha:.1f}, {beta:.1f})" # Use formatting

    # Prevent plotting issues with non-positive alpha/beta
    if alpha <= 0 or beta <= 0:
         ax.text(0.5, 0.5, 'Invalid parameters\nCannot plot Beta(≤0, ≤0)',
                 horizontalalignment='center', verticalalignment='center', transform=ax.transAxes)
         ax.set_title(plot_title, fontsize=10)
         ax.set_xlabel("Probability of Heads (r)", fontsize=9)
         ax.set_ylabel("Density", fontsize=9)
    else:
        # Calculate PDF using the RENAMED distribution object
        try:
            # Use the renamed import here:
            pdf = beta_distribution.pdf(x_pdf, alpha, beta) # <-- CORRECTED
            # Handle potential inf values if alpha/beta are very small
            pdf = np.nan_to_num(pdf, nan=0.0, posinf=np.nanmax(pdf[np.isfinite(pdf)]) if np.any(np.isfinite(pdf)) else 1.0, neginf=0.0) # Replace inf with max finite value or 1
        except ValueError:
             pdf = np.zeros_like(x_pdf) # Fallback

        ax.plot(x_pdf, pdf, color='dodgerblue', linewidth=2)
        ax.fill_between(x_pdf, pdf, color='dodgerblue', alpha=0.3)

        # Add mean marker safely
        if alpha > 0 and beta > 0:
             mean_val = alpha / (alpha + beta)
             try:
                  # Use the renamed import here:
                  peak_y = beta_distribution.pdf(mean_val, alpha, beta) # <-- CORRECTED
                  if np.isfinite(peak_y) and peak_y >= 0:
                     # Ensure vlines max isn't inf/nan
                     ymax_vline = peak_y if np.isfinite(peak_y) else ax.get_ylim()[1]
                     ax.vlines(mean_val, 0, ymax_vline , color='red', linestyle='--', label=f'Mean ≈ {mean_val:.2f}')
                     ax.legend(fontsize=8)
             except ValueError:
                  pass # Ignore if pdf calculation fails for mean

        ax.set_title(plot_title, fontsize=10)
        ax.set_xlabel("Probability of Heads (r)", fontsize=9)
        ax.set_ylabel("Density", fontsize=9)
        ax.set_ylim(bottom=0)
        ax.grid(True, linestyle=':', alpha=0.7)

    plt.tight_layout()

    # --- Convert plot to NumPy array ---
    buf = io.BytesIO()
    fig.savefig(buf, format='png')
    plt.close(fig) # IMPORTANT: Close the figure to release memory
    buf.seek(0)
    img = Image.open(buf)
    img_array = np.array(img)
    buf.close()
    # --- End Conversion ---

    return img_array

# --- Gradio Action Functions ---

def reset_simulation(selected_prior_name):
    """Resets the state based on the selected prior."""
    # print(f"Resetting with prior: {selected_prior_name}") # Debug print
    if selected_prior_name not in priors:
         selected_prior_name = prior_names[0] # Default fallback

    prior_a, prior_b = priors[selected_prior_name]
    initial_state = {
        "prior_a": prior_a,
        "prior_b": prior_b,
        "current_a": prior_a,
        "current_b": prior_b,
        "history": []
    }
    history_str = "No tosses yet."
    params_str = f"Current Posterior: Beta({prior_a:.1f}, {prior_b:.1f})"
    plot_image = generate_plot_image(prior_a, prior_b, title_prefix="Prior: ") # Pass numeric params
    # print("Reset complete.") # Debug print
    return initial_state, history_str, params_str, plot_image

def perform_next_toss(current_state):
    """Performs one toss, updates state, history, params, and plot image."""
    # print(f"Performing next toss. Current state: {current_state}") # Debug print
    if not isinstance(current_state, dict) or "current_a" not in current_state:
        print("Warning: Invalid state detected in perform_next_toss. Resetting.") # Debug print
        # Need to ensure reset doesn't fail here now
        try:
            current_state, _, _, _ = reset_simulation(prior_names[0]) # Use first prior as default
        except Exception as e:
             print(f"ERROR during reset within perform_next_toss: {e}")
             # Cannot proceed - maybe return error messages/default plot?
             error_plot = generate_plot_image(1, 1, title_prefix="ERROR State Invalid - ") # Default plot on error
             return current_state, "Error: Invalid state", "Error", error_plot


    # Simulate a coin toss
    toss_result = random.randint(0, 1) # 0=Tails, 1=Heads

    # Update history list in state
    new_history = current_state.get("history", []) + [toss_result]
    current_state["history"] = new_history

    # Update parameters in state
    current_a = current_state.get("current_a", 1.0) # Default if missing
    current_b = current_state.get("current_b", 1.0) # Default if missing

    if toss_result == 1: # Heads
        current_a += 1
    else: # Tails
        current_b += 1
    current_state["current_a"] = current_a
    current_state["current_b"] = current_b

    # Generate outputs
    history_str = ", ".join(['H' if t == 1 else 'T' for t in new_history])
    if not history_str: history_str = "No tosses yet."

    params_str = f"Current Posterior: Beta({current_a:.1f}, {current_b:.1f})"
    plot_image = generate_plot_image(current_a, current_b, title_prefix="Posterior: ") # Pass numeric params
    # print("Toss complete.") # Debug print
    return current_state, history_str, params_str, plot_image

# --- Build Gradio Interface ---
# (The gr.Blocks interface code remains the same as the previous version)
with gr.Blocks(theme=gr.themes.Soft()) as app:
    gr.Markdown(
        """
        # Bayesian Coin Toss Simulation 🪙
        Visualize how a Beta distribution (representing belief about a coin's bias)
        updates after each simulated coin toss (Heads or Tails).
        Select a prior belief, then press 'Next Toss' or 'Reset'.
        """
    )

    # Hidden state to store current alpha, beta, and history list
    state = gr.State(value=None) # Initial value is None, will be set by load/change

    with gr.Row():
        prior_selector = gr.Dropdown(
            label="Select Prior Belief",
            choices=prior_names,
            value=prior_names[0] # Default selection
        )

    with gr.Row():
        next_button = gr.Button("Next Toss", variant="primary")
        reset_button = gr.Button("Reset")

    with gr.Row():
         history_output = gr.Textbox(label="Toss History", value="Initializing...", interactive=False, scale=2)
         params_output = gr.Textbox(label="Posterior Parameters", value="Initializing...", interactive=False, scale=1)

    # Use gr.Image for the plot output
    plot_output = gr.Image(label="Posterior Distribution", type="numpy", value=None) # Expect NumPy array

    # --- Event Listeners ---
    # 1. Initialize or reset when the prior selection changes
    prior_selector.change(
        fn=reset_simulation,
        inputs=[prior_selector],
        outputs=[state, history_output, params_output, plot_output],
        queue=False # Prevent queuing if rapid changes happen
    )

    # 2. Initialize when the app loads - runs reset_simulation with the default dropdown value
    app.load(
        fn=reset_simulation,
        inputs=[prior_selector],
        outputs=[state, history_output, params_output, plot_output],
    )

    # 3. Update when "Next Toss" is clicked
    next_button.click(
        fn=perform_next_toss,
        inputs=[state], # Pass the current state
        outputs=[state, history_output, params_output, plot_output] # Get updated state back
    )

    # 4. Reset when "Reset" is clicked
    reset_button.click(
        fn=reset_simulation,
        inputs=[prior_selector], # Reset based on the *currently selected* prior
        outputs=[state, history_output, params_output, plot_output]
    )

# --- Launch the App ---
app.launch(debug=True) # debug=True helps see errors in console