app.py

import streamlit as st
import pandas as pd
import matplotlib.pyplot as plt

st.set_page_config(page_title="Smart Group Expense Tracker", layout="centered")

st.title("💸 Smart Group Expense Tracker")
st.caption("Roommates & Trips Edition - Track expenses, split fairly, and settle up easily!")

# --- Session State Init ---
if "participants" not in st.session_state:
    st.session_state.participants = []
if "expenses" not in st.session_state:
    st.session_state.expenses = []

# --- Add Participants ---
with st.expander("➕ Add Participants"):
    new_name = st.text_input("Participant Name", placeholder="e.g. Bilal")
    if st.button("Add Participant") and new_name:
        if new_name not in st.session_state.participants:
            st.session_state.participants.append(new_name)
        else:
            st.warning("Participant already exists!")

# --- Show Participants ---
if st.session_state.participants:
    st.markdown("### 👥 Participants")
    st.write(", ".join(st.session_state.participants))
else:
    st.info("Add at least one participant to start tracking.")

# --- Log New Expense ---
st.divider()
st.markdown("### 🧾 Log New Expense")
if st.session_state.participants:
    col1, col2 = st.columns(2)
    with col1:
        payer = st.selectbox("Paid By", st.session_state.participants)
    with col2:
        amount = st.number_input("Amount (Rs)", min_value=0.0, step=0.5)
    reason = st.text_input("For What?", placeholder="e.g. Dinner, Transport")
    if st.button("Add Expense"):
        if amount > 0 and payer:
            st.session_state.expenses.append({
                "payer": payer,
                "amount": amount,
                "reason": reason
            })
        else:
            st.warning("Please enter a valid amount and payer.")

# --- Display Expense Log ---
if st.session_state.expenses:
    df = pd.DataFrame(st.session_state.expenses)
    st.subheader("📋 Expense Log")
    st.dataframe(df)

    # --- Optional Tip Calculation ---
    st.subheader("💡 Add Tip")
    tip_percent = st.slider("Tip %", 0, 25, 0)
    tip_amount = df["amount"].sum() * (tip_percent / 100)
    total = df["amount"].sum() + tip_amount
    st.markdown(f"**💰 Total Spent (with Tip): Rs. {total:.2f}**")

    # --- Final Balances ---
    per_person = total / len(st.session_state.participants)
    paid = df.groupby("payer")["amount"].sum()
    balance = paid - per_person
    summary = pd.DataFrame({
        "Paid": paid,
        "Balance": balance
    }).fillna(0)

    st.subheader("💳 Final Balances")
    st.dataframe(summary.style.format({"Paid": "Rs. {:.2f}", "Balance": "Rs. {:.2f}"}))
    st.caption("🔹 Positive balance = Gets money back | Negative = Owes money")

    # --- Pie Chart ---
    st.subheader("📊 Who Paid What?")
    fig, ax = plt.subplots()
    ax.pie(paid, labels=paid.index, autopct='%1.1f%%', startangle=90)
    ax.axis('equal')
    st.pyplot(fig)

    # --- Download CSV ---
    st.subheader("⬇️ Download Expense Log")
    csv = df.to_csv(index=False).encode("utf-8")
    st.download_button(
        label="Download CSV",
        data=csv,
        file_name="group_expenses.csv",
        mime="text/csv"
    )

else:
    st.info("No expenses logged yet.")

# --- Footer ---
st.markdown("---")
st.markdown(
    "<p style='text-align: center; font-size: 14px;'>Made with ❤️ by <b>Bilal Hussain</b></p>",
    unsafe_allow_html=True
)