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diff --git a/.gitignore b/.gitignore
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+__pycache__
+*.db
diff --git a/app.py b/app.py
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+++ b/app.py
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+#!/usr/bin/env python3
+"""Minimal personal expense tracker. Python 3 stdlib only."""
+
+import os
+import random
+import re
+import sqlite3
+import string
+from datetime import datetime, timezone
+from http.server import BaseHTTPRequestHandler, HTTPServer
+from urllib.parse import parse_qs, urlparse
+
+DB = "expenses.db"
+PAGE_SIZE = 20
+BASE_URL = os.environ.get("EXPENSES_BASE_URL", "http://localhost:8000").rstrip("/")
+
+
+def get_db():
+    conn = sqlite3.connect(DB)
+    conn.row_factory = sqlite3.Row
+    conn.execute("PRAGMA foreign_keys = ON")
+    return conn
+
+
+def init_db():
+    conn = get_db()
+    conn.executescript("""
+        CREATE TABLE IF NOT EXISTS groups (
+            id TEXT PRIMARY KEY
+        );
+        CREATE TABLE IF NOT EXISTS users (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            group_id TEXT NOT NULL REFERENCES groups(id),
+            name TEXT NOT NULL
+        );
+        CREATE TABLE IF NOT EXISTS expenses (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            group_id TEXT NOT NULL REFERENCES groups(id),
+            paid_by INTEGER NOT NULL REFERENCES users(id),
+            description TEXT NOT NULL,
+            amount REAL NOT NULL,
+            created_at TEXT NOT NULL
+        );
+        CREATE TABLE IF NOT EXISTS expense_shares (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            expense_id INTEGER NOT NULL REFERENCES expenses(id),
+            user_id INTEGER NOT NULL REFERENCES users(id),
+            percentage REAL NOT NULL
+        );
+        CREATE INDEX IF NOT EXISTS idx_expenses_group ON expenses(group_id);
+        CREATE INDEX IF NOT EXISTS idx_shares_expense ON expense_shares(expense_id);
+    """)
+    conn.commit()
+    conn.close()
+
+
+def gen_id():
+    return "".join(random.choices(string.ascii_lowercase + string.digits, k=8))
+
+
+def compute_debts(group_id):
+    """Compute net balances: positive = owed money, negative = owes money."""
+    conn = get_db()
+    users = conn.execute("SELECT * FROM users WHERE group_id=?", (group_id,)).fetchall()
+
+    balances = {u["id"]: 0.0 for u in users}
+
+    # Credit payers
+    for row in conn.execute(
+        "SELECT paid_by, SUM(amount) as total FROM expenses WHERE group_id=? GROUP BY paid_by",
+        (group_id,),
+    ).fetchall():
+        balances[row["paid_by"]] += row["total"]
+
+    # Debit shares
+    for row in conn.execute(
+        "SELECT es.user_id, SUM(e.amount * es.percentage / 100.0) as owed FROM expense_shares es JOIN expenses e ON es.expense_id=e.id WHERE e.group_id=? GROUP BY es.user_id",
+        (group_id,),
+    ).fetchall():
+        balances[row["user_id"]] -= row["owed"]
+
+    conn.close()
+
+    # Simplify debts
+    user_map = {u["id"]: u["name"] for u in users}
+    debts = []
+    creditors = [(uid, bal) for uid, bal in balances.items() if bal > 0.01]
+    debtors = [(uid, -bal) for uid, bal in balances.items() if bal < -0.01]
+    creditors.sort(key=lambda x: -x[1])
+    debtors.sort(key=lambda x: -x[1])
+
+    i, j = 0, 0
+    while i < len(debtors) and j < len(creditors):
+        amount = min(debtors[i][1], creditors[j][1])
+        debts.append((user_map[debtors[i][0]], user_map[creditors[j][0]], amount))
+        debtors[i] = (debtors[i][0], debtors[i][1] - amount)
+        creditors[j] = (creditors[j][0], creditors[j][1] - amount)
+        if debtors[i][1] < 0.01:
+            i += 1
+        if creditors[j][1] < 0.01:
+            j += 1
+    return debts
+
+
+CSS = """
+*{box-sizing:border-box;margin:0;}
+body{font-family:system-ui,sans-serif;max-width:900px;margin:0 auto;padding:1rem;color:#222;background:#fafafa}
+h1,h2{margin-bottom:.5rem}
+a{color:#0066cc}
+form{margin-bottom:1rem}
+label{display:block;margin:.3rem 0 .1rem;font-size:1rem}
+input[type=text],input[type=number],input[type=date],select{width:100%;padding:.4rem;border:1px solid #ccc;border-radius:4px;font-size:1rem}
+input[type=date]{min-width:100%;height:33px;}
+button{padding:.5rem 1rem;background:#0066cc;color:#fff;border:none;border-radius:4px;cursor:pointer;font-size:1rem;margin-top:.5rem}
+button:hover{background:#0052a3}
+.section{background:#fff;border:1px solid #e0e0e0;border-radius:6px;padding:1rem;margin-bottom:1rem}
+table{width:100%;border-collapse:collapse;font-size:1rem}
+th,td{text-align:left;padding:.2rem .4rem .2rem 0;white-space:nowrap}
+tr{border-bottom:1px solid #eee}
+td:nth-child(3),th:nth-child(3){max-width:300px;white-space:normal;word-break:break-word}
+.shares{display:flex;flex-direction:column;gap:.3rem}
+.share-item{display:flex;align-items:center;gap:.3rem}
+.share-item input[type=number]{width:4rem;margin-left:auto}
+.row{display:flex;gap:.5rem}.row>div{flex:1}
+.btn-del{background:none;border:none;color:#c00;cursor:pointer;padding:0;margin:0;}
+.actions{white-space:nowrap;vertical-align:middle}
+.actions a,.actions button{padding:8px;font-size:1.2rem;text-decoration:none;}
+.actions form{margin:0;}
+.page-nav{display:flex;gap:1rem;justify-content:center;margin-top:.5rem}
+.debt{padding:.3rem 0}
+.nav{margin-bottom:1rem;display:flex;align-items:center}
+.nav h1{margin:0}
+.nav .links{margin-left:auto;display:flex;gap:1rem}
+.landing{margin:0 -1rem}
+@media(min-width:700px){.landing{display:grid;grid-template-columns:1fr 1fr;gap:1rem;margin:0}.landing .log{grid-column:1/3}}
+select{height:33px;}
+"""
+
+
+def html(title, body, group_id=None):
+    nav = ""
+    if group_id:
+        nav = f'<div class="nav"><h1>{title} <small style="font-weight:normal;font-size:.5em;color:#888">{group_id}</small></h1><div class="links"><a href="/{group_id}">Expenses</a> <a href="/{group_id}/settings">Settings</a></div></div>'
+    return f"""<!DOCTYPE html><html><head><meta charset="utf-8"><meta name="viewport" content="width=device-width,initial-scale=1"><meta name="view-transition" content="same-origin"><title>{title} - {group_id}</title><style>{CSS}</style></head><body>{nav}{body}</body></html>"""
+
+
+def escape(s):
+    return (
+        s.replace("&", "&amp;")
+        .replace("<", "&lt;")
+        .replace(">", "&gt;")
+        .replace('"', "&quot;")
+    )
+
+
+class Handler(BaseHTTPRequestHandler):
+    def do_GET(self):
+        parsed = urlparse(self.path)
+        path = parsed.path.rstrip("/") or "/"
+        qs = parse_qs(parsed.query)
+
+        if path == "/new":
+            return self.new_group()
+
+        m = re.match(r"^/([a-z0-9]+)$", path)
+        if m:
+            return self.landing(m.group(1), qs)
+
+        m = re.match(r"^/([a-z0-9]+)/settings$", path)
+        if m:
+            return self.settings(m.group(1))
+
+        self.send_response(302)
+        self.send_header("Location", "/new")
+        self.end_headers()
+
+    def do_POST(self):
+        length = int(self.headers.get("Content-Length", 0))
+        body = self.rfile.read(length).decode()
+        data = parse_qs(body)
+
+        path = urlparse(self.path).path.rstrip("/")
+
+        m = re.match(r"^/([a-z0-9]+)/add$", path)
+        if m:
+            return self.add_expense(m.group(1), data)
+
+        m = re.match(r"^/([a-z0-9]+)/add-user$", path)
+        if m:
+            return self.add_user(m.group(1), data)
+
+        m = re.match(r"^/([a-z0-9]+)/delete/(\d+)$", path)
+        if m:
+            return self.delete_expense(m.group(1), int(m.group(2)))
+
+        self.send_response(400)
+        self.end_headers()
+
+    def respond(self, code, content):
+        self.send_response(code)
+        self.send_header("Content-Type", "text/html; charset=utf-8")
+        self.end_headers()
+        self.wfile.write(content.encode())
+
+    def new_group(self):
+        gid = gen_id()
+        conn = get_db()
+        conn.execute("INSERT INTO groups(id) VALUES(?)", (gid,))
+        conn.commit()
+        conn.close()
+        self.send_response(302)
+        self.send_header("Location", f"/{gid}/settings")
+        self.end_headers()
+
+    def landing(self, gid, qs):
+        conn = get_db()
+        group = conn.execute("SELECT * FROM groups WHERE id=?", (gid,)).fetchone()
+        if not group:
+            conn.close()
+            return self.respond(404, html("Not Found", "<h1>Group not found</h1>"))
+
+        users = conn.execute("SELECT * FROM users WHERE group_id=?", (gid,)).fetchall()
+        page = int(qs.get("page", ["1"])[0])
+        offset = (page - 1) * PAGE_SIZE
+        total = conn.execute(
+            "SELECT COUNT(*) as c FROM expenses WHERE group_id=?", (gid,)
+        ).fetchone()["c"]
+        expenses = conn.execute(
+            "SELECT e.*, u.name as payer FROM expenses e JOIN users u ON e.paid_by=u.id WHERE e.group_id=? ORDER BY e.created_at DESC LIMIT ? OFFSET ?",
+            (gid, PAGE_SIZE, offset),
+        ).fetchall()
+
+        # Get shares for prefilling (from copy param or last expense)
+        copy_id = qs.get("copy", [None])[0]
+        copy_exp = None
+        prefill_shares = {}
+        prefill_desc = ""
+        prefill_amount = ""
+        prefill_payer = None
+        prefill_date = datetime.now(timezone.utc).strftime("%Y-%m-%d")
+
+        if copy_id:
+            copy_exp = conn.execute(
+                "SELECT * FROM expenses WHERE id=? AND group_id=?", (copy_id, gid)
+            ).fetchone()
+        if copy_exp:
+            prefill_desc = copy_exp["description"]
+            prefill_amount = f"{copy_exp['amount']:.2f}"
+            prefill_payer = copy_exp["paid_by"]
+            prefill_date = copy_exp["created_at"][:10]
+            for s in conn.execute(
+                "SELECT user_id, percentage FROM expense_shares WHERE expense_id=?",
+                (copy_exp["id"],),
+            ).fetchall():
+                prefill_shares[s["user_id"]] = s["percentage"]
+        else:
+            last_exp = conn.execute(
+                "SELECT id FROM expenses WHERE group_id=? ORDER BY created_at DESC LIMIT 1",
+                (gid,),
+            ).fetchone()
+            if last_exp:
+                for s in conn.execute(
+                    "SELECT user_id, percentage FROM expense_shares WHERE expense_id=?",
+                    (last_exp["id"],),
+                ).fetchall():
+                    prefill_shares[s["user_id"]] = s["percentage"]
+
+        # Add expense form
+        form = '<div class="section"><h2>Add expense</h2>'
+        if not users:
+            form += (
+                "<p>Add users in <a href='/{}/settings'>settings</a> first.</p>".format(
+                    gid
+                )
+            )
+        else:
+            form += f'<form method="post" action="/{gid}/add">'
+            form += '<label>Paid by</label><select name="paid_by">'
+            for u in users:
+                sel = "selected" if u["id"] == prefill_payer else ""
+                form += f'<option value="{u["id"]}" {sel}>{escape(u["name"])}</option>'
+            form += "</select>"
+            form += f'<label>Description</label><input type="text" name="description" value="{escape(prefill_desc)}" required>'
+            form += f'<div class="row"><div><label>Amount</label><input type="number" name="amount" step="0.01" min="0.01" value="{prefill_amount}" required></div>'
+            form += f'<div><label>Date</label><input type="date" name="date" value="{prefill_date}" required></div></div>'
+            form += '<label>Split between</label><div class="shares">'
+            for u in users:
+                checked = (
+                    "checked"
+                    if (not prefill_shares or u["id"] in prefill_shares)
+                    else ""
+                )
+                if u["id"] in prefill_shares:
+                    p = prefill_shares[u["id"]]
+                    pv = int(p) if p == int(p) else f"{p:.2f}"
+                    pct_val = f' value="{pv}"'
+                else:
+                    pct_val = ""
+                form += f'<div class="share-item"><input type="checkbox" name="share_{u["id"]}" value="1" {checked}>'
+                form += f"<span>{escape(u['name'])}</span>"
+                form += f'<input type="number" name="pct_{u["id"]}" step="0.01" min="0" max="100" required{pct_val}> %</div>'
+            form += "</div>"
+            form += "<button>Add Expense</button></form>"
+        form += "</div>"
+
+        # Debt status
+        debts = compute_debts(gid)
+        debt_html = '<div class="section"><h2>Balances</h2>'
+        if not debts:
+            debt_html += "<p>All settled up!</p>"
+        else:
+            for frm, to, amt in debts:
+                debt_html += f'<div class="debt"><strong>{escape(frm)}</strong> owes <strong>{escape(to)}</strong> {amt:.2f}</div>'
+        debt_html += "</div>"
+
+        # Expense log
+        log_html = '<div class="section log"><h2>Expenses</h2>'
+        if expenses:
+            log_html += "<table><tr><th>Date</th><th>Payer</th><th>Description</th><th>Amount</th><th></th></tr>"
+            for e in expenses:
+                d = e["created_at"][:10]
+                log_html += f"<tr><td>{d}</td><td>{escape(e['payer'])}</td><td>{escape(e['description'])}</td><td>{e['amount']:.2f}</td>"
+                log_html += (
+                    f'<td class="actions"><a href="/{gid}?copy={e["id"]}">✎</a> '
+                )
+                log_html += f'<form method="post" action="/{gid}/delete/{e["id"]}" style="display:inline"><button type="submit" class="btn-del">&times;</button></form></td></tr>'
+            log_html += "</table>"
+            # Pagination
+            pages = (total + PAGE_SIZE - 1) // PAGE_SIZE
+            if pages > 1:
+                log_html += '<div class="page-nav">'
+                if page > 1:
+                    log_html += f'<a href="/{gid}?page={page - 1}">&laquo; Prev</a>'
+                log_html += f" Page {page}/{pages} "
+                if page < pages:
+                    log_html += f'<a href="/{gid}?page={page + 1}">Next &raquo;</a>'
+                log_html += "</div>"
+        else:
+            log_html += "<p>No expenses yet.</p>"
+        log_html += "</div>"
+
+        body = f'<div class="landing">{form}{debt_html}{log_html}</div>'
+        conn.close()
+        self.respond(200, html("Expenses", body, gid))
+
+    def settings(self, gid):
+        conn = get_db()
+        group = conn.execute("SELECT * FROM groups WHERE id=?", (gid,)).fetchone()
+        if not group:
+            conn.close()
+            return self.respond(404, html("Not Found", "<h1>Group not found</h1>"))
+        users = conn.execute("SELECT * FROM users WHERE group_id=?", (gid,)).fetchall()
+        conn.close()
+
+        user_list = ""
+        for u in users:
+            user_list += f"<li>{escape(u['name'])}</li>"
+
+        body = f"""<div class="section"><h2>Members</h2>
+        <ul style="margin:1rem 0">{user_list if user_list else "<li>No members yet</li>"}</ul>
+        <form method="post" action="/{gid}/add-user" style="margin-top:1rem;display:flex;gap:.5rem;align-items:end">
+        <div style="flex:1"><label>Add member</label><input type="text" name="name" required></div>
+        <button>Add</button></form></div>
+        <div class="section"><h2>Share Link</h2><p>Anyone with this link can view, add and delete expenses in this group:</p>
+        <a href="{BASE_URL}/{gid}" style="display:block;margin:.5rem 0">{BASE_URL}/{gid}</a></div>
+        <div class="section"><h2>New Group</h2><p>Start a separate expense group with its own members and history.</p><a href="/new" style="display:block;margin:.5rem 0">Create new group</a></div>"""
+        self.respond(200, html("Settings", body, gid))
+
+    def add_expense(self, gid, data):
+        paid_by = int(data["paid_by"][0])
+        description = data["description"][0].strip()
+        amount = round(float(data["amount"][0]), 2)
+
+        conn = get_db()
+        users = conn.execute("SELECT * FROM users WHERE group_id=?", (gid,)).fetchall()
+
+        # Determine participants and percentages
+        participants = []
+        for u in users:
+            if f"share_{u['id']}" in data:
+                pct_val = data.get(f"pct_{u['id']}", [""])[0].strip()
+                participants.append((u["id"], float(pct_val) if pct_val else 0))
+
+        if not participants:
+            conn.close()
+            self.send_response(302)
+            self.send_header("Location", f"/{gid}")
+            self.end_headers()
+            return
+
+        final = [(uid, pct) for uid, pct in participants if pct > 0]
+
+        now = data.get("date", [datetime.now(timezone.utc).strftime("%Y-%m-%d")])[0]
+        cur = conn.execute(
+            "INSERT INTO expenses(group_id, paid_by, description, amount, created_at) VALUES(?,?,?,?,?)",
+            (gid, paid_by, description, amount, now),
+        )
+        exp_id = cur.lastrowid
+        for uid, pct in final:
+            conn.execute(
+                "INSERT INTO expense_shares(expense_id, user_id, percentage) VALUES(?,?,?)",
+                (exp_id, uid, pct),
+            )
+        conn.commit()
+        conn.close()
+
+        self.send_response(302)
+        self.send_header("Location", f"/{gid}")
+        self.end_headers()
+
+    def add_user(self, gid, data):
+        name = data["name"][0].strip()
+        if name:
+            conn = get_db()
+            conn.execute("INSERT INTO users(group_id, name) VALUES(?,?)", (gid, name))
+            conn.commit()
+            conn.close()
+        self.send_response(302)
+        self.send_header("Location", f"/{gid}/settings")
+        self.end_headers()
+
+    def delete_expense(self, gid, exp_id):
+        conn = get_db()
+        conn.execute("DELETE FROM expense_shares WHERE expense_id=?", (exp_id,))
+        conn.execute("DELETE FROM expenses WHERE id=? AND group_id=?", (exp_id, gid))
+        conn.commit()
+        conn.close()
+        self.send_response(302)
+        self.send_header("Location", f"/{gid}")
+        self.end_headers()
+
+    def address_string(self):
+        return self.client_address[0]
+
+    def log_message(self, format, *args):
+        pass  # Suppress request logs
+
+
+if __name__ == "__main__":
+    init_db()
+    port = 8000
+    print(f"Running on http://localhost:{port}")
+    print("Visit http://localhost:{}/new to create a group".format(port))
+    HTTPServer(("", port), Handler).serve_forever()