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1from fastapi import FastAPI, HTTPException, Header
2from pydantic import BaseModel
3from typing import Optional, Dict, List
4import uuid
5import uvicorn
6 
7app = FastAPI()
8 
9users = {}
10groups = {}
11expenses = {}
12tokens = {}
13 
14user_id_counter = 1
15group_id_counter = 1
16expense_id_counter = 1
17 
18class SignupRequest(BaseModel):
19    username: str
20    password: str
21 
22class LoginRequest(BaseModel):
23    username: str
24    password: str
25 
26class CreateGroupRequest(BaseModel):
27    name: str
28    member_ids: List[int]
29 
30class RecordExpenseRequest(BaseModel):
31    group_id: int
32    paid_by_user_id: int
33    amount: float
34    currency: str = "USD"
35    splits: List[dict]  # [{"user_id": 1, "percentage": 50}, ...]
36 
37def get_current_user(authorization: Optional[str] = Header(None)):
38    if not authorization:
39        raise HTTPException(status_code=401, detail="No auth header")
40    token = authorization.replace("Bearer ", "")
41    if token not in tokens:
42        raise HTTPException(status_code=401, detail="Invalid token")
43    return tokens[token]
44 
45@app.post("/signup")
46def signup(req: SignupRequest):
47    global user_id_counter
48    for u in users.values():
49        if u["username"] == req.username:
50            raise HTTPException(status_code=400, detail="Username exists")
51    user_id = user_id_counter
52    user_id_counter += 1
53    users[user_id] = {"id": user_id, "username": req.username, "password": req.password}
54    return {"user_id": user_id, "username": req.username}
55 
56@app.post("/login")
57def login(req: LoginRequest):
58    for u in users.values():
59        if u["username"] == req.username and u["password"] == req.password:
60            token = str(uuid.uuid4())
61            tokens[token] = u["id"]
62            return {"token": token}
63    raise HTTPException(status_code=401, detail="Invalid credentials")
64 
65@app.get("/users/{user_id}")
66def get_user(user_id: int, authorization: Optional[str] = Header(None)):
67    get_current_user(authorization)
68    if user_id not in users:
69        raise HTTPException(status_code=404, detail="User not found")
70    return users[user_id]
71 
72@app.post("/groups")
73def create_group(req: CreateGroupRequest, authorization: Optional[str] = Header(None)):
74    global group_id_counter
75    get_current_user(authorization)
76    for mid in req.member_ids:
77        if mid not in users:
78            raise HTTPException(status_code=400, detail=f"User {mid} not found")
79    group_id = group_id_counter
80    group_id_counter += 1
81    groups[group_id] = {"id": group_id, "name": req.name, "member_ids": req.member_ids, "expenses": []}
82    return groups[group_id]
83 
84@app.get("/groups/{group_id}")
85def get_group(group_id: int, authorization: Optional[str] = Header(None)):
86    get_current_user(authorization)
87    if group_id not in groups:
88        raise HTTPException(status_code=404, detail="Group not found")
89    return groups[group_id]
90 
91@app.post("/expenses/record")
92def record_expense(req: RecordExpenseRequest, authorization: Optional[str] = Header(None)):
93    global expense_id_counter
94    get_current_user(authorization)
95    if req.group_id not in groups:
96        raise HTTPException(status_code=404, detail="Group not found")
97    group = groups[req.group_id]
98    if req.paid_by_user_id not in group["member_ids"]:
99        raise HTTPException(status_code=400, detail="Payer not in group")
100    total_pct = sum(s["percentage"] for s in req.splits)
101    if abs(total_pct - 100.0) > 0.01:
102        raise HTTPException(status_code=400, detail="Percentages must sum to 100")
103    for s in req.splits:
104        if s["user_id"] not in group["member_ids"]:
105            raise HTTPException(status_code=400, detail=f"User {s['user_id']} not in group")
106    expense_id = expense_id_counter
107    expense_id_counter += 1
108    expense = {
109        "id": expense_id,
110        "group_id": req.group_id,
111        "paid_by_user_id": req.paid_by_user_id,
112        "amount": req.amount,
113        "currency": req.currency,
114        "splits": req.splits
115    }
116    expenses[expense_id] = expense
117    group["expenses"].append(expense_id)
118    return expense
119 
120@app.get("/groups/{group_id}/balances")
121def get_balances(group_id: int, authorization: Optional[str] = Header(None)):
122    get_current_user(authorization)
123    if group_id not in groups:
124        raise HTTPException(status_code=404, detail="Group not found")
125    group = groups[group_id]
126    balances = {uid: 0.0 for uid in group["member_ids"]}
127    for eid in group["expenses"]:
128        exp = expenses[eid]
129        for split in exp["splits"]:
130            user_id = split["user_id"]
131            percentage = split["percentage"]
132            owes_amount = exp["amount"] * (percentage / 100.0)
133            balances[user_id] -= owes_amount
134            balances[exp["paid_by_user_id"]] += owes_amount
135    result = []
136    for uid, balance in balances.items():
137        result.append({"user_id": uid, "username": users[uid]["username"], "balance": round(balance, 2), "currency": "USD"})
138    return result
139 
140if __name__ == "__main__":
141    uvicorn.run(app, host="127.0.0.1", port=8000)

1fastapi
2uvicorn