# app/api/v1/endpoints/costs.py import logging from fastapi import APIRouter, Depends, HTTPException, status from sqlalchemy import select from sqlalchemy.ext.asyncio import AsyncSession from sqlalchemy.orm import Session, selectinload from decimal import Decimal, ROUND_HALF_UP, ROUND_DOWN from typing import List from app.database import get_transactional_session from app.auth import current_active_user from app.models import ( User as UserModel, Group as GroupModel, List as ListModel, Expense as ExpenseModel, Item as ItemModel, UserGroup as UserGroupModel, SplitTypeEnum, ExpenseSplit as ExpenseSplitModel, Settlement as SettlementModel, SettlementActivity as SettlementActivityModel # Added ) from app.schemas.cost import ListCostSummary, GroupBalanceSummary, UserCostShare, UserBalanceDetail, SuggestedSettlement from app.schemas.expense import ExpenseCreate from app.crud import list as crud_list from app.crud import expense as crud_expense from app.core.exceptions import ListNotFoundError, ListPermissionError, UserNotFoundError, GroupNotFoundError logger = logging.getLogger(__name__) router = APIRouter() def calculate_suggested_settlements(user_balances: List[UserBalanceDetail]) -> List[SuggestedSettlement]: """ Calculate suggested settlements to balance the finances within a group. This function takes the current balances of all users and suggests optimal settlements to minimize the number of transactions needed to settle all debts. Args: user_balances: List of UserBalanceDetail objects with their current balances Returns: List of SuggestedSettlement objects representing the suggested payments """ # Create list of users who owe money (negative balance) and who are owed money (positive balance) debtors = [] # Users who owe money (negative balance) creditors = [] # Users who are owed money (positive balance) # Threshold to consider a balance as zero due to floating point precision epsilon = Decimal('0.01') # Sort users into debtors and creditors for user in user_balances: # Skip users with zero balance (or very close to zero) if abs(user.net_balance) < epsilon: continue if user.net_balance < Decimal('0'): # User owes money debtors.append({ 'user_id': user.user_id, 'user_identifier': user.user_identifier, 'amount': -user.net_balance # Convert to positive amount }) else: # User is owed money creditors.append({ 'user_id': user.user_id, 'user_identifier': user.user_identifier, 'amount': user.net_balance }) # Sort by amount (descending) to handle largest debts first debtors.sort(key=lambda x: x['amount'], reverse=True) creditors.sort(key=lambda x: x['amount'], reverse=True) settlements = [] # Iterate through debtors and match them with creditors while debtors and creditors: debtor = debtors[0] creditor = creditors[0] # Determine the settlement amount (the smaller of the two amounts) amount = min(debtor['amount'], creditor['amount']).quantize(Decimal('0.01'), rounding=ROUND_HALF_UP) # Create settlement record if amount > Decimal('0'): settlements.append( SuggestedSettlement( from_user_id=debtor['user_id'], from_user_identifier=debtor['user_identifier'], to_user_id=creditor['user_id'], to_user_identifier=creditor['user_identifier'], amount=amount ) ) # Update balances debtor['amount'] -= amount creditor['amount'] -= amount # Remove users who have settled their debts/credits if debtor['amount'] < epsilon: debtors.pop(0) if creditor['amount'] < epsilon: creditors.pop(0) return settlements @router.get( "/lists/{list_id}/cost-summary", response_model=ListCostSummary, summary="Get Cost Summary for a List", tags=["Costs"], responses={ status.HTTP_403_FORBIDDEN: {"description": "User does not have permission to access this list"}, status.HTTP_404_NOT_FOUND: {"description": "List or associated user not found"} } ) async def get_list_cost_summary( list_id: int, db: AsyncSession = Depends(get_transactional_session), current_user: UserModel = Depends(current_active_user), ): """ Retrieves a calculated cost summary for a specific list, detailing total costs, equal shares per user, and individual user balances based on their contributions. The user must have access to the list to view its cost summary. Costs are split among group members if the list belongs to a group, or just for the creator if it's a personal list. All users who added items with prices are included in the calculation. """ logger.info(f"User {current_user.email} requesting cost summary for list {list_id}") # 1. Verify user has access to the target list try: await crud_list.check_list_permission(db=db, list_id=list_id, user_id=current_user.id) except ListPermissionError as e: logger.warning(f"Permission denied for user {current_user.email} on list {list_id}: {str(e)}") raise except ListNotFoundError as e: logger.warning(f"List {list_id} not found when checking permissions for cost summary: {str(e)}") raise # 2. Get the list with its items and users list_result = await db.execute( select(ListModel) .options( selectinload(ListModel.items).options(selectinload(ItemModel.added_by_user)), selectinload(ListModel.group).options(selectinload(GroupModel.member_associations).options(selectinload(UserGroupModel.user))), selectinload(ListModel.creator) ) .where(ListModel.id == list_id) ) db_list = list_result.scalars().first() if not db_list: raise ListNotFoundError(list_id) # 3. Get or create an expense for this list expense_result = await db.execute( select(ExpenseModel) .where(ExpenseModel.list_id == list_id) .options(selectinload(ExpenseModel.splits)) ) db_expense = expense_result.scalars().first() if not db_expense: # Create a new expense for this list total_amount = sum(item.price for item in db_list.items if item.price is not None and item.price > Decimal("0")) if total_amount == Decimal("0"): return ListCostSummary( list_id=db_list.id, list_name=db_list.name, total_list_cost=Decimal("0.00"), num_participating_users=0, equal_share_per_user=Decimal("0.00"), user_balances=[] ) # Create expense with ITEM_BASED split type expense_in = ExpenseCreate( description=f"Cost summary for list {db_list.name}", total_amount=total_amount, list_id=list_id, split_type=SplitTypeEnum.ITEM_BASED, paid_by_user_id=db_list.creator.id ) db_expense = await crud_expense.create_expense(db=db, expense_in=expense_in, current_user_id=current_user.id) # 4. Calculate cost summary from expense splits participating_users = set() user_items_added_value = {} total_list_cost = Decimal("0.00") # Get all users who added items for item in db_list.items: if item.price is not None and item.price > Decimal("0") and item.added_by_user: participating_users.add(item.added_by_user) user_items_added_value[item.added_by_user.id] = user_items_added_value.get(item.added_by_user.id, Decimal("0.00")) + item.price total_list_cost += item.price # Get all users from expense splits for split in db_expense.splits: if split.user: participating_users.add(split.user) num_participating_users = len(participating_users) if num_participating_users == 0: return ListCostSummary( list_id=db_list.id, list_name=db_list.name, total_list_cost=Decimal("0.00"), num_participating_users=0, equal_share_per_user=Decimal("0.00"), user_balances=[] ) # This is the ideal equal share, returned in the summary equal_share_per_user_for_response = (total_list_cost / Decimal(num_participating_users)).quantize(Decimal("0.01"), rounding=ROUND_HALF_UP) # Sort users for deterministic remainder distribution sorted_participating_users = sorted(list(participating_users), key=lambda u: u.id) user_final_shares = {} if num_participating_users > 0: base_share_unrounded = total_list_cost / Decimal(num_participating_users) # Calculate initial share for each user, rounding down for user in sorted_participating_users: user_final_shares[user.id] = base_share_unrounded.quantize(Decimal("0.01"), rounding=ROUND_DOWN) # Calculate sum of rounded down shares sum_of_rounded_shares = sum(user_final_shares.values()) # Calculate remaining pennies to be distributed remaining_pennies = int(((total_list_cost - sum_of_rounded_shares) * Decimal("100")).to_integral_value(rounding=ROUND_HALF_UP)) # Distribute remaining pennies one by one to sorted users for i in range(remaining_pennies): user_to_adjust = sorted_participating_users[i % num_participating_users] user_final_shares[user_to_adjust.id] += Decimal("0.01") user_balances = [] for user in sorted_participating_users: # Iterate over sorted users items_added = user_items_added_value.get(user.id, Decimal("0.00")) # current_user_share is now the precisely calculated share for this user current_user_share = user_final_shares.get(user.id, Decimal("0.00")) balance = items_added - current_user_share user_identifier = user.name if user.name else user.email user_balances.append( UserCostShare( user_id=user.id, user_identifier=user_identifier, items_added_value=items_added.quantize(Decimal("0.01"), rounding=ROUND_HALF_UP), amount_due=current_user_share.quantize(Decimal("0.01"), rounding=ROUND_HALF_UP), balance=balance.quantize(Decimal("0.01"), rounding=ROUND_HALF_UP) ) ) user_balances.sort(key=lambda x: x.user_identifier) return ListCostSummary( list_id=db_list.id, list_name=db_list.name, total_list_cost=total_list_cost.quantize(Decimal("0.01"), rounding=ROUND_HALF_UP), num_participating_users=num_participating_users, equal_share_per_user=equal_share_per_user_for_response, # Use the ideal share for the response field user_balances=user_balances ) @router.get( "/groups/{group_id}/balance-summary", response_model=GroupBalanceSummary, summary="Get Detailed Balance Summary for a Group", tags=["Costs", "Groups"], responses={ status.HTTP_403_FORBIDDEN: {"description": "User does not have permission to access this group"}, status.HTTP_404_NOT_FOUND: {"description": "Group not found"} } ) async def get_group_balance_summary( group_id: int, db: AsyncSession = Depends(get_transactional_session), current_user: UserModel = Depends(current_active_user), ): """ Retrieves a detailed financial balance summary for all users within a specific group. It considers all expenses, their splits, and all settlements recorded for the group. The user must be a member of the group to view its balance summary. """ logger.info(f"User {current_user.email} requesting balance summary for group {group_id}") # 1. Verify user is a member of the target group group_check = await db.execute( select(GroupModel) .options(selectinload(GroupModel.member_associations)) .where(GroupModel.id == group_id) ) db_group_for_check = group_check.scalars().first() if not db_group_for_check: raise GroupNotFoundError(group_id) user_is_member = any(assoc.user_id == current_user.id for assoc in db_group_for_check.member_associations) if not user_is_member: raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=f"User not a member of group {group_id}") # 2. Get all expenses and settlements for the group expenses_result = await db.execute( select(ExpenseModel) .where(ExpenseModel.group_id == group_id) .options(selectinload(ExpenseModel.splits).selectinload(ExpenseSplitModel.user)) ) expenses = expenses_result.scalars().all() settlements_result = await db.execute( select(SettlementModel) .where(SettlementModel.group_id == group_id) .options( selectinload(SettlementModel.paid_by_user), selectinload(SettlementModel.paid_to_user) ) ) settlements = settlements_result.scalars().all() # Fetch SettlementActivities related to the group's expenses # This requires joining SettlementActivity -> ExpenseSplit -> Expense settlement_activities_result = await db.execute( select(SettlementActivityModel) .join(ExpenseSplitModel, SettlementActivityModel.expense_split_id == ExpenseSplitModel.id) .join(ExpenseModel, ExpenseSplitModel.expense_id == ExpenseModel.id) .where(ExpenseModel.group_id == group_id) .options(selectinload(SettlementActivityModel.payer)) # Optional: if you need payer details directly ) settlement_activities = settlement_activities_result.scalars().all() # 3. Calculate user balances user_balances_data = {} # Initialize UserBalanceDetail for each group member for assoc in db_group_for_check.member_associations: if assoc.user: user_balances_data[assoc.user.id] = { "user_id": assoc.user.id, "user_identifier": assoc.user.name if assoc.user.name else assoc.user.email, "total_paid_for_expenses": Decimal("0.00"), "initial_total_share_of_expenses": Decimal("0.00"), "total_amount_paid_via_settlement_activities": Decimal("0.00"), "total_generic_settlements_paid": Decimal("0.00"), "total_generic_settlements_received": Decimal("0.00"), } # Process Expenses for expense in expenses: if expense.paid_by_user_id in user_balances_data: user_balances_data[expense.paid_by_user_id]["total_paid_for_expenses"] += expense.total_amount for split in expense.splits: if split.user_id in user_balances_data: user_balances_data[split.user_id]["initial_total_share_of_expenses"] += split.owed_amount # Process Settlement Activities (SettlementActivityModel) for activity in settlement_activities: if activity.paid_by_user_id in user_balances_data: user_balances_data[activity.paid_by_user_id]["total_amount_paid_via_settlement_activities"] += activity.amount_paid # Process Generic Settlements (SettlementModel) for settlement in settlements: if settlement.paid_by_user_id in user_balances_data: user_balances_data[settlement.paid_by_user_id]["total_generic_settlements_paid"] += settlement.amount if settlement.paid_to_user_id in user_balances_data: user_balances_data[settlement.paid_to_user_id]["total_generic_settlements_received"] += settlement.amount # Calculate Final Balances final_user_balances = [] for user_id, data in user_balances_data.items(): initial_total_share_of_expenses = data["initial_total_share_of_expenses"] total_amount_paid_via_settlement_activities = data["total_amount_paid_via_settlement_activities"] adjusted_total_share_of_expenses = initial_total_share_of_expenses - total_amount_paid_via_settlement_activities total_paid_for_expenses = data["total_paid_for_expenses"] total_generic_settlements_received = data["total_generic_settlements_received"] total_generic_settlements_paid = data["total_generic_settlements_paid"] net_balance = ( total_paid_for_expenses + total_generic_settlements_received ) - (adjusted_total_share_of_expenses + total_generic_settlements_paid) # Quantize all final values for UserBalanceDetail schema user_detail = UserBalanceDetail( user_id=data["user_id"], user_identifier=data["user_identifier"], total_paid_for_expenses=total_paid_for_expenses.quantize(Decimal("0.01"), rounding=ROUND_HALF_UP), # Store adjusted_total_share_of_expenses in total_share_of_expenses total_share_of_expenses=adjusted_total_share_of_expenses.quantize(Decimal("0.01"), rounding=ROUND_HALF_UP), # Store total_generic_settlements_paid in total_settlements_paid total_settlements_paid=total_generic_settlements_paid.quantize(Decimal("0.01"), rounding=ROUND_HALF_UP), total_settlements_received=total_generic_settlements_received.quantize(Decimal("0.01"), rounding=ROUND_HALF_UP), net_balance=net_balance.quantize(Decimal("0.01"), rounding=ROUND_HALF_UP) ) final_user_balances.append(user_detail) # Sort by user identifier final_user_balances.sort(key=lambda x: x.user_identifier) # Calculate suggested settlements suggested_settlements = calculate_suggested_settlements(final_user_balances) return GroupBalanceSummary( group_id=db_group_for_check.id, group_name=db_group_for_check.name, user_balances=final_user_balances, suggested_settlements=suggested_settlements )