Expense Sharing System

Arguments have the documented types. Money is integer cents; IDs and member names are case-sensitive strings. No floating-point conversion is required. Inputs must be copied where retained. Returned value records/lists must be fresh, and instances must be independent. There is no member deletion, expense editing, currency conversion, or external payment processing.

Prototype: implement split_equal(). Require total >= 0 and a nonempty list of distinct, nonempty participant names; otherwise raise std::invalid_argument. Return shares in lexicographic member order. Give each participant total / count, and give the first total % count sorted members one extra cent. Zero shares are valid. Do not mutate the input list.

Final: implement ExpenseSharing(members), where members is a nonempty list of distinct, nonempty names; invalid constructor input raises std::invalid_argument. Start every member at zero. The class manages one shared operation-ID namespace across expenses and settlements.

• add_expense(): require a nonempty operation ID, total > 0, a payer in the group, and a nonempty list of distinct group members. Invalid data raises std::invalid_argument unchanged. Split cents using the prototype's rule. Credit the payer with total, debit each participant with their share, and return true for a newly accepted expense. The payer may be absent from participants.
• settle(): require a nonempty ID, two distinct group members, and amount > 0; invalid structure raises std::invalid_argument. For a new ID, require balance[debtor] <= -amount and balance[creditor] >= amount, otherwise raise std::invalid_argument unchanged. Increase the debtor balance by amount and decrease the creditor balance by amount. Return true on acceptance.
• Both mutation methods recognize exact retries after structural validation and before applying any balance-dependent checks. A repeated accepted operation returns false without mutation. For expense identity, participant order is ignored; payer, total, and participant membership must match. For settlements, debtor, creditor, and amount must match. Reusing any accepted ID with different data or a different method raises std::invalid_argument. Failed requests do not consume IDs.
• balances(): return every member, including zero balances, in lexicographic key order. Positive means owed money; negative means owes money. The sum must always be zero.
• suggest_settlements(): use copies of balances. Sort debtors and creditors independently by name. Match the first remaining debtor to the first remaining creditor for the smaller remaining amount; advance each exhausted side and repeat. Return (debtor, creditor, cents) tuples in that generated order. Omit zero amounts. Reading suggestions never applies them. Return [] when all balances are zero; no minimum-transfer claim is required.

Every invalid request preserves balances and the operation registry.

The public C++ API below is authoritative for field types and return values. erase is used where another language might use a method named delete, because delete is a C++ keyword. Returned records are value snapshots with the documented named fields; mutating a record or its nested vectors cannot affect the service.

inline std::map<std::string,Int> split_equal(Int total,Strings participants);
class ExpenseSharing { public:
 explicit ExpenseSharing(const Strings& members);
 bool add_expense(const std::string& id,const std::string& payer,Int total,Strings participants);
 bool settle(const std::string& id,const std::string& debtor,const std::string& creditor,Int amount);
 std::map<std::string,Int> balances() const;
 Vec<std::tuple<std::string,std::string,Int>> suggest_settlements() const;
};

Compile and run from this directory:

c++ -std=c++20 -O2 -Wall -Wextra -pedantic -pthread main.cpp -o practice
./practice --stage prototype
./practice --stage final

Edit exercise.hpp. Both stages start unfinished and must report failures until implemented. Keep main.cpp as the executable contract. The runner returns nonzero for a failed check, an unexpected exception, an invalid stage, or an empty suite. C++20 and only the standard library are required.