Expense Splitter

Assumptions you may make

• One currency; amounts are whole minor units (cents) held in a long, never a floating-point number
• A split that cannot be divided exactly must place the leftover cents deterministically
• Single process; persistence is out of scope

Things the brief expects to exist

Candidates, not a design. How they relate is yours to decide.

Group

Holds members and the expenses recorded between them

Member

Identifies a person in the group

Expense

Records who paid, how much, and how it is to be split

SplitStrategy

Turns a total and a list of participants into exact shares

Ledger

Keeps each member's running net balance

SettlementPlanner

Proposes transfers that clear every balance

Transfer

A suggested or recorded payment from one member to another

What this exercise is teaching

• Add a new way of splitting without editing the expense or ledger code
• Keep balances exact, so that every member's balance always sums to zero across the group
• Separate the record of what happened from the suggestion of who should pay whom

Once it works, make it survive these

• Add split by shares, where one member has two shares and everyone else one
• Support expenses in several currencies using a fixed exchange rate per expense
• Allow an expense to be edited, with balances updated correctly