Expense Sharing System

Ownership and transitions

Integer cents preserve conservation exactly. Divide by participant count and distribute remainder cents to the first sorted names; floating-point rounding can invent or lose cents. Store a canonical operation fingerprint so participant order does not affect expense retries. Exact settlement replay precedes balance checks because the first success changed those balances. Suggestions pair alphabetic debtors/creditors on copies, with deterministic output and no minimum-transfer claim.

JavaScript boundaries

Use native classes with per-instance state and plain object/array snapshots. Object assignment shares a reference; copy retained inputs and every outward mutable container, including nested arrays. null means absence. Map handles arbitrary string keys safely, including "__proto__"; do not confuse a missing key with zero or an empty string. Domain validation uses RangeError. Numeric inputs must satisfy Number.isSafeInteger, so booleans, fractions, infinities, and NaN are invalid where an integer is required. All documented aggregate results and timestamp-plus-duration sums fit the safe-integer range. Strings use exact case-sensitive equality; lexical order compares JavaScript UTF-16 code units. Calls are serialized and synchronous.

const internal = ["initial"];
const snapshot = [...internal];
snapshot.length = 0;
console.assert(internal.length === 1);

A detached snapshot protects ownership; the service still has to validate and commit a coherent transition.