Bridge · From syntax to engineering · 40 MIN

Project · Apply each ledger event once

A repeated identifier must keep the same meaning.

Clients may repeat a request after losing the response. Store the meaning of the first event under its ID. A repeat with the same amount has no additional effect; a repeat with a different amount is a conflict. A Map implements this for a single in-memory batch. Real services need durable uniqueness, transaction boundaries, an expiry policy, and owner-scoped keys. The downloadable Expense API project builds on this model so you can inspect the HTTP behavior and test duplicate requests.

Write a small contract first, then test how the implementation behaves at its boundaries.

Read the example

const seen = new Map([["evt-1", 10]]);
console.log(seen.get("evt-1"));

10

Your challenge

Input is a list of {id:string,amount:integer}. Apply the first event for each id; ignore exact repeats and list ids of conflicting repeats in encounter order. Return {total, appliedIds, conflicts}. Do not mutate input.

Solution cost: Expected O(n). time · O(u + f), unique ids plus conflicts. space

Common trap

Checking if seen.get(id) is truthy loses the valid amount zero.

Further reading: HTTP method idempotency