Practice Mode • Backend & APIs • hard

Message Cursor Page

280 points

Return a stable page of messages.

Problem

You are implementing return a stable page of messages. The input is one JSON value. Return exactly the JSON or scalar result described by the examples, with stable ordering and no extra logging. Return a deterministic cursor page over a stable compound ordering. Explain your reasoning and complexity during interview review; the judge evaluates only the returned result.

Input contract

One UTF-8 JSON value matching the structures shown in the examples.

Output contract

One deterministic JSON value or scalar with no explanatory text.

Target complexity

O(n log n) time, O(page) output space

Judge protocol

function-stdin-json-v1

Examples

Example 1

Input: {"limit":2,"rows":[{"createdAt":1,"id":"id1"},{"createdAt":2,"id":"id2"},{"createdAt":3,"id":"id3"},{"createdAt":4,"id":"id4"},{"createdAt":5,"id":"id5"},{"createdAt":6,"id":"id6"},{"createdAt":7,"id":"id7"},{"createdAt":8,"id":"id8"},{"createdAt":9,"id":"id9"}]}
Output: {"items":[{"createdAt":1,"id":"id1"},{"createdAt":2,"id":"id2"}],"nextCursor":"2|id2"}

This example demonstrates the core backend apis invariant.

Example 2

Input: {"limit":3,"rows":[]}
Output: {"items":[],"nextCursor":null}

This example covers the empty or minimum boundary.

Constraints

Input is valid UTF-8 JSON and is at most 64 KB.
Output must be deterministic for identical input.
Do not use network access, persistent filesystem writes, subprocesses, or environment secrets.
Handle the empty or minimum boundary shown in the examples.

Progressive hints

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