Design a packet reassembler API

Q: Design a packet reassembler API

This question evaluates API design, stream-oriented buffer management, stateful packet assembly, input validation, concurrency control, testing strategy, and time/space complexity reasoning.

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This is a medium difficulty System Design question, commonly asked during Technical Screen rounds at Bloomberg.

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This question is commonly asked for Software Engineer candidates at Bloomberg during technical interviews.

Question

Design a PacketAssembler API for Streaming Fragments

Context and Assumptions

You are building a small component that assembles packets from a byte stream.
Each packet begins with a 1-byte header that encodes the payload length N (0–255). The header byte is not part of the payload.
The stream may arrive fragmented arbitrarily; a single call may contain parts of a packet or multiple full packets back-to-back.

Example interaction:

packetit(0x04, 'a', 'b', 'c') -> no output (header 4 indicates a 4-byte payload; only 3 bytes arrived)
packetit('d') -> 'abcd' (completes the prior packet)

Requirements

Specify the class and method signatures of a PacketAssembler API.
Describe state management: buffering, expected size, and how fragments are handled.
Behavior when receiving multiple consecutive packets within one or many calls.
Handling of invalid headers and oversized input (e.g., header value exceeds a configured maximum).
Define when/how state resets after emission and on errors.
Thread-safety considerations and options.
Provide tests (unit-style examples) that exercise common and edge cases.
Analyze time and space complexity.

Design a packet reassembler API

Quick Overview

Design a PacketAssembler API for Streaming Fragments

Context and Assumptions

Requirements

Solution

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