Design a keyboard and mouse input system

Q: Design a keyboard and mouse input system

This question evaluates a candidate's competency in designing a cross-platform input processing layer, encompassing event modeling and ordering, IME/composition handling, pointer and selection semantics, deterministic event logging and replay, and resilience to lost or duplicated events.

Q: How do I approach System Design interview questions?

System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

Q: What difficulty level is this interview question?

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

Q: What role is this question designed for?

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

Question

System Design: Input Processing and Text Reconstruction

Context

You are designing a cross-platform input processing layer for a text editor that reconstructs exactly what a user typed from raw keyboard and mouse events. The system must support standard keyboards, mice/trackpads, and IME/composition for non-Latin input. Assume a single-user session per editor instance, possibly with multiple input devices (e.g., external keyboard + laptop keyboard + mouse).

Functional Requirements

Event capture and handling
- Keyboard: keydown/keyup, key repeat, modifiers (Shift/Ctrl/Alt/Meta/CapsLock), backspace/delete.
- Pointer: cursor movement, mouse down/up/move, selection via drag, double/triple click.
- Clipboard: copy, cut, paste.
- IME/composition: compositionstart/update/end and pre-edit text.
Ordering and time
- Maintain precise timestamps and a total order across devices (keyboard, mouse).
APIs
- recordEvent(event)
- getCurrentText()
- getSelectionRange()
- undo()
- redo()
Persistence and replay
- Persist an event log and support deterministic replay to derive final text.
Reliability and resilience
- Debounce where appropriate; handle lost/duplicated events.
- Define data models for events and sessions.

Deliverables

High-level components and their responsibilities.
Core data structures (document, selection, history, logging).
Recovery strategy after crashes.
Testing strategy (functional, property-based, fuzzing, IME).
Performance considerations (latency, throughput, memory).

Design a keyboard and mouse input system

Quick Overview

System Design: Input Processing and Text Reconstruction

Context

Functional Requirements

Deliverables

Solution

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