Design Microsoft Word — LLD with Undo/Redo

Problem Statement

Design a simplified version of Microsoft Word that supports:

1. Multiple font types (Arial, Times New Roman, etc.)
2. Font styles (Bold, Italic, Underline) — can be combined
3. Font sizes
4. Text formatting and paragraphs
5. Inline images
6. Undo / Redo functionality

The design should be extensible — adding new formatting options or features should not require modifying existing code.

Requirements

• Support rich text with mixed formatting within a single paragraph
• Images can be placed inline with text
• Undo/Redo should work for any operation (typing, formatting changes, image insertion)
• Design for extensibility (Open/Closed principle)

What the Interviewer Expects

1. Document structure — Composite Pattern:

   • Document contains Page[]
   • Page contains Block[] (paragraphs, images)
   • Paragraph contains TextRun[] (each run has uniform formatting)
   • TextRun has content + FormattingStyle

2. Formatting — Strategy/Decorator Pattern:

   • FormattingStyle object with font, size, bold, italic, underline
   • Styles can be composed/stacked
   • New styles added without changing existing code

3. Undo/Redo — Command Pattern:

   • Every action is a Command object: execute() and undo()
   • CommandHistory maintains two stacks (undo stack, redo stack)
   • Executing new action clears redo stack
   • Types: InsertTextCommand, DeleteTextCommand, FormatCommand, InsertImageCommand

4. Extensibility:

   • Adding "Strikethrough" = new style flag, no existing code changes
   • Adding "Table" = new Block subclass, composites handle it naturally

Follow-ups

1. How would you handle collaborative editing (multiple cursors)?
2. How would you persist the document? What serialization format?
3. How would you implement "group undo" (undo a bulk formatting change as one step)?
4. What's the memory cost of storing every command? How would you optimize for large documents?
5. How would you add a plugin system for third-party formatting extensions?