Jane Street·Machine Learning Engineer·Technical Phone Screen·Senior

Senior

May 2026

Summary

Jane Street ML Engineer interview with a meaty coding problem centered on building a code folding system from scratch. The problem was well-specified but had enough edge cases in the visibility logic to keep you busy for a while.

Questions Asked (1)

Implement a CodeEditor class that supports indentation-based code folding: a render() method that shows visible lines with display numbers and block-header markers, plus shrink(d) and expand(d) operations that collapse or restore blocks based on the display line number from the most recent render.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

This took me longer to get right than I expected.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and edge cases, then design a data structure that efficiently maps display line numbers to underlying code lines and supports folding operations. Implement the class with a focus on correctness, and discuss time/space complexity and potential optimizations.

Pro tip: Demonstrate awareness of real-world code editors by mentioning that folding operations should be idempotent and that the display line number refers to the most recent render, which may change after each operation.

1. Clarify requirements and edge cases

Ask questions to understand the exact behavior: how blocks are defined (indentation levels), what happens when shrinking/expanding invalid lines, and whether nested folds are supported.

2. Design data structures

Choose a representation for the code lines, indentation levels, and fold state. Consider using a tree or a list with metadata to track visible lines and collapsed blocks.

3. Implement render()

Iterate through lines, skipping those inside collapsed blocks, and output visible lines with display numbers and block-header markers (e.g., '...' for collapsed blocks).

4. Implement shrink(d) and expand(d)

Map the display line number d to the corresponding code line, identify the block to collapse/expand, update the fold state, and ensure subsequent renders reflect the change.

5. Analyze complexity and discuss trade-offs

Evaluate time and space complexity of each operation, and discuss potential improvements like caching or incremental updates for large files.

Key Points to Mention

Definition of a block: lines with greater indentation than the header line.
Handling nested blocks: collapsing a block should hide all its sub-blocks.
Mapping display line numbers to actual line indices, especially after folds.
Edge cases: invalid display line numbers, expanding an already expanded block, shrinking a non-header line.
Time complexity: render O(n), shrink/expand O(n) or O(log n) with appropriate data structures.
Space complexity: O(n) for storing lines and fold state.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.