Implement an AST Differ for Code Changes
Last updated: May 21, 2025
Quick Overview
Implement an Abstract Syntax Tree (AST) differ that takes two AST representations of source code as input and computes a minimal edit script to transform the first AST into the second. The output should detail the necessary operations at the node level, including additions, deletions, and modifications of functions, statements, and expressions, ensuring that the transformation is efficient and accurate.
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May 21, 202515
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Implement an Abstract Syntax Tree (AST) differ that takes two AST representations of source code as input and computes a minimal edit script to transform the first AST into the second. The output should detail the necessary operations at the node level, including additions, deletions, and modifications of functions, statements, and expressions, ensuring that the transformation is efficient and accurate.
Cursor needs to understand code changes at a structural level for features like intelligent code review and context retrieval. Line-level diffs miss semantic meaning. For example, reformatting a function changes every line but does not change the AST. This question tests your ability to work with tree structures and edit distance algorithms.
What the Interviewer Expects
- Define a meaningful node comparison function that captures semantic equivalence
- Implement a tree edit distance algorithm
- Generate a minimal edit script of insert, delete, update, and move operations
- Handle common cases efficiently like function reordering
- Discuss the time complexity and practical optimizations
Key Topics to Cover
How to Approach This
- Clarify input constraints and edge cases before writing code.
- Walk through your approach verbally and confirm with the interviewer before coding.
- Start with a brute force solution, then optimize. Mention time and space complexity.
- Test your solution with examples, including edge cases like empty input or duplicates.
- Consider common patterns: sliding window, two pointers, hash map, BFS/DFS, dynamic programming.
Possible Follow-up Questions
- How would you handle a function that was moved to a different file?
- How would you make the diff human-readable?
- What is the practical performance on files with thousands of AST nodes?
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Practice DSA ProblemsSample Answer
Problem Analysis
To solve the problem of transforming one AST into another, we can utilize a tree edit distance algorithm. This approach is appropriate because it allows us to efficiently compute the minimal set of op...
Approach
- Node Comparison Function: Define a function to compare AST nodes by their type and relevant attributes (e.g., function name, parameters). This will help identify if two nodes are semantically e...
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