Explain strings, moves, and concurrency
Company: xAI
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: medium
Interview Round: Technical Screen
##### Question
What is a string in programming languages? What fields are stored in a typical struct string and how would you implement one yourself? What is the time complexity of copying a string? How can move operations be made more efficient? In Rust, does move only modify the reference? What is the difference between a thread and an asynchronous coroutine?
Quick Answer: This question evaluates understanding of string data structures, ownership and move semantics (including Rust-specific move behavior), and concurrency models such as threads versus asynchronous coroutines, testing competencies in memory management, performance analysis, and concurrent programming.
You are given a list of operations on named strings. Each string is represented only by its length. Compute the total number of character copies performed. Allowed operations (tokens are space-separated):
- new x n: Create/overwrite variable x with a string of length n. Cost: n (copying n characters).
- copy x y: Set x to a copy of y. Cost: len(y).
- move x y: Move y into x (x takes y's contents, y becomes empty). Cost: 0.
- concat x y z: Set x to y+z. Cost: len(y)+len(z).
- mconcat x y z: Move-concatenate y and z into x; x becomes y+z and both y and z become empty. Cost: 0.
Overwriting a variable discards its previous content at no cost. For mconcat, the destination x will be different from y and z. Inputs will reference only existing variables. Return the total copy cost.
Input to the function is a list of operation strings in the above formats.
Constraints
- 1 <= len(ops) <= 200000
- Variable names match [A-Za-z_][A-Za-z0-9_]*
- 0 <= n <= 10^9 for new x n
- mconcat x y z will have x distinct from y and z
- All reads reference variables that have been defined earlier
- Sum of all referenced lengths can be up to 10^12; use 64-bit or big integers
- Process should be O(len(ops)) time and O(U) space where U is number of variables
Hints
- Track only lengths, not string contents.
- Use a dictionary mapping variable names to current lengths.
- copy and concat add the source lengths to the total cost.
- move sets the destination length and empties the source with zero cost.
- mconcat sets destination to sum of source lengths and empties both sources with zero cost.