Explain and Implement Strings
Company: xAI
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: medium
Interview Round: Technical Screen
##### Question
What is a string in programming languages? Inside a struct string{} in Rust, what fields are stored and how would you design and implement one yourself? If you copy a string, what is the time complexity of the operation? How can move semantics be made more efficient? In Rust, is a move simply a reference update?
Overview: This question evaluates understanding of string data structures, memory layout, ownership and move semantics in Rust, and algorithmic complexity related to copying and performance.
Simulate a simplified dynamically-allocated string type and compute the total number of bytes copied by the system. Each string has a length L and a capacity C. Capacity is always at least 1. You are given a list of operations on named variables. Operations:
- make name len: Create a new string variable with length len and capacity equal to the smallest power of two >= max(1, len).
- copy src dst: Deep copy src into a new variable dst. This copies exactly src.length bytes. The new dst has length src.length and capacity max(1, src.length). src remains valid.
- move src dst: Move src into a new variable dst (shallow, ownership transfer). No bytes are copied. dst takes src's length and capacity. src becomes invalid and cannot be used until re-created.
- append name k: Increase the string's length by k. If length + k exceeds capacity, reallocate by repeatedly doubling capacity until it is >= new length; this reallocation copies exactly the old length bytes once. Then set length to length + k.
No operation will reference a variable that does not exist or is invalid, and destinations for make/copy/move do not already exist. Return the total number of bytes copied across all operations (deep copies and reallocations only).
Constraints
- 1 <= len(ops) <= 200000
- 0 <= len, k <= 10^9
- All operations are valid as per the rules (e.g., src exists, dst does not yet exist; moved-from variables are not used until re-created).
- Capacity doubling is by repeated multiplication by 2 until capacity >= required length.
- Bytes counted are only from deep copies (copy) and internal reallocations during append; appending new external data itself does not count.
Hints
- Track for each variable its (length, capacity) and whether it is alive.
- For append that exceeds capacity, add the old length once to the total and then double capacity until sufficient.
- copy adds exactly src.length to the total; move adds 0 and invalidates the source.