The Shift-to-Middle array is a C++ data structure presented as a potential alternative to std::deque for scenarios requiring frequent insertions and deletions at both ends. It aims to improve performance by reducing the overhead associated with std::deque's segmented architecture. Instead of using fixed-size blocks, the Shift-to-Middle array employs a single contiguous block of memory. When insertions at either end cause the data to reach one edge of the allocated memory, the entire array is shifted towards the center of the allocated space, creating free space on both sides. This strategy aims to amortize the cost of reallocating and copying elements, potentially outperforming std::deque when frequent insertions and deletions occur at both ends. The author provides benchmarks suggesting performance gains in these specific scenarios.
The blog post showcases efficient implementations of hash tables and dynamic arrays in C, prioritizing speed and simplicity over features. The hash table uses open addressing with linear probing and a power-of-two size, offering fast lookups and insertions. Resizing is handled by allocating a larger table and rehashing all elements, a process triggered when the table reaches a certain load factor. The dynamic array, built atop realloc, doubles in capacity when full, ensuring amortized constant-time appends while minimizing wasted space. Both examples emphasize practical performance over complex optimizations, providing clear and concise code suitable for embedding in performance-sensitive applications.
Hacker News users discuss the practicality and efficiency of Chris Wellons' C implementations of hash tables and dynamic arrays. Several commenters praise the clear and concise code, finding it a valuable learning resource. Some debate the choice of open addressing over separate chaining for the hash table, with proponents of open addressing citing better cache locality and less memory overhead. Others highlight the importance of proper hash functions and the potential performance degradation with high load factors in open addressing. A few users suggest alternative approaches, such as using C++ containers or optimizing for specific use cases, while acknowledging the educational value of Wellons' straightforward C examples. The discussion also touches on the trade-offs of manual memory management and the challenges of achieving both simplicity and performance.
Summary of Comments ( 97 )
https://news.ycombinator.com/item?id=43456669
Hacker News users discussed the performance implications and niche use cases of the Shift-to-Middle array. Some doubted the benchmarks, suggesting they weren't representative of real-world workloads or that
std::dequewas being used improperly. Others pointed out the potential advantages in specific scenarios like embedded systems or game development where memory allocation is critical. The lack of iterator invalidation during insertion/deletion was noted as a benefit, but some considered the overall data structure too niche to be widely useful, especially given the existing, well-optimizedstd::deque. The maintainability and understandability of the code, compared to the standard library implementation, were also questioned.The Hacker News post titled "Shift-to-Middle Array: A Faster Alternative to Std:Deque?" (https://news.ycombinator.com/item?id=43456669) sparked a discussion with several interesting comments. Many commenters focused on the niche use cases where this data structure might be beneficial and questioned the broad claim of superiority over
std::deque.Several commenters pointed out the potential advantages of the "shift-to-middle" array in specific situations. One commenter highlighted its usefulness for implementing a fixed-size circular buffer where elements are frequently added and removed from both ends. They suggested that this data structure might outperform
std::dequein such a scenario because it avoids memory allocations and deallocations. Another user echoed this sentiment, emphasizing that the shift-to-middle array's contiguous memory layout could be particularly advantageous for cache performance when dealing with a fixed-size buffer.However, many comments expressed skepticism about the general claim of being "faster" than
std::deque. Some users pointed out the overhead associated with shifting elements in the middle of the array, which could outweigh the benefits in many common use cases. One commenter argued thatstd::dequeis highly optimized and already uses a similar strategy of managing chunks of memory, making it unlikely that the shift-to-middle array would offer significant improvements in most scenarios. Another user mentioned the potential complexity and difficulty in implementing the shift-to-middle array correctly, which could introduce subtle bugs and negate any performance gains.The discussion also touched upon the importance of benchmarking and real-world testing to validate the performance claims. One commenter stressed the need for rigorous benchmarks comparing the shift-to-middle array against
std::dequein various use cases. Another user suggested that the performance characteristics might vary depending on the specific hardware and compiler used.Finally, some comments discussed alternative data structures that might be more suitable for specific use cases. One commenter mentioned the "ring buffer" as a potential alternative for fixed-size circular buffer scenarios. Another user suggested exploring specialized libraries optimized for specific data structures and algorithms.
In summary, the comments on the Hacker News post expressed both interest in the potential advantages of the shift-to-middle array and skepticism about its general applicability as a faster alternative to
std::deque. The discussion highlighted the importance of considering specific use cases, performing rigorous benchmarks, and exploring alternative data structures before making broad performance claims.