Performant Code

Performant Code

How to write code that won't timeout on large inputs.

Think About Scale First

Before writing code, ask: how big is the data?

I/O Optimization

Large files

• mmap (C: mmap(), Python: mmap.mmap()) — map file into memory, OS handles paging
• Buffered binary reads — fread() in C, open(f, 'rb').read(chunk) in Python
• NEVER read a 500MB file line-by-line with fgets() when you need random access

Writing output

• Buffer writes — don't call write() for every byte
• Use fwrite() or sys.stdout.buffer.write() for binary output
• Flush only when needed

Algorithm Complexity

• O(n) beats O(n log n) beats O(n²) — always
• Nested loops on large data = timeout. Restructure to single pass + hash map
• Sorting is O(n log n) — only sort if you need to
• Use hash maps/sets for lookup instead of linear search
• Pre-compute what you can outside loops

Language-Specific Tips

C

• Use mmap() for large file access
• -O2 or -O3 for compiler optimizations