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?
| Data size | Approach |
|---|---|
| < 1 MB | Load into memory, any approach works |
| 1-100 MB | Load into memory, but use efficient algorithms |
| 100 MB - 1 GB | Stream/mmap, avoid loading entirely into memory |
| > 1 GB | Streaming only, chunk-based processing |
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()orsys.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 -O2or-O3for compiler optimizations- Avoid
malloc()/free()in tight loops — pre-allocate - Use
memcpy()instead of byte-by-byte copying - Integer arithmetic > floating point when possible
Python
- Use
numpyfor numerical work (100x faster than pure Python loops) collections.Counter,defaultdict— avoid manual counting- List comprehensions > explicit loops
struct.unpack()for binary parsingsubprocess.run()>os.system()- For heavy computation: consider writing a small C program instead
General
- Profile before optimizing — find the actual bottleneck
- If a program hangs, it's likely: infinite loop, deadlock, or I/O bound on huge data
- If a program is slow, check: algorithm complexity, I/O pattern, memory allocation
Constraints Awareness
- If the task says "< 5000 bytes" — count your bytes, use
wc -c - If there's a time limit — test with actual data, not toy inputs
- If there's a memory limit — don't load everything into RAM
- Always verify constraints BEFORE declaring done
