python <=> c ++跨语言哈希

Question

I am writing a python program to look up a file. The file was created by a C++ program with a hash as filename ( std::hash<std::string> hash_fn ). I know the string from which the hash was created, but I can't find a python hash function that produces the same hash (I tried hash() and all in hashlib ). As an example, the string

file:///home/ubuntu/Untitled.skp

should give the hash:

3133433022

Unfortunately I don't have control over the C++ program, only about the python script (or eventual python extensions). Is it possible to find or implement the same hash function that C++ uses? Or should I try another approach?

Answer 1

For the most part, internal hash functions like std::hash in C++ or the Python hash function are not designed for external use. When you're designing such systems, strictly specify the hash function used, and implement it in both systems.

If it's too late for this, and you've already used std::hash , then about all you can do is find the sources for it (which depending on the compiler, may not be available), back engineer them to find the hashing algorithm used, specify it as your hash, and reimplement it in whatever languages needed. (You need to implement it in your own code, because it could potentially change in the next release of your compiler.)

Answer 2

To be compatible, use a known hash on both sides, like SHA-1. Python has it builtin, and if c++ doesn't have it built in, there are many libraries that do.

Answer 3

If you have control over the C++ part, you can make sure you use the same hashing algorithm on both sides. Alternatively, you can always have a look at the implementation of the hash_fn and try to re-implement the same functionality in Python.

Otherwise it may be quite hard to try to match the hashing functionality.