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zephyrproject-rtos / zephyr

Primary Git Repository for the Zephyr Project. Zephyr is a new generation, scalable, optimized, secure RTOS for multiple hardware architectures.
https://docs.zephyrproject.org
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`sys_hashmap` user defined equality function #76238

Open lucaato opened 2 months ago

lucaato commented 2 months ago

Is your enhancement proposal related to a problem? Please describe. I was trying to store strings as keys in a hashmap and quickly ran into the problem that only uint64_t can be stored in the map, and by looking at the implementation it's clear that this can't be solved, even by providing a custom hash function.

Describe the solution you'd like If possible a new field of struct sys_hashmap could be added to allow defining an equality function. If specified this function could be used to compare the keys otherwise we could fallback to using the == operator. Edit: this would not work out of the box for the cxx implementation of the map. Since that code does not even use the hash_func defined by the user it shouldn't be a problem.

Describe alternatives you've considered Haven't considered alternatives, I just want to know what maintainers think about it.

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cfriedt commented 1 month ago

Using strings as keys would be a welcome addition.

The current hash map implementation only supports 32-bit hashes. However, on 64-bit architectures, a string (char *, const char *, or const char *const) uses a 64-bit pointer.

Additionally, strings as keys would likely need dynamic allocation (unless only string literals were used as keys), and I think this would likely need to be handled outside of the hash map itself.

So I think some additional work is needed. I'll comment on the PR as well.