hamt
sdsl-lite
hamt | sdsl-lite | |
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7 | 5 | |
261 | 2,175 | |
- | - | |
6.9 | 0.0 | |
3 months ago | 12 months ago | |
C | C++ | |
MIT License | GNU General Public License v3.0 or later |
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hamt
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Visual Introduction to Hash-Array Mapped Tries (HAMTs)
This isn't a very good explanation. The wikipedia article isn't great either. I like this description:
https://github.com/mkirchner/hamt#persistent-hash-array-mapp...
The name does tell you quite a bit about what these are:
* Hash - rather than directly using the keys to navigate the structure, the keys are hashed, and the hashes are used for navigation. This turns potentially long, poorly-distributed keys into short, well-distributed keys. However, that does mean you have to compute a hash on every access, and have to deal with hash collisions. The mkirchner implementation above calls collisions "hash exhaustion", and deals with them using some generational hashing scheme. I think i'd fall back to collision lists until that was conclusively proven to be too slow.
* Trie - the tree is navigated by indexing nodes using chunks of the (hash of the) key, rather than comparing the keys in the node
* Array mapped - sparse nodes are compressed, using a bitmap to indicate which logical slots are occupied, and then only storing those. The bitmaps live in the parent node, rather than the node itself, i think? Presumably helps with fetching.
A HAMT contains a lot of small nodes. If every entry is a bitmap plus a pointer, then it's two words, and if we use five-bit chunks, then each node can be up to 32 entries, but i would imagine the majority are small, so a typical node might be 64 bytes. I worry that doing a malloc for each one would end up with a lot of overhead. Are HAMTs often implemented with some more custom memory management? Can you allocate a big block and then carve it up?
Could you do a slightly relaxed HAMT where nodes are not always fully compact, but sized to the smallest suitable power of two entries? That might let you use some sort of buddy allocation scheme. It would also let you insert and delete without having to reallocate the node. Although i suppose you can already do that by mapping a few empty slots.
- Show HN: A hash array-mapped trie implementation in C
- Ask HN: What are some 'cool' but obscure data structures you know about?
sdsl-lite
- SDSL – Succinct Data Structure Library
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Ask HN: What are some 'cool' but obscure data structures you know about?
Succinct Data Structures [0] [1]. It encompass many different underlying data structure types but the overarching idea is that you want small data size while still keeping "big O" run time.
In other words, data structures that effectively reach a 'practical' entropy lower bound while still keeping asymptotic run time.
[0] https://en.wikipedia.org/wiki/Succinct_data_structure
[1] https://github.com/simongog/sdsl-lite
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SDSL-RS: A Rust interface for the C++ Succinct Data Structure Library.
The book mentioned in another comment is probably the best way to go. But FYI, documentation for some data structures include references. An SDSL-lite example can be found here. And its equivalent in SDSL-RS can be found here.
What are some alternatives?
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CPython - The Python programming language
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