us
stutter
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us
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Ask HN: What are some 'cool' but obscure data structures you know about?
It might be easier to think about it as a stack, rather than a tree. Each element of the stack represents a subtree -- a perfect binary tree. If you ever have two subtrees of height k, you merge them together into one subtree of height k+1. Your stack might already have another subtree of height k+1; if so, you repeat the process, until there's at most one subtree of each height.
This process is isomorphic to binary addition. Worked example: let's start with a single leaf, i.e. a subtree of height 0. Then we "add" another leaf; since we now have a pair of two equally-sized leaves, we merge them into one subtree of height 1. Then we add a third leaf; now this one doesn't have a sibling to merge with, so we just keep it. Now our "stack" contains two subtrees: one of height 1, and one of height 0.
Now the isomorphism: we start with the binary integer 1, i.e. a single bit at index 0. We add another 1 to it, and the 1s "merge" into a single 1 bit at index 1. Then we add another 1, resulting in two 1 bits at different indices: 11. If we add one more bit, we'll get 100; likewise, if we add another leaf to our BNT, we'll get a single subtree of height 2. Thus, the binary representation of the number of leaves "encodes" the structure of the BNT.
This isomorphism allows you to do some neat tricks, like calculating the size of a Merkle proof in 3 asm instructions. There's some code here if that helps: https://github.com/lukechampine/us/blob/master/merkle/stack....
You could also check out section 5.1 of the BLAKE3 paper: https://github.com/BLAKE3-team/BLAKE3-specs/blob/master/blak...
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My proposal to the Foundation: add first-class S3 provider support
This isn't what I'm asking for - I don't care if it's baked into us, exists as a backend for minio, uses PseudoKV https://github.com/lukechampine/us/issues/67, or whatever the case may be. I see no value in sending any third party my private data in an unencrypted form (uploading to your server, even if over HTTPS, you got my data).
stutter
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Ask HN: What are some 'cool' but obscure data structures you know about?
This. Roughly a year ago I got interested in efficient immutability for my write-from-scratch-in-C Lisp [0] and started to write a HAMT implementation in C [1], along with a (somewhat hacky, you have been warned) benchmarking suite [2].
The docs are only 70% done (in particular the "putting it all together" part is missing) but it has been a really interesting and enlightening journey so far and can only recommend embarking on this path to everyone.
[0]: https://github.com/mkirchner/stutter
What are some alternatives?
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sdsl-lite - Succinct Data Structure Library 2.0
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pyroscope - Continuous Profiling Platform. Debug performance issues down to a single line of code [Moved to: https://github.com/grafana/pyroscope]
gring - Golang circular linked list with array backend
FusionCache - FusionCache is an easy to use, fast and robust cache with advanced resiliency features and an optional distributed 2nd level.
ctrie-java - Java implementation of a concurrent trie
asami - A graph store for Clojure and ClojureScript