hamt
asami
hamt | asami | |
---|---|---|
7 | 6 | |
261 | 626 | |
- | 0.0% | |
6.9 | 0.0 | |
3 months ago | about 2 years ago | |
C | Clojure | |
MIT License | Eclipse Public License 1.0 |
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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?
asami
- Ask HN: What are some 'cool' but obscure data structures you know about?
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Ask HN: Why are relational DBs are the standard instead of graph-based DBs?
Unlike some other commenters, I agree that graph models are usually a better fit for most data than relational models. There's been some interesting work in recent years developing this idea: in the Clojure world there's Datomic, XTDB, and a host of competitors, all of which build on work from Semantic Web/SPARQL/triplestores and logic programming. Some are even intended to be used as primary datastores: they support some amount of schema and constraints, have well-defined consistency and ACID guarantees, etc. This makes them unlike graph databases like Neo4J and others, which fill an architectural role more like Elasticsearch as a read-optimization tool. Here's an interesting talk making a case for triple-based databases.
- Introduction to the Asami Graph Database
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How to query Datomic, Datascript, Asami, or other graph databases
Despite the documentation that exists, I've heard many people who have been confused about how to query Datomic, Datascript, Asami, or other graph databases. So I've made an attempt at explaining it https://github.com/threatgrid/asami/wiki/Introduction
- Introduction (To Graph Databases)
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Asami
The first Graph implementation for Asami was a simple in-memory data structure, described in my ClojureD talk. The code for this appears in asami.index. This file started much smaller (as referenced above), but has since expanded with the needs extended functionality, such as transactions, and transitive closure operations.
What are some alternatives?
AspNetCoreDiagnosticScenarios - This repository has examples of broken patterns in ASP.NET Core applications
datascript - Immutable database and Datalog query engine for Clojure, ClojureScript and JS
multiversion-concurrency-contro
crux - General purpose bitemporal database for SQL, Datalog & graph queries. Backed by @juxt [Moved to: https://github.com/xtdb/xtdb]
RVS_Generic_Swift_Toolbox - A Collection Of Various Swift Tools, Like Extensions and Utilities
datahike - A durable Datalog implementation adaptable for distribution.
multiversion-concurrency-control - Implementation of multiversion concurrency control, Raft, Left Right concurrency Hashmaps and a multi consumer multi producer Ringbuffer, concurrent and parallel load-balanced loops, parallel actors implementation in Main.java, Actor2.java and a parallel interpreter
datalevin - A simple, fast and versatile Datalog database
CPython - The Python programming language
Apache AGE - Graph database optimized for fast analysis and real-time data processing. It is provided as an extension to PostgreSQL. [Moved to: https://github.com/apache/age]
pyroscope - Continuous Profiling Platform. Debug performance issues down to a single line of code [Moved to: https://github.com/grafana/pyroscope]
naga - Datalog based rules engine