Remix.run Logo
zvrba 10 months ago

I implemented an academic paper (join tree) in C# here https://github.com/zvrba/Pfm

It provides the opposite: a mutable CoW data structure that is extremely cheap to "fork" so that all subsequent updates occur only on the new "fork" and are invisible to the old "fork".

zelphirkalt 10 months ago | parent [-]

Isn't that the point of well designed persistent data structures?

zvrba 10 months ago | parent [-]

Depends on how you define "persistent data structure". In most definitions that I've encountered, a new version is made after each update. This code makes a new version only when you explicitly request it with Fork(). This allows you to

- Use the data structure as a "standard" tree, sharing one instance across threads and use locking for thread-safety

- Use it as a fully-persistent structure by calling "Fork" before every modification

- Or anything in between

I needed the 3rd case: a cache manager gives out a forked view of the actual cache contents to its clients. Thus the cache is always in control of the "master" copy, and if a client modifies its own fork, the cache and other clients are not affected.