LFU implemented in very basic (no interior mutability) Rust.

Not so long ago a paper on O(1) LFU design (http://dhruvbird.com/lfu.pdf) was pretty popular in caching world. While LFU isn't (on it's own) that good of a general cache algorithm, people do like bragging about implementing a O(1) algorithm :)

The original paper outlines the algorithm using doubly linked list. That's both interesting as a problem for learning Rust (because doubly linked lists are deemed hard) syntax and a counter example of how to implement something in Rust. I came to that conclusion comparing version with doubly linked lists that requires use of interior mutability pattern in Rust. That in turn means runtime vs. compile time checking and generally makes for worse performance overall due to simple problems such as poor compiler-level optimizations.

So as an exercise in writing idiomatic performant Rust that's easy to read, argue about and still correct I added this version.

The design principles are fairly simple:

• store data about keys and how frequent they are using FrequencyNode struct
• store said structs in a Vec<FrequencyNode>
• at the moment frequency simply maps to index in that list
• values for keys are stored in a HashMap using Item struct
• Item itself simply contains the current index of FrequencyNode storing the key and Bytes field for data

Assuming Rust's HashMap get, insert and remove operations are O(1) this boils down to question of what's the added complexity coming from storing frequency data in Vec:

• get means we need to grab the Item from HashMap, pull key from Vec<FrequencyNode>, assign it to next Vec<FrequencyNode> and incrment it's index
• set is simply insert into HashMap and add key to Vec<FrequencyNode> at index 0
• only worrying point at the moment is adding new FrequencyNode> since that's O(m) where m is Vec.len()

Well, it's written using Vec, HashMap and tokio Bytes. Thus it's limited to being oportunistically (expected/amortized) O(1) rather than guaranteed O(1).

Complexities for Vec and HashMap are sourced from here

At minimum cache has to provide two key functions:

• get - internally this means one HashMap.get, Vec.get_mut and Vec.push operations
• insert - internally this means HashMap.get and Vec.get_mut and Vec.push operations

with Vec.get being O(1) and HashMap.get being expected O(1) the only problem is Vec.push which is amortized O(1) in this case.

To read if HashMap makes sense you can read on it here