weight-cache

A cache that holds a limited number of key-value pairs. When the capacity of the cache is exceeded, the least-recently-used (where "used" means a look-up or putting the pair into the cache) pair is automatically removed.

Contrary to the lru-cache crate (which this crate is heavily inspired by!), the capacity is not the number of items in the cache, but can be given by an arbitrary criterion by implementing Weighable for the value type V. A straight-forward example of this would be to use the allocated size of the object, and provide a total capacity which must not be exceeded by the cache.

Examples

use weight_cache::{Weighable, WeightCache};
use std::num::NonZeroUsize;

#[derive(PartialEq, Debug)]
enum Food {
    Milk { milliliters: usize },
    Cucumber { pieces: usize },
    Meat { grams: usize },
    Potato { pieces: usize },
    Crab { grams: usize },
}

impl Weighable for Food {
    fn measure(value: &Self) -> usize {
        match value {
            Food::Milk { milliliters } => milliliters * 104 / 100,
            Food::Cucumber { pieces } => pieces * 158,
            Food::Meat { grams } => *grams,
            Food::Potato { pieces } => pieces * 175,
            Food::Crab { grams } => *grams,
        }
    }
}

let mut cache = WeightCache::new(NonZeroUsize::new(500).unwrap());

// Can't put too much in!
assert!(cache.put(0, Food::Meat { grams: 600 }).is_err());
assert!(cache.is_empty());

cache.put(1, Food::Milk { milliliters: 100 }).unwrap();
assert!(!cache.is_empty());
assert_eq!(*cache.get(&1).unwrap(), Food::Milk { milliliters: 100 });

cache.put(2, Food::Crab { grams: 300 }).unwrap();
assert_eq!(*cache.get(&2).unwrap(), Food::Crab { grams: 300 });
assert_eq!(*cache.get(&1).unwrap(), Food::Milk { milliliters: 100 });

cache.put(3, Food::Potato { pieces: 2 }).unwrap();
assert_eq!(*cache.get(&3).unwrap(), Food::Potato { pieces: 2});
assert!(cache.get(&2).is_none()); // 1 has been touched last
assert_eq!(*cache.get(&1).unwrap(), Food::Milk { milliliters: 100 });

Feature flags

• metrics: Enables metric gathering on the cache. Register a prometheus::Registry with a call to WeightCache::register; set a custom metric namespace with WeightCache::new_with_namespace

License: MIT OR Apache-2.0