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HashCow is a Rust HashMap implementation with copy-on-write keys and values.

Originally built for optimizing the Purple Protocol, this library provides a way to link HashMaps in memory that have duplicate entries. Instead of the duplicate data, it is instead borrowed and it is only cloned when mutation is needed.

Using HashCow

use hashcow::{Form, CowHashMap};

let mut hm: CowHashMap<str, [u8]> = CowHashMap::new();

// We insert an owned value in the map
hm.insert_owned("key".to_owned(), vec![1, 2, 3]);
assert_eq!(hm.entry_form(&"key").unwrap(), Form::Owned);

// We now create a clone with borrowed fields
let mut hm_clone = hm.borrow_fields();
assert_eq!(hm_clone.entry_form(&"key").unwrap(), Form::Borrowed);

// On mutation, the borrowed entry is cloned
let entry = hm_clone.get_mut(&"key").unwrap();

// We now mutate the cloned value
*entry = vec![4, 5, 6];
assert_eq!(hm_clone.entry_form(&"key").unwrap(), Form::Owned);

// The two maps now have different entries for the same key
assert_eq!(hm.get(&"key").unwrap(), &[1, 2, 3]);
assert_eq!(hm_clone.get(&"key").unwrap(), &[4, 5, 6]);


We welcome anyone wishing to contribute to HashCow! Check out the issues section of the repository before starting out.


HashCow is licensed under the MIT license.

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