compact-dict is a fast hashmap based dictionary implemented in Mojo 🔥.

Although the dictionary is fast (currently it is about 10x faster than the std Dict) its main concern is with reducing memory footprint.

We introduce two self sufficient modules:

• string_dict where the key type of the dictionary is a String
• generic_dict which allows keys to be of any type conforming with Keyable trait

Both modules expose a Dict struct which has the following compile time parametrization options:

• Value type can be any type conforming with CollectionElement trait
• We use a fast hash function as default, but you can provide your own hash function
• By setting the KeyCountType to a lower unsigned DType e.g. (DType.uint8 or DType.uint16) we can reduce the memory footprint. The type needs to be able to represent number of keys
• By setting the KeyOffsetType to a lower unsigned DType we can reduce the memory footprint even further. The type needs to be able to represent the sum of all key bytes
• Set destructive to False if you don't intend to delete keys from the dict. This way we do not waste space for deleted flags
• Set caching_hashes to False in order to reduce memory footprint by not caching the hash values. Keep in mind that this change slows down the rehashing process

The Dict can be instantiated with a capacity value. Default is set to 16, min capacity is 8. If you know the number of elements ahead of time set it, this will avoid rehashing and might improve memory footprint.

Sample code for generic dict:

from generic_dict import Dict, Keyable, KeysBuilder
from testing import assert_equal

@value
struct Person(Keyable):
    var name: String
    var age: Int

    fn accept[T: KeysBuilder](self, inout keys_builder: T):
        keys_builder.add_buffer[DType.int8](self.name._as_ptr(), len(self.name))
        keys_builder.add(Int64(self.age))

fn test_person_dict() raises:
    let p1 = Person("Maxim", 42)
    let p2 = Person("Maximilian", 62)
    let p3 = Person("Alex", 25)
    let p4 = Person("Maria", 28)
    let p5 = Person("Daria", 13)
    let p6 = Person("Max", 31)

    var d = Dict[Int]()
    d.put(p1, 1)
    d.put(p2, 11)
    d.put(p3, 111)
    d.put(p4, 1111)
    d.put(p5, 11111)
    d.put(p6, 111111)

    assert_equal(d.get(p1, 0), 1)
    assert_equal(d.get(p2, 0), 11)
    assert_equal(d.get(p3, 0), 111)
    assert_equal(d.get(p4, 0), 1111)
    assert_equal(d.get(p5, 0), 11111)
    assert_equal(d.get(p6, 0), 111111)

Note:

Due to a bug in Mojo 24.1 generic_dict module does not compile Bug report modularml/mojo#1858