hashmap and mutable values and efficiency

[jesse99]

As far as I can tell there is no way to efficiently use a hashmap with a mutable value. Consider a type like

type pets = hashmap<str, dvec<str>>;

How can new elements be efficiently added to the dvec?

[jesse99]

Here is a test case for the above

// rustc --test test.rs && ./test
use std;
import std::map::hashmap;
import core::dvec::*;

type pets = hashmap<str, dvec<str>>;

fn broken_add(pets: pets, owner: str, name: str)
{
    alt pets.find(owner)
    {
        option::some(owns)
        {
            // owns is a copy so this is not very useful
            owns.push(name);
        }
        option::none
        {
            pets.insert(owner, dvec::from_vec([mut name]));
        }
    }
}

fn inefficient_add(pets: pets, owner: str, name: str)
{
    alt pets.find(owner)
    {
        option::some(owns)
        {
            // this does work, but there are at least two copies of a
            // potentially very large vector
            owns.push(name);
            pets.insert(owner, copy(owns));
        }
        option::none
        {
            pets.insert(owner, dvec::from_vec([mut name]));
        }
    }
}

#[test]
fn broken()
{
    let pets = std::map::str_hash();
    broken_add(pets, "Dr Evil", "Mr. Bigglesworth");
    broken_add(pets, "Dr Evil", "Sharks with frickin' laser beams");
    broken_add(pets, "Pinocchio", "Figaro");

    assert pets.size() == 2u;
    assert pets.get("Dr Evil").len() == 2u;        // this fails
    assert pets.get("Pinocchio").len() == 1u;
}

#[test]
fn inefficient()
{
    let pets = std::map::str_hash();
    inefficient_add(pets, "Dr Evil", "Mr. Bigglesworth");
    inefficient_add(pets, "Dr Evil", "Sharks with frickin' laser beams");
    inefficient_add(pets, "Pinocchio", "Figaro");

    assert pets.size() == 2u;
    assert pets.get("Dr Evil").len() == 2u;
    assert pets.get("Pinocchio").len() == 1u;
}

[nikomatsakis]

Yes I want to refactor the API so that it is based on a with() primitive that gives you in-place access to the value rather than get() and find(). For now, I would suggest using @dvec as the value type.

[jesse99]

Ya, that works.

[catamorphism]

Seems like this can be closed? @nikomatsakis , if you want to open an issue for your refactoring, please do :-)