How do I efficiently build a vector and an index of that vector while processing a data stream?

Question

I have a struct Foo:

struct Foo {
    v: String,
    // Other data not important for the question
}

I want to handle a data st

Answer 1

not increase the run time by using Rc or RefCell.

@Shepmaster already demonstrated accomplishing this using unsafe, once you have I would encourage you to check how much Rc actually would cost you. Here is a full version with Rc:

use std::{
    collections::{hash_map::Entry, HashMap},
    rc::Rc,
};

#[derive(Debug)]
struct Foo {
    v: Rc,
}

#[derive(Debug)]
struct Collection {
    vec: Vec,
    index: HashMap, usize>,
}

impl Foo {
    fn new(s: &str) -> Foo {
        Foo {
            v: s.into(),
        }
    }
}

impl Collection {
    fn new() -> Collection {
        Collection {
            vec: Vec::new(),
            index: HashMap::new(),
        }
    }

    fn insert(&mut self, foo: Foo) {
        match self.index.entry(foo.v.clone()) {
            Entry::Occupied(o) => panic!(
                "Duplicate entry for: {}, {:?} inserted before {:?}",
                foo.v,
                o.get(),
                foo
            ),
            Entry::Vacant(v) => v.insert(self.vec.len()),
        };
        self.vec.push(foo)
    }
}

fn main() {
    let mut collection = Collection::new();

    for foo in vec![Foo::new("Hello"), Foo::new("World"), Foo::new("Go!")] {
        collection.insert(foo)
    }

    println!("{:?}", collection);
}