/// intentionally does not implement copy or clone
/// which should make it harder to misuse;
/// the idea is to generally try to guarantee all IDs
/// point to something valid, although duplicate
/// gets around this if needed
#[derive(Eq, Hash, PartialEq, Debug)]
pub struct ID(usize);

#[derive(Default)]
pub struct IDTracker {
    free: Vec<ID>,
    cur: usize,
}

impl IDTracker {
    pub fn new() -> Self {
        Self::default()
    }

    #[allow(clippy::should_implement_trait)]
    pub fn next(&mut self) -> ID {
        if let Some(id) = self.free.pop() {
            return id;
        }
        let id = ID(self.cur);
        self.cur += 1;
        id
    }

    pub fn free(&mut self, id: ID) {
        self.free.push(id);
    }
}

impl ID {
    /// this must be used carefully to make sure
    /// all IDs are still valid references;
    /// named weirdly to indicate this.
    /// generally should not be used in "user" code
    pub fn duplicate(&self) -> Self {
        Self(self.0)
    }
}