#[repr(C)]
#[derive(Eq, Hash, PartialEq, Debug, Clone, Copy, bytemuck::Zeroable)]
pub struct Id<I = u64>(I);

unsafe impl<I: Copy + bytemuck::Zeroable + 'static> bytemuck::Pod for Id<I> {}

pub struct IdTracker<I = u64> {
    free: Vec<Id<I>>,
    cur: Id<I>,
}

impl<I: IdNum> IdTracker<I> {
    #[allow(clippy::should_implement_trait)]
    pub fn next(&mut self) -> Id<I> {
        if let Some(id) = self.free.pop() {
            return id;
        }
        let next = self.cur.next();
        std::mem::replace(&mut self.cur, next)
    }

    #[allow(dead_code)]
    pub fn free(&mut self, id: Id<I>) {
        self.free.push(id);
    }
}

impl<I: IdNum> Id<I> {
    #[allow(dead_code)]
    /// for debug purposes; should this be exposed?
    /// generally you want to use labels with widgets
    pub(crate) fn raw(id: I) -> Self {
        Self(id)
    }
    pub fn idx(&self) -> usize {
        self.0.idx()
    }
    pub fn next(&self) -> Id<I> {
        Self(self.0.next())
    }
    pub const fn preset(value: I) -> Self {
        Self(value)
    }
}

impl<I: IdNum> Default for IdTracker<I> {
    fn default() -> Self {
        Self {
            free: Vec::new(),
            cur: Id(I::first()),
        }
    }
}

pub trait IdNum {
    fn first() -> Self;
    fn next(&self) -> Self;
    fn idx(&self) -> usize;
}

impl IdNum for u64 {
    fn first() -> Self {
        0
    }

    fn next(&self) -> Self {
        self + 1
    }

    fn idx(&self) -> usize {
        *self as usize
    }
}

impl IdNum for u32 {
    fn first() -> Self {
        0
    }

    fn next(&self) -> Self {
        self + 1
    }

    fn idx(&self) -> usize {
        *self as usize
    }
}