use std::mem::transmute;
use std::sync::atomic::{AtomicU32, Ordering};

use crate::util::{gen_mask_64, BitMask, NumberExt};

static NEXT: AtomicU32 = AtomicU32::new(1);

const ID_BITS: BitMask<u64> = BitMask::new(gen_mask_64!(32..=63));
const KIND_BITS: BitMask<u64> = BitMask::new(gen_mask_64!(0..=1));

#[derive(Debug, Clone, Copy, Hash, PartialEq, Eq, PartialOrd, Ord)]
#[repr(u8)]
pub enum Kind
{
    Entity = 2,
    Component = 1,
}

/// Unique entity/component ID.
#[derive(Debug, Clone, Copy, Hash, PartialEq, Eq, PartialOrd, Ord)]
pub struct Uid
{
    inner: u64,
}

impl Uid
{
    /// Returns a new unique entity/component ID.
    pub fn new_unique(kind: Kind) -> Self
    {
        let id = NEXT.fetch_add(1, Ordering::Relaxed);

        Self {
            inner: ID_BITS.field_prep(id as u64) | KIND_BITS.field_prep(kind as u64),
        }
    }

    #[must_use]
    pub fn kind(&self) -> Kind
    {
        // SAFETY: The kind bits cannot be invalid since they are set using the Kind enum
        // in the new_unique function
        unsafe { transmute(self.inner.field_get(KIND_BITS) as u8) }
    }
}