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

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

// Bit 0 and 1 for the kind
const KIND_BITS: u64 = 0x03;

#[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_part = NEXT.fetch_add(1, Ordering::Relaxed);

        Self {
            inner: ((id_part as u64) << 32) | kind as u64,
        }
    }

    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 & KIND_BITS) as u8) }
    }
}