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use std::collections::BTreeSet;
use std::error;
use std::fmt;

// 4 billion entities ought to be enough for anybody.
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub struct Entity(pub u32);

impl Entity {
    #[inline]
    pub fn index(&self) -> usize {
        self.0 as usize
    }
}

impl From<u32> for Entity {
    #[inline]
    fn from(value: u32) -> Self {
        Entity(value)
    }
}

pub struct Allocator {
    count: u32,
    freed: BTreeSet<Entity>,
}

impl Allocator {
    #[inline]
    pub fn new() -> Self {
        Self {
            count: 0,
            freed: BTreeSet::new(),
        }
    }

    #[inline]
    pub fn alloc(&mut self) -> Entity {
        if let Some(entity) = self.freed.pop_first() {
            entity
        } else {
            assert!(self.count < u32::MAX, "out of entities");

            let entity = self.count;
            self.count += 1;
            Entity(entity)
        }
    }

    #[inline]
    pub fn free(&mut self, entity: Entity) -> Result<(), Error> {
        if entity.0 >= self.count {
            Err(Error::Unallocated)
        } else if self.freed.insert(entity) {
            Ok(())
        } else {
            Err(Error::DoubleFree)
        }
    }
}

#[derive(Debug, PartialEq)]
pub enum Error {
    Unallocated,
    DoubleFree,
}

impl fmt::Display for Error {
    #[inline]
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            Error::Unallocated => f.write_str("an unallocated entity cannot be freed"),
            Error::DoubleFree => f.write_str("an entity cannot be freed twice"),
        }
    }
}

impl error::Error for Error {}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn alloc_free_and_reuse() {
        let mut entity_allocator = Allocator::new();

        let e0 = entity_allocator.alloc();
        let e1 = entity_allocator.alloc();

        assert_eq!(e0, Entity(0));
        assert_eq!(e1, Entity(1));

        entity_allocator.free(e0).unwrap();

        let e2 = entity_allocator.alloc();
        let e3 = entity_allocator.alloc();

        assert_eq!(e2, Entity(0));
        assert_eq!(e3, Entity(2));

        // Double free an entity.
        entity_allocator.free(e3).unwrap();
        let err = entity_allocator.free(e3).unwrap_err();

        assert_eq!(err, Error::DoubleFree);

        // Free an unallocated entity.
        let err = entity_allocator.free(Entity(99)).unwrap_err();

        assert_eq!(err, Error::Unallocated);
    }
}