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pub struct Arena {
    container: Vec<u8>,
    capacity: usize,
    offset: usize,
}

impl<'a> Arena {
    pub fn new(size: usize) -> Self {
        Arena {
            container: Vec::with_capacity(size),
            capacity: size,
            offset: 0,
        }
    }
    pub fn alloc<T>(&mut self, value: T) -> Option<&'a mut T> {
        let size = std::mem::size_of::<T>();
        if self.offset + size > self.capacity {
            return None;
        }
        for _ in 0..size {
            self.container.push(0u8);
        }
        let ptr: *mut u8 = &mut self.container[self.offset];
        let ptr = ptr.cast::<T>();
        self.offset += size;
        unsafe {
            *ptr = value;
            Some(&mut *ptr)
        }
    }
}

#[test]
fn single_allocation() {
    let mut arena = Arena::new(1024);
    let value = arena.alloc(7).unwrap();
    assert_eq!(*value, 7);
}

#[test]
fn out_of_space() {
    let mut arena = Arena::new(4);
    arena.alloc(7).unwrap();
    let value = arena.alloc(7);
    assert_eq!(value, None);
}

#[test]
fn multiple_allocations() {
    let mut arena = Arena::new(1024);
    let value_78 = arena.alloc(78).unwrap();
    let value_42 = arena.alloc(42).unwrap();

    {
        let value_17  = arena.alloc(17).unwrap();
        assert_eq!(*value_17, 17);
    }
    // value_17 is out of scope, and so cannot be used
    // but it hasn't been freed

    assert_eq!(*value_78, 78);
    assert_eq!(*value_42, 42);
    assert_eq!(*value_78 + *value_42, 78 + 42);

    // All values are now freed in one giant batch
}