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