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// Port of box3d-cpp-reference/src/id_pool.h and src/id_pool.c
//
// An index allocator with a free list: ids are dense while allocated and
// recycled LIFO. The C b3Array(int) free array maps to Vec<i32>.
//
// SPDX-FileCopyrightText: 2025 Erin Catto
// SPDX-License-Identifier: MIT
/// (b3IdPool)
#[derive(Debug, Clone, Default)]
pub struct IdPool {
/// Free-list of recycled ids. C: `b3Array(int) freeArray` → `Vec<i32>`.
pub(crate) free_array: Vec<i32>,
pub(crate) next_index: i32,
}
impl IdPool {
/// (b3CreateIdPool)
pub fn new() -> IdPool {
IdPool {
// C: b3Array_Reserve(pool.freeArray, 32) — capacity 32, count 0.
free_array: Vec::with_capacity(32),
next_index: 0,
}
}
/// (b3DestroyIdPool)
pub fn destroy(&mut self) {
*self = IdPool {
free_array: Vec::new(),
next_index: 0,
};
}
/// (b3AllocId)
pub fn alloc_id(&mut self) -> i32 {
if let Some(id) = self.free_array.pop() {
return id;
}
let id = self.next_index;
self.next_index += 1;
id
}
/// (b3FreeId)
///
/// C has a dead shrink branch (`if id == nextIndex`) that is unreachable
/// under the asserts above it (`id < nextIndex`). Always push, matching
/// the reachable C path and box2d-rust.
pub fn free_id(&mut self, id: i32) {
debug_assert!(self.next_index > 0);
debug_assert!(0 <= id && id < self.next_index);
self.free_array.push(id);
}
/// (b3GetIdCount)
pub fn id_count(&self) -> i32 {
self.next_index - self.free_array.len() as i32
}
/// (b3GetIdCapacity)
pub fn id_capacity(&self) -> i32 {
self.next_index
}
/// (b3GetIdBytes)
pub fn id_bytes(&self) -> i32 {
// C: b3Array_ByteCount(a) = capacity * sizeof(*data)
(self.free_array.capacity() * core::mem::size_of::<i32>()) as i32
}
/// Debug check that `id` is currently free. (b3ValidateFreeId)
pub fn validate_free_id(&self, id: i32) {
if self.free_array.contains(&id) {
return;
}
debug_assert!(false, "id {id} is not free");
}
}
#[cfg(test)]
mod tests {
use super::*;
// id_pool has no dedicated C unit test; this locks in the allocator behavior.
#[test]
fn alloc_free_recycle() {
let mut pool = IdPool::new();
assert_eq!(pool.alloc_id(), 0);
assert_eq!(pool.alloc_id(), 1);
assert_eq!(pool.alloc_id(), 2);
assert_eq!(pool.id_count(), 3);
assert_eq!(pool.id_capacity(), 3);
// Free list is LIFO: last freed id is reused first.
pool.free_id(1);
pool.validate_free_id(1);
assert_eq!(pool.id_count(), 2);
assert_eq!(pool.alloc_id(), 1);
assert_eq!(pool.id_count(), 3);
// Exhausted free list resumes bump allocation.
assert_eq!(pool.alloc_id(), 3);
assert_eq!(pool.id_capacity(), 4);
pool.destroy();
assert_eq!(pool.id_count(), 0);
}
}