This is an efficient data structure that can be used to manage index slots for bindless GPU resources. It can also be used for any other purpose which requires tracking resources.
It works similarily to an ordinary bitmap with the difference that finding the first available slot is very fast. The tree can be sparse so very big indices are possible (if indices need to be assigned manually).
Internally it's implemented as an 32/64-ary tree of 32/64 bit integers storing the bits. Leaf nodes are flattened to contain 4096 available bit positions.
For managing 16 777 216 resources the tree has only 3 levels and requires calculating 3 ctz/BitScanForward intrinsics.
Simply include bitmap_tree.hpp and that's it!
// Create new tree. It can dynamically expand as necessary
bmt::tree_t<uint64_t> tree;
// This will reserve the first available index
auto idx = tree.allocate();
// We can test if the index is reserved
assert(tree.is_allocated(idx));
// We can deallocate
tree.deallocate(idx);
// We can allocate a specific index. Since the tree is sparse
// this won't be too problematic memory wise like normal bitmap
// even if the index is very big. However the level of the tree
// might increase.
tree.allocate_at(10000000000ull);
// We can get the total slots allocated
assert(tree.allocated_slots() == 1);
// The current maximum capacity can be aquired with this.
// The tree will automatically expand when the total
// allocated slots reach the capacity
assert(tree.current_capacity() == 4096);