typedef struct SpaceEntryHandle SpaceEntryHandle;
struct SpaceEntryHandle {
    u64 idx;
    u64 gen;
};

typedef struct SpaceCellNode SpaceCellNode;
typedef struct SpaceEntry SpaceEntry;
struct SpaceEntry {
    b32 valid;
    SpaceEntryHandle handle;

    SpaceCellNode *first_node;
    SpaceCellNode *last_node;

    Aabb aabb;
    EntId ent;

    SpaceEntry *next_free;
};

/* Links a cell to a entry.
 * Acts as both a node in the list of entries contained by the cell, and a node in the list of cells containing the entry. */
typedef struct SpaceCell SpaceCell;
typedef struct SpaceCellNode SpaceCellNode;
struct SpaceCellNode {
    SpaceEntry *entry;
    SpaceCell *cell;

    /* For list of all entries contained by cell */
    SpaceCellNode *prev_in_cell;
    SpaceCellNode *next_in_cell;

    /* For list of all cells containing entry */
    SpaceCellNode *prev_in_entry;
    SpaceCellNode *next_in_entry;

    SpaceCellNode *next_free;
};

typedef struct SpaceCellBin SpaceCellBin;
typedef struct SpaceCell SpaceCell;
struct SpaceCell {
    b32 valid;
    V2i32 pos;

    SpaceCellNode *first_node;
    SpaceCellNode *last_node;

    SpaceCellBin *bin;
    SpaceCell *prev_in_bin;
    SpaceCell *next_in_bin;

    SpaceCell *next_free;
};

typedef struct SpaceCellBin SpaceCellBin;
struct SpaceCellBin {
    SpaceCell *first_cell;
    SpaceCell *last_cell;
};

typedef struct Space Space;
struct Space {
    b32 valid;
    f32 cell_size;

    Arena *cell_arena;
    SpaceCellBin *bins;
    i32 num_bins;
    i32 num_bins_sqrt;
    SpaceCell *first_free_cell;
    SpaceCellNode *first_free_cell_node;

    Arena *entry_arena;
    u64 num_entries_reserved;
    SpaceEntry *entries;
    SpaceEntry *first_free_entry;
};

typedef struct SpaceIter SpaceIter;
struct SpaceIter {
    Aabb aabb;
    Space *space;
    V2i32 cell_start;
    V2i32 cell_end;
    V2i32 cell_cur;
    SpaceCellNode *prev;
};

/* ========================== *
 * Nil
 * ========================== */

INLINE SpaceEntry *space_entry_nil(void)
{
    extern READONLY SpaceEntry _g_space_entry_nil;
    return &_g_space_entry_nil;
}

INLINE SpaceCell *space_cell_nil(void)
{
    extern READONLY SpaceCell _g_space_cell_nil;
    return &_g_space_cell_nil;
}

INLINE Space *space_nil(void)
{
    extern READONLY Space _g_space_nil;
    return &_g_space_nil;
}

/* ========================== *
 * Space
 * ========================== */

Space *space_alloc(f32 cell_size, u32 num_bins_sqrt);
void space_release(Space *space);
void space_reset(Space *space);

Space *space_from_entry(SpaceEntry *entry);

/* ========================== *
 * Cell
 * ========================== */

SpaceCell *space_get_cell(Space *space, V2i32 cell_pos);

/* ========================== *
 * Entry
 * ========================== */

SpaceEntry *space_entry_from_handle(Space *space, SpaceEntryHandle handle);
SpaceEntry *space_entry_alloc(Space *space, EntId entity);
void space_entry_release(SpaceEntry *entry);
void space_entry_update_aabb(SpaceEntry *entry, Aabb new_aabb);

/* ========================== *
 * Iter
 * ========================== */

SpaceIter space_iter_begin_aabb(Space *space, Aabb aabb);
SpaceEntry *space_iter_next(SpaceIter *iter);
#define space_iter_end(i)