////////////////////////////////////////////////////////////
//~ Space entry types

Struct(PP_SpaceEntryKey)
{
    u64 idx;
    u64 gen;
};

Struct(PP_SpaceEntry)
{
    b32 valid;
    PP_SpaceEntryKey key;

    struct PP_SpaceCellNode *first_node;
    struct PP_SpaceCellNode *last_node;

    Aabb aabb;
    PP_EntKey ent;

    PP_SpaceEntry *next_free;
};

////////////////////////////////////////////////////////////
//~ Space cell types

/* 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. */
Struct(PP_SpaceCellNode)
{
    PP_SpaceEntry *entry;
    struct PP_SpaceCell *cell;

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

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

    PP_SpaceCellNode *next_free;
};

Struct(PP_SpaceCell)
{
    b32 valid;
    Vec2I32 pos;

    PP_SpaceCellNode *first_node;
    PP_SpaceCellNode *last_node;

    struct PP_SpaceCellBin *bin;
    PP_SpaceCell *prev_in_bin;
    PP_SpaceCell *next_in_bin;

    PP_SpaceCell *next_free;
};

Struct(PP_SpaceCellBin)
{
    PP_SpaceCell *first_cell;
    PP_SpaceCell *last_cell;
};

////////////////////////////////////////////////////////////
//~ Space types

Struct(PP_Space)
{
    b32 valid;
    f32 cell_size;

    Arena *cell_arena;
    PP_SpaceCellBin *bins;
    i32 num_bins;
    i32 num_bins_sqrt;
    PP_SpaceCell *first_free_cell;
    PP_SpaceCellNode *first_free_cell_node;

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

Struct(PP_SpaceIter)
{
    Aabb aabb;
    PP_Space *space;
    Vec2I32 cell_start;
    Vec2I32 cell_end;
    Vec2I32 cell_cur;
    PP_SpaceCellNode *prev;
};

////////////////////////////////////////////////////////////
//~ Nil types

/* Offset in bytes from start of space struct to start of entry array (assume adjacently allocated) */
#define PP_SpaceEntriesOffset (sizeof(PP_Space) + (sizeof(PP_Space) % alignof(PP_SpaceEntry)))

/* Accessed via NilEnt() */
extern Readonly PP_SpaceEntry PP_nil_space_entry;
extern Readonly PP_SpaceCell PP_nil_space_cell;
extern Readonly PP_Space PP_nil_space;

////////////////////////////////////////////////////////////
//~ Nil helpers

Inline PP_SpaceEntry *PP_NilSpaceEntry(void)
{
    extern Readonly PP_SpaceEntry PP_nil_space_entry;
    return &PP_nil_space_entry;
}

Inline PP_SpaceCell *PP_NilSpaceCell(void)
{
    extern Readonly PP_SpaceCell PP_nil_space_cell;
    return &PP_nil_space_cell;
}

Inline PP_Space *PP_NilSpace(void)
{
    extern Readonly PP_Space PP_nil_space;
    return &PP_nil_space;
}

////////////////////////////////////////////////////////////
//~ Space

PP_Space *PP_AcquireSpace(f32 cell_size, u32 num_bins_sqrt);
void PP_ReleaseSpace(PP_Space *space);
void PP_ResetSpace(PP_Space *space);
PP_Space *PP_SpaceFromEntry(PP_SpaceEntry *entry);

////////////////////////////////////////////////////////////
//~ Cell

Vec2I32 PP_SpaceCellCoordsFromWorldCoords(f32 cell_size, Vec2 world_pos);
i32 PP_SpaceBinIndexFromCellCoords(PP_Space *space, Vec2I32 cell_pos);
PP_SpaceCell *PP_SpaceCellFromCellPos(PP_Space *space, Vec2I32 cell_pos);
void PP_AcquireSpaceCellNode(Vec2I32 cell_pos, PP_SpaceEntry *entry);
void PP_ReleaseSpaceCellNode(PP_SpaceCellNode *n);

////////////////////////////////////////////////////////////
//~ Entry

PP_SpaceEntry *PP_SpaceEntryFromKey(PP_Space *space, PP_SpaceEntryKey key);
PP_SpaceEntry *PP_AcquireSpaceEntry(PP_Space *space, PP_EntKey ent);
void PP_ReleaseSpaceEntry(PP_SpaceEntry *entry);
void PP_UpdateSpaceEntryAabb(PP_SpaceEntry *entry, Aabb new_aabb);

////////////////////////////////////////////////////////////
//~ Iter

PP_SpaceIter PP_BeginSpaceIterAabb(PP_Space *space, Aabb aabb);
PP_SpaceEntry *PP_NextSpaceIterAabb(PP_SpaceIter *iter);
#define PP_EndSpaceIter(i)