Best data structure for an immutable persistent 3D grid

Question

I\'m experimenting with writing a game in a functional programming style, which implies representing the game state with a purely functional, immutable data structures.

Answer 1

I'd try an octtree. The boundary coordinates of each node are implicit in structure placement, and each nonterminal node keep 8 subtree but no data. You can thus 'unioning' to gain space.

I think that Immutable and Unbounded are (generally) conflicting requirements.
Anyway... to grow a octtree you must must replace the root.

Other requirement you pose should be met.