Prolog: searching for sublists of length>=3

Question

I'd like to produce a predicate which is able to check whether a list will divide into sublists each containing at least three atoms.

For example:

findgroups([1,2,3,4,5,6,7,8,9],L1).

Has solutions such as:

L1=[[1,2,3],[4,5,6],[7,8,9]]
L1=[[1,2,3,4],[5,6,7,8,9]]
L1=[[1,2,3,4,5],[6,7,8,9]]
L1=[[1,2,3,4,5,6],[7,8,9]]

However, the following will fail:

findgroups([1,2,3,4,5]).

I have written a few predicates based on splitting lists into two sub terms using append, but I'm confused about handling multiple sublists.

Thank you very much for your help

With Best wishes J

Answer 1

Maybe this very simple solution could do

findgroups([], []).
findgroups(L, [[A,B,C|D]|Gs]) :-
    append([A,B,C|D], R, L),
    findgroups(R, Gs).

The last one solution is not requested, but it's correct, since you don't rule out a group of only 1 element

?- findgroups([1,2,3,4,5,6,7,8,9], Gs).
Gs = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] ;
Gs = [[1, 2, 3], [4, 5, 6, 7, 8, 9]] ;
Gs = [[1, 2, 3, 4], [5, 6, 7, 8, 9]] ;
Gs = [[1, 2, 3, 4, 5], [6, 7, 8, 9]] ;
Gs = [[1, 2, 3, 4, 5, 6], [7, 8, 9]] ;
Gs = [[1, 2, 3, 4, 5, 6, 7, 8|...]] ;
false.

To match exactly the examples, I would write

findgroups(L, Gs) :- findgroups_(L, Gs), Gs = [_,_|_].
findgroups_([], []).
findgroups_(L, [[A,B,C|D]|Gs]) :-
    append([A,B,C|D], R, L),
    findgroups_(R, Gs).