comb.erl

-module(comb).
-export([permutations/1]).

%%
%% permutations(L): generates a list with all the permutations of the elements of the list.
%%
%% For each element in the list:
%%  a) combine this element with the permutations of the rest of the list.
%%  b) store the results
%%
-spec permutations([T]) -> [[T]].

permutations([]) -> [[]];
permutations(List) -> permutations(List, List, []).

permutations([], _List, Store) -> Store;
permutations([Head|Tail], List, Store) ->
    HeadPermutations = combine(Head, permutations(lists:delete(Head, List))),
    permutations(Tail, List, Store ++ HeadPermutations).

%%
%% combine(X, [L]): inserts element X as the head of each list 
%%
-spec combine(T, [[T]]) -> [[T]].
combine(Item, List) -> combine(Item, List, []).

combine(_Item, [], Store) -> lists:reverse(Store);
combine(Item, [Head|Tail], Store) ->
    combine(Item, Tail, [[Item|Head] | Store]).