Permutations: Difference between revisions

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@@ Line 906: / Line 906: @@
 permute([]) -> [[]];
-permute(L) -> permute(L, []).
+permute(L) -> zipper(L, []).
 % Use zipper to pick up first element of permutation
-permute([], _) -> [];
+zipper([], _) -> [];
-permute([H|T], R) ->
+zipper([H|T], R) ->
   % place current member in front of all permutations
   % of rest of set - both sides of zipper
-  prepend(H, permute(lists:reverse(R, T)))
+  prepend(H, permute(lists:reverse(R, T)),
-  % go further in zipper
+    % pass zipper state for continuation
-  ++ permute(T, [H|R]).
+    T, [H|R]).
-prepend(_, []) -> [];
+prepend(_, [], T, R) -> zipper(T, R); % continue in zipper
-prepend(X, [H|T]) -> [[X|H] | prepend(X, T)].
+prepend(X, [H|T], ZT, ZR) -> [[X|H] | prepend(X, T, ZT, ZR)].
 </lang>