Later PDXErlang intersection implementation w/ Tim. Much more performant.

gistfile1.erl

-module(larray).
-include_lib("eunit/include/eunit.hrl").
-export([intersection/2]).

intersection(ListA, ListB) ->
  Numbers = lists:foldl( fun(N, Ranking) -> dict:update_counter(N, 1, Ranking) end , dict:new(), ListA ++ ListB),
  lists:usort([Unique || Unique <- dict:fetch_keys(Numbers), dict:fetch(Unique, Numbers) > 1]).

%% helper functions to create big lists of random integers
large_random_array(N, MaxValue) ->
  large_random_array(N, MaxValue, []).
large_random_array(0, _, L) ->
  L;
large_random_array(N, MaxValue, L) -> 
  large_random_array(N - 1, MaxValue, [random:uniform(MaxValue) | L]).

intersection_test_() ->
  ListA = lists:usort(large_random_array(100000, 1000000)),
  ListB = lists:usort(large_random_array(100000, 1000000)),
  Result = lists:usort(sets:to_list(sets:intersection(sets:from_list(ListA), sets:from_list(ListB)))),
  ?_assertEqual(Result, intersection(ListA,ListB)).

More performant but much less Erlangy. Needs some pattern matching.