Game of Life

src/life.erl

@@ -0,0 +1,46 @@
+-module(life).
+-import(lists, [flatmap/2]).
+-import(sets, [from_list/1, to_list/1, is_element/2]).
+-export([neighbours/1, next_step/1, frequencies/1]).
+
+neighbours({X, Y}) ->
+    [{X + DX, Y + DY} || DX <- [-1, 0, 1], DY <- [-1, 0, 1], {DX, DY} =/= {0, 0}].
+
+next_step(Cells) ->
+    Nbs = flatmap(fun neighbours/1, to_list(Cells)),
+    NewCells = [C || {C, N} <- frequencies(Nbs),
+                     (N == 3) or ((N == 2) and is_element(C, Cells))],
+    from_list(NewCells).
+
+frequencies(List) -> frequencies(List, []).
+frequencies([], Acc) -> Acc;
+frequencies([X|Xs], Acc) ->
+    case lists:keyfind(X, 1, Acc) of
+        {X, F} -> frequencies(Xs, lists:keyreplace(X, 1, Acc, {X, F+1}));
+        false  -> frequencies(Xs, lists:keystore(X, 1, Acc, {X, 1}))
+    end.
+
+%
+% Unit tests
+%
+
+-include_lib("eunit/include/eunit.hrl").
+
+frequencies_test() ->
+    ?assertEqual([{1,2}, {2,2}, {3,3}, {4,1}],
+        frequencies([1, 2, 3, 2, 3, 4, 1, 3])).
+
+neighbours_test() ->
+    ?assertEqual([{0,1}, {0,2}, {0,3}, {1,1}, {1,3}, {2,1}, {2,2}, {2,3}],
+        neighbours({1, 2})).
+
+blinker_test() ->
+    assert_next_step([{2,3}, {3,3}, {4,3}], [{3,2}, {3,3}, {3,4}]),
+    assert_next_step([{3,2}, {3,3}, {3,4}], [{2,3}, {3,3}, {4,3}]).
+
+beehive_test() ->
+    assert_next_step([{3,2}, {2,3}, {2,4}, {3,5}, {4,4}, {4,3}],
+        [{3,2}, {2,3}, {2,4}, {3,5}, {4,4}, {4,3}]).
+
+assert_next_step(ListAfter, ListBefore) ->
+    ?assertEqual(from_list(ListAfter), next_step(from_list(ListBefore))).