--- a/rps.erl	Wed Feb 26 02:28:07 2014 +0900
+++ b/rps.erl	Wed Feb 26 15:06:12 2014 +0900
@@ -1,5 +1,5 @@
 -module(rps).
--import(rps3, [get_rules/0, beats/1]).
+-import(rps3, [get_rules/0, winning_hand/1]).
 -export([
     go/0,
     winner/1,
@@ -15,18 +15,17 @@
 % rock, rock, paper     -> {paper, Who}
 
 winner(Choices) when length(Choices) > 1 ->
-    HSet = sets:from_list([Hand || {Hand, _} <- Choices]),
-    RSet = sets:from_list(get_rules()),
-    Absents = sets:subtract(RSet, HSet),
-    case sets:size(Absents) of
-        1 ->
-            Absent = hd(sets:to_list(Absents)),
-            Unbeaten = beats(Absent),
-            Winners = [{Hand, Who} || {Hand, Who} <- Choices, Hand =:= Unbeaten],
-            case length(Winners) of
-                1 -> hd(Winners);
-                _ -> undefined
-            end;
+    PlayersByHand = lists:foldl(
+        fun({Hand, Who}, Acc) ->
+            dict:append(Hand, Who, Acc)
+        end, dict:new(), Choices),
+    WHand = winning_hand([Hand || {Hand, _} <- Choices]),
+    Players = if
+        WHand =:= undefined -> [];
+        WHand =/= undefined -> dict:fetch(WHand, PlayersByHand)
+    end,
+    case length(Players) of
+        1 -> {WHand, hd(Players)};
         _ -> undefined
     end.
 

--- a/rps3.erl	Wed Feb 26 02:28:07 2014 +0900
+++ b/rps3.erl	Wed Feb 26 15:06:12 2014 +0900
@@ -1,5 +1,5 @@
 -module(rps3).
--export([get_rules/0, beats/1, test_beats/0]).
+-export([get_rules/0, beats/1, test_beats/0, test_loses/0, winning_hand/1, test_winning_hand/0]).
 
 get_rules() -> [rock, paper, scissors].
 
@@ -15,9 +15,52 @@
 beats(H, [_|T]) ->
     beats(H, T).
 
+loses(H) ->
+    beats(H, lists:reverse(get_rules())).
+
 test_beats() ->
     lists:foreach(
-        fun(C) ->
-            io:format("~w beats ~w.~n", [C, beats(C)])
+        fun(H) ->
+            io:format("~w beats ~w.~n", [H, beats(H)])
+        end,
+        get_rules()).
+
+test_loses() ->
+    lists:foreach(
+        fun(H) ->
+            io:format("~w loses to ~w.~n", [H, loses(H)])
         end,
         get_rules()).
+
+% winner for 3 hands is:
+% rock, rock, rock      -> rock
+% rock, paper, scissors -> undefined
+% rock, paper, paper    -> paper
+% rock, rock, paper     -> paper
+
+winning_hand(Hands) ->
+    HSet = sets:from_list(Hands),
+    sets:fold(
+        fun(Hand, Acc) ->
+            case sets:is_element(loses(Hand), HSet) of
+                true -> Acc;
+                false -> Hand
+            end
+        end, undefined, HSet).
+
+test_winning_hand() ->
+    Cases = [
+             [rock, rock, rock],
+             [rock, paper, scissors],
+             [rock, paper, paper],
+             [rock, rock, paper],
+             [rock, paper],
+             [paper, scissors],
+             [scissors, rock]
+            ],
+    lists:foreach(
+        fun(Hands) ->
+            Winner = winning_hand(Hands),
+            io:format("~w wins in ~w.~n", [Winner, Hands])
+        end,
+        Cases).