Rock Paper Scissors exercise from part 3 of https://www.futurelearn.com/courses/functional-programming-erlang

rockpaperscissors.erl

-module (rockpaperscissors).
-compile(export_all).

-include_lib("eunit/include/eunit.hrl").

beat(rock)     -> paper;
beat(paper)    -> scissors;
beat(scissors) -> rock;
beat(_)        -> error("expected rock, paper or scissors").

loose(rock)     -> paper;
loose(paper)    -> scissors;
loose(scissors) -> rock;
loose(_)        -> error("expected rock, paper or scissors").

% I thought we could return a 1 or 2
% for player 1 or 2 respectively, and a 0 for draw
% but I decided it would be better to return the
% winning atom, e.g. `rock` (or `draw`) and let the players
% workout for themselves the winner.
% Then when I realised I needed to return a 1 for player one
% winning, -1 for player 2 winning and 0 for a draw, for use
% with `tournament/2` it became easier to just return that number

result(P1,P1) ->
  0;
result(P1,P2) ->
  case beat(P2) =:= P1 of
    true  -> 1;
    false -> -1
  end.

tournament(P1s, P2s) ->
  Results = lists:zipwith(fun result/2, P1s, P2s),
  lists:sum(Results).

result_test_() ->
  [
    ?_assertEqual(1,  result(rock,scissors)),
    ?_assertEqual(0,  result(rock,rock)),
    ?_assertEqual(-1, result(rock,paper)),
    ?_assertEqual(-1, result(paper,scissors)),
    ?_assertEqual(1,  result(paper,rock)),
    ?_assertEqual(0,  result(paper,paper)),
    ?_assertEqual(-1, result(scissors,rock)),
    ?_assertEqual(1,  result(scissors,paper)),
    ?_assertEqual(0,  result(scissors,scissors))
  ].

tournament_test_() ->
  [
    ?_assertEqual(0, tournament([rock], [rock])),
    ?_assertEqual(1, tournament([rock], [scissors])),
    ?_assertEqual(-1, tournament([rock], [paper])),
    ?_assertEqual(-1, tournament([rock,rock,paper,paper],[rock,paper,scissors,rock]))
  ].