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Learn You Some Erlang / musicians.erl

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-module(musicians).
-behaviour(gen_server).

-export([start_link/2, stop/1]).
-export([init/1, handle_call/3, handle_cast/2,
         handle_info/2, code_change/3, terminate/2]).

-record(state, {name="", role, skill=good}).
-define(DELAY, 750).

start_link(Role, Skill) ->
    gen_server:start_link({local, Role}, ?MODULE, [Role, Skill], []).

stop(Role) -> gen_server:call(Role, stop).

init([Role, Skill]) ->
    %% To know when the parent shuts down
    process_flag(trap_exit, true),
    %% sets a seed for random number generation for the life of the process
    %% uses the current time to do it. Unique value guaranteed by now()
    random:seed(now()),
    TimeToPlay = random:uniform(3000),
    Name = pick_name(),
    StrRole = atom_to_list(Role),
    io:format("Musician ~s, playing the ~s entered the room~n",
              [Name, StrRole]),
    {ok, #state{name=Name, role=StrRole, skill=Skill}, TimeToPlay}.

handle_call(stop, _From, S=#state{}) ->
    {stop, normal, ok, S};
handle_call(_Message, _From, S) ->
    {noreply, S, ?DELAY}.

handle_cast(_Message, S) ->
    {noreply, S, ?DELAY}.

handle_info(timeout, S = #state{name=N, skill=good}) ->
    io:format("~s produced sound!~n",[N]),
    {noreply, S, ?DELAY};
handle_info(timeout, S = #state{name=N, skill=bad}) ->
    case random:uniform(5) of
        1 ->
            io:format("~s played a false note. Uh oh~n",[N]),
            {stop, bad_note, S};
        _ ->
            io:format("~s produced sound!~n",[N]),
            {noreply, S, ?DELAY}
    end;
handle_info(_Message, S) ->
    {noreply, S, ?DELAY}.

code_change(_OldVsn, State, _Extra) ->
    {ok, State}.

terminate(normal, S) ->
    io:format("~s left the room (~s)~n",[S#state.name, S#state.role]);
terminate(bad_note, S) ->
    io:format("~s sucks! kicked that member out of the band! (~s)~n",
              [S#state.name, S#state.role]);
terminate(shutdown, S) ->
    io:format("The manager is mad and fired the whole band! "
              "~s just got back to playing in the subway~n",
              [S#state.name]);
terminate(_Reason, S) ->
    io:format("~s has been kicked out (~s)~n", [S#state.name, S#state.role]).

%% Yes, the names are based off the magic school bus characters
%% 10 names!
pick_name() ->
    %% the seed must be set for the random functions. Use within the
    %% process that started with init/1
    lists:nth(random:uniform(10), firstnames())
    ++ " " ++
    lists:nth(random:uniform(10), lastnames()).

firstnames() ->
    ["Valerie", "Arnold", "Carlos", "Dorothy", "Keesha",
     "Phoebe", "Ralphie", "Tim", "Wanda", "Janet"].

lastnames() ->
    ["Frizzle", "Perlstein", "Ramon", "Ann", "Franklin",
     "Terese", "Tennelli", "Jamal", "Li", "Perlstein"].