race.erl

#!/usr/bin/env escript
%% -*- erlang -*-
%%! +P 100000000
-module(race).
-mode(compile).
-export([main/1]).

main(Args) ->
    [NumThreads, NumRunners] = [list_to_integer(Arg) || Arg <- Args],
    Parent = self(),
    RacerA = spawn_link(fun() -> race(Parent, NumRunners, NumThreads) end),
    RacerB = spawn_link(fun() -> race(Parent, NumRunners, NumThreads) end),
    receive
        {RacerA, Res} ->
            io:format("A won: ~p~n", [Res]);
        {RacerB, Res} ->
            io:format("B won: ~p~n", [Res])
    end,
    halt(0).


race(Parent, NumRunners, NumThreads) ->
    io:format("making tracks~n"),
    First = make_tracks(self(), 0, NumThreads),
    io:format("sending runners~n"),
    send_runners(First, NumRunners),
    io:format("waiting for finishers~n"),
    Parent ! {self(), recv_runners(NumRunners)}.

make_tracks(Parent, 0, N) ->
    Pid = spawn_link(fun() -> last_track(Parent) end),
    make_tracks(Pid, 1, N);
make_tracks(Parent, N, N) ->
    spawn_link(fun() -> track(Parent) end);
make_tracks(Parent, N, Max) ->
    Pid = spawn_link(fun() -> track(Parent) end),
    make_tracks(Pid, N+1, Max).


send_runners(_, 0) -> ok;
send_runners(Pid, N) ->
    Pid ! {from, self(), N},
    send_runners(Pid, N-1).

track(To) ->
    receive
        {from, Parent, N} ->
            To ! {from, self(), N},
            track(To, Parent)
    end.

track(To, Parent) ->
    receive
        {from, Parent, N} -> To ! {from, self(), N};
        {back, N} -> Parent ! {back, N}
    end,
    track(To, Parent).

last_track(To) ->
    receive
        {from, Parent, N} -> Parent ! {back, N}
    end,
    last_track(To).

recv_runners(N) -> recv_runners(N,[]).

recv_runners(0, Acc) -> lists:reverse(Acc);
recv_runners(N, Acc) ->
    receive
        {back, Runner} -> recv_runners(N-1, [Runner|Acc])
    end.