Week 1 - Process and Messages (futurelearn.com)

palin.erl

-module(palin).
-compile(export_all).

% Starting a single server %
% erl> Pid = palin:start().
% erl> palin:check_palindrome(Pid, "bob").
% true
%
% Start multi server %
% erl> Pid = palin:start(5).
% erl> palin:check_palindrome(Pid, "bob").
% true

% == Client ==
start() -> spawn(?MODULE, server, []).
start(N) -> lb_start(N).

check_palindrome(Pid, String) ->
  Pid ! {self(), {check, String}},
  receive
    true ->
      io:format("~p is a palindrome~n", [String]);
    false ->
      io:format("~p is not a palindrome~n", [String])
  end.

% == Server ==
server() ->
  receive
    {From, {check, Msg}} ->
      From ! palindrome(Msg),
      server();
    _ ->
      io:format("Stopping~n")
  end.

server(0) ->
  io:format("Must pass number of servers to start");
server(N) ->
  lists:map(fun (_) -> start() end, lists:seq(1, N)).

% Load Balancer (Not really but just using many workers and sends work them in order)
% Would normally move to its own module
lb_start(N) ->
  Workers = server(N),
  spawn(?MODULE, lb, [Workers]).

lb([WorkerPid | Workers]) ->
  receive
    stop ->
      stop;
    Msg ->
      WorkerPid ! Msg,
      lb(Workers ++ [WorkerPid])
  end.

% palindrome problem
%
% palindrome("Madam I\'m Adam.") = true
% Code below is provide via the https://www.futurelearn.com/courses/concurrent-programming-erlang course
% to get students some base code to wrap in processes.

palindrome(Xs) ->
    palin(nocaps(nopunct(Xs))).

nopunct([]) ->
    [];
nopunct([X|Xs]) ->
    case lists:member(X,".,\ ;:\t\n\'\"") of
	true ->
	    nopunct(Xs);
	false ->
	    [ X | nopunct(Xs) ]
    end.

nocaps([]) ->
    [];
nocaps([X|Xs]) ->
    [ nocap(X) | nocaps(Xs) ].

nocap(X) ->
    case $A =< X andalso X =< $Z of
	true ->
	    X+32;
	false ->
	    X
    end.

% literal palindrome

palin(Xs) ->
    Xs == reverse(Xs).

reverse(Xs) ->
    shunt(Xs,[]).

shunt([],Ys) ->
    Ys;
shunt([X|Xs],Ys) ->
    shunt(Xs,[X|Ys]).