%%%
%%% lfm.erl
%%% Copyright (C) 2009 James Lee
%%% 
%%% This program is free software; you can redistribute it and/or
%%% modify it under the terms of the GNU General Public License
%%% as published by the Free Software Foundation; either version 2
%%% of the License, or (at your option) any later version.
%%% 
%%% This program is distributed in the hope that it will be useful,
%%% but WITHOUT ANY WARRANTY; without even the implied warranty of
%%% ERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
%%% GNU General Public License for more details.
%%% 
%%% You should have received a copy of the GNU General Public License
%%% along with this program; if not, write to the Free Software
%%% Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
%%% 02110-1301, USA.
%%%
%%% 
%%% Save streams from the last.fm flash player:
%%%
%%% 1. Add "127.0.0.1 play.last.fm" to /etc/hosts
%%% 2. Start the proxy with: Listen = lfm:start().
%%% 3. Click play on the flash player.
%%% 4. As soon as the download starts close the player.
%%% 5. When the download finishes, stop the proxy with: lfm:stop(Listen).
%%%

-module(lfm).
-author('James Lee <jlee@thestaticvoid.com>').

-export([start/0, stop/1]).
-export([accept_loop/2]).

-include_lib("kernel/src/inet_dns.hrl").

start() ->
	ok = inet_db:add_ns({4,2,2,1}),	% add a dns server to be used later
	{ok, Listen} = gen_tcp:listen(80, [binary, {packet, line}, {active, false}]),
	spawn(?MODULE, accept_loop, [Listen, 0]),
	Listen.

stop(Listen) ->
	gen_tcp:close(Listen).

accept_loop(Listen, Num) ->
	case gen_tcp:accept(Listen) of
		{ok, Socket} ->
			spawn(?MODULE, accept_loop, [Listen, Num + 1]),
			loop(Socket, "", [], Num);

		{error, closed} ->
			ok
	end.

loop(Socket, Host, Lines, Num) ->
	case gen_tcp:recv(Socket, 0) of
		{ok, Line = <<"\r\n">>} ->	% we've reached the end of the http headers
			gen_tcp:close(Socket),
			relay(Host, lists:reverse([Line|Lines]), Num);

		{ok, Line = <<"Host: ", HostBin/binary>>} ->	% note down the host from the headers to relay to
			loop(Socket, binary_to_list(HostBin) -- "\r\n", [Line|Lines], Num);

		{ok, Line} ->
			loop(Socket, Host, [Line|Lines], Num)
	end.

relay(Host, Lines, Num) ->
	% open a connection to the specified host based on a dns lookup
	% /etc/hosts is modified to for this proxy to work so we must look up
	% the real ip in dns to avoid an infinite loop
	{ok, Socket} = gen_tcp:connect(lookupa(Host), 80, [binary, {packet, line}, {active, false}]),
	gen_tcp:send(Socket, Lines),
	receive_response(Socket, Lines, Num).

receive_response(Socket, Lines, Num) ->
	case gen_tcp:recv(Socket, 0) of
		{ok, <<"\r\n">>} ->	% we've reached the end of the http response...
			% so open a file to save the content in
			Filename = "/tmp/" ++ integer_to_list(Num) ++ ".mp3",
			{ok, File} = file:open(Filename, write),
			io:format("Receiving ~p...~n", [Filename]),
			receive_mp3(Socket, File);

		{ok, <<"Location: ", LocationBin/binary>>} ->	% when we get a redirect...
			% replace the old headers with the info from the redirect location
			{match, [HostBin, FileBin]} = re:run(LocationBin, "^http://([^/]*)(/.*)\r\n$", [{capture, all_but_first, binary}]),
			NewLines = [<<"GET ", FileBin/binary, " HTTP/1.1\r\n">>, <<"Host: ", HostBin/binary, "\r\n">> | lists:nthtail(2, Lines)],

			% and start a new http session
			gen_tcp:close(Socket),
			relay(binary_to_list(HostBin), NewLines, Num);

		{ok, _Other} ->
			receive_response(Socket, Lines, Num);

		{error, closed} ->
			ok
	end.

receive_mp3(Socket, File) ->
	case gen_tcp:recv(Socket, 0) of
		{ok, Line} ->
			file:write(File, Line),
			receive_mp3(Socket, File);

		{error, closed} ->
			file:close(File),
			io:format("Finished receiving.~n")
	end.


%%
%% DNS lookup code from
%% http://technicalmusings.blogspot.com/2007/08/erlang-to-stress-test-dns.html
%%
lookupa(Domain) ->
	{ok, #dns_rec{anlist = Ans}} = inet_res:nslookup(Domain, 1, a),
	finda(Ans).	

finda([#dns_rr{type=?S_A, data = Addr} | _]) -> Addr;
finda([_ | T]) -> finda(T);
finda(_Other) -> none.