%%
%% Load balancer
%%
%% Author: Krzysztof Kliś <krzysztof.klis@gmail.com>
%%
%% 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 3 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
%% MERCHANTABILITY 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, see <http://www.gnu.org/licenses/>.
%%

-module(balancer).

-export([load/0, show/1, pick/1]).

-define(TIMEOUT, 3000).

% Return node() system load.
load() ->
    load_result(cpu_sup:avg1()).

load_result(0) ->
    cpu_sup:start(),
    {load, node(), cpu_sup:avg1()};
load_result(N) ->
    {load, node(), N}.

% Show load of all nodes
show([]) ->
    show([node()]);
show(L) ->
    {Results, _} = rpc:multicall(L, balancer, load, [], ?TIMEOUT),
    [{Node, Load} || {load, Node, Load} <- Results].

% Pick a node with lowest system load from list L.
pick([]) ->
    [N] = show([node()]),
    N;
pick(L) ->
    case show(L) of
        [] ->
            pick([]);
        [H|T] ->
            N = select(H, T),
            N
    end.

select(N, []) ->
    N;
select(N, L) ->
    [H|T] = L,
    {_,Load} = N,
    {_,Load1} = H,
    case Load1 < Load of
	true ->
	    N1 = H;
	false ->
	    N1 = N
    end,
    select(N1, T).