% Copyright 2010 Cloudant
% 
% Licensed under the Apache License, Version 2.0 (the "License"); you may not
% use this file except in compliance with the License. You may obtain a copy of
% the License at
%
%   http://www.apache.org/licenses/LICENSE-2.0
%
% Unless required by applicable law or agreed to in writing, software
% distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
% WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
% License for the specific language governing permissions and limitations under
% the License.

-module(rexi_server).
-behaviour(gen_server).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2,
    code_change/3, calls/1, set_call_buffer_size/1]).

-export([start_link/0, init_p/2, init_p/3]).

-include_lib("eunit/include/eunit.hrl").

-record(st, {
    workers = ets:new(workers, [private, {keypos,2}]),
    err_cache = queue:new(),
    err_cache_max = 20,
    err_cache_len = 0

}).

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

init([]) ->
    {ok, #st{}}.

calls(N) ->
    {ok, gen_server:call(?MODULE,{calls, N},infinity)}.

set_call_buffer_size(N) ->
    {ok, gen_server:call(?MODULE,{cache_max, N},infinity)}.

handle_call({calls, N}, _From, #st{err_cache=Q}=St) ->
    ErrList = lists:sublist(lists:reverse(queue:to_list(Q)),1,N),
    {reply,ErrList,St};

handle_call({cache_max, N}, _From, #st{err_cache_len=Len,
                                       err_cache_max=Max, err_cache=Q}=St) ->
    {NewQ,NewLen} = case N < Len of
           true ->
               List = queue:to_list(Q),
               {queue:from_list(lists:sublist(List,(Len-N)+1,Len)),
                N};
           false ->
               {Q,Len}
           end,
    {reply, Max, St#st{err_cache_max=N, err_cache_len=NewLen, err_cache=NewQ}};
handle_call(_Request, _From, St) ->
    {reply, ignored, St}.


handle_cast({doit, From, MFA}, St) ->
    handle_cast({doit, From, undefined, MFA}, St);

handle_cast({doit, From, Nonce, MFA}, #st{workers=Workers} = St) ->
    {LocalPid, Ref} = spawn_monitor(?MODULE, init_p, [From, MFA, Nonce]),
    {noreply, St#st{workers = add_worker({LocalPid, Ref, From}, Workers)}};


handle_cast({kill, FromRef}, #st{workers=Workers} = St) ->
    case find_worker_from(FromRef, Workers) of
    {Pid, KeyRef, {_, FromRef}} ->
        erlang:demonitor(KeyRef),
        exit(Pid, kill),
        {noreply, St#st{workers = remove_worker(KeyRef, Workers)}};
    false ->
        {noreply, St}
    end.

handle_info({'DOWN', Ref, process, _, normal}, #st{workers=Workers} = St) ->
    {noreply, St#st{workers = remove_worker(Ref, Workers)}};

handle_info({'DOWN', Ref, process, Pid, Reason}, #st{workers=Workers,
                                                    err_cache=Q,
                                                    err_cache_max=Max,
                                                    err_cache_len=Len} = St) ->
    case find_worker(Ref, Workers) of
    {Pid, Ref, From} ->

        {Error,{M,F,A},Nonce,Stack} = Reason,

        {NewQ,NewLen} =
        case Len >= Max of
        true ->
            Q1 = queue:drop(Q),
            {Q1,Len};
        _ ->
            {Q,Len+1}
        end,
        NewQ1 = queue:in([now(),From,Nonce,M,F,A],NewQ),
        notify_caller(From, {Error,Stack}),
        {noreply, St#st{workers = remove_worker(Ref, Workers),
                       err_cache=NewQ1,
                       err_cache_len=NewLen}};
    false ->
        {noreply, St}
    end;

handle_info(_Info, St) ->
    {noreply, St}.

terminate(_Reason, St) ->
    ets:foldl(fun({Pid, _, _}, _) -> exit(Pid,kill) end, nil, St#st.workers),
    ok.

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

init_p(From, MFA) ->
    init_p(From, MFA, undefined).

%% @doc initializes a process started by rexi_server.
-spec init_p({pid(), reference()}, {atom(), atom(), list()}, string()) -> any().
init_p(From, {M,F,A}, Nonce) ->
    put(rexi_from, From),
    put(initial_call, {M,F,length(A)}),
    try apply(M, F, A) catch exit:normal -> ok; Class:Reason ->
        Stack = clean_stack(),
        error_logger:error_report([{?MODULE, Nonce, {Class, Reason}}, Stack]),
        exit({Reason,{M,F,A},Nonce,Stack})
    end.

%% internal

clean_stack() ->
    lists:map(fun({M,F,A}) when is_list(A) -> {M,F,length(A)}; (X) -> X end,
        erlang:get_stacktrace()).

add_worker(Worker, Tab) ->
    ets:insert(Tab, Worker), Tab.

remove_worker(Ref, Tab) ->
    ets:delete(Tab, Ref), Tab.

find_worker(Ref, Tab) ->
    case ets:lookup(Tab, Ref) of [] -> false; [Worker] -> Worker end.

find_worker_from(Ref, Tab) ->
    case ets:match_object(Tab, {'_', '_', {'_', Ref}}) of
    [] ->
        false;
    [Worker] ->
        Worker
    end.

notify_caller({Caller, Ref}, Reason) ->
    Caller ! {Ref, {rexi_EXIT, Reason}}.