-module(ephp_mem).

-author('manuel@altenwald.com').

-compile([{no_auto_import, [get/1]}]).

-include("ephp.hrl").

-export([start_link/0, stop/0, get/1, get_with_links/1, remove/1, set/2, add_link/1,
         add/1]).

-record(mem, {data :: mixed(), links = 0 :: non_neg_integer()}).

-spec start_link() -> {ok, module()}.
%% @doc starts the memory storage system for referenced data.
start_link() ->
    case erlang:get(get_id()) of
        undefined ->
            erlang:put(get_id(), array:new({default, free}));
        _Mem ->
            ok
    end,
    {ok, get_id()}.

-spec get_id() -> module().
%% @hidden
%% @doc retrieve the name of the ID in use to storage the information in the
%%      dictionary process.
%% @end
get_id() ->
    ?MODULE.

-spec stop() -> ok.
%% @doc remove the information about the links for the referenced data.
stop() ->
    Ref = get_id(),
    erlang:erase(Ref),
    ok.

-spec get(mem_ref()) -> any().
%% @doc get the content for a specific MemId.
get(#mem_ref{mem_id = MemId}) ->
    Ref = get_id(),
    Mem = erlang:get(Ref),
    case array:get(MemId, Mem) of
        free ->
            throw(segmentation_fault);
        #mem{data = Data} ->
            Data
    end.

-spec get_with_links(mem_ref()) -> {any(), non_neg_integer()}.
%% @doc get the content for a specific MemId and the number of links.
get_with_links(#mem_ref{mem_id = MemId}) ->
    Ref = get_id(),
    Mem = erlang:get(Ref),
    case array:get(MemId, Mem) of
        free ->
            throw(segmentation_fault);
        #mem{data = Data, links = Links} ->
            {Data, Links}
    end.

-spec remove(mem_ref()) -> ok.
%% @doc removes an entry given by MemId in the storage data.
remove(#mem_ref{mem_id = MemId}) ->
    Ref = get_id(),
    Mem = erlang:get(Ref),
    NewMem =
        case array:get(MemId, Mem) of
            free ->
                throw(segmentation_fault);
            #mem{links = Links} = _MemData when Links =< 1 ->
                array:set(MemId, free, Mem);
            #mem{links = Links} = MemData ->
                array:set(MemId, MemData#mem{links = Links - 1}, Mem)
        end,
    erlang:put(Ref, NewMem),
    ok.

-spec set(mem_ref(), Data :: any()) -> ok.
%% @doc stores the element in the position required.
set(#mem_ref{mem_id = MemId}, Data) ->
    Ref = get_id(),
    Mem = erlang:get(Ref),
    MemData =
        case array:get(MemId, Mem) of
            free ->
                #mem{data = Data};
            #mem{} = MD ->
                MD#mem{data = Data}
        end,
    NewMem = array:set(MemId, MemData, Mem),
    erlang:put(Ref, NewMem),
    ok.

-spec add_link(mem_ref()) -> ok.
%% @doc increase the links attribute for a specific MemId.
add_link(#mem_ref{mem_id = MemId}) ->
    Ref = get_id(),
    Mem = erlang:get(Ref),
    NewMem =
        case array:get(MemId, Mem) of
            free ->
                throw(segmentation_fault);
            #mem{links = Links} = MemData ->
                array:set(MemId, MemData#mem{links = Links + 1}, Mem)
        end,
    erlang:put(Ref, NewMem),
    ok.

-spec add(Data :: any()) -> mem_ref().
%% @doc adds information for the storage and returns the MemId to access later
%%      to that information.
%% @end
add(Data) ->
    MemData = #mem{data = Data},
    Ref = get_id(),
    Mem = erlang:get(Ref),
    #mem_ref{mem_id = MemId} = MemRef = search_empty(Mem),
    NewMem = array:set(MemId, MemData, Mem),
    erlang:put(Ref, NewMem),
    MemRef.

-spec search_empty(array:array()) -> mem_ref().
%% @hidden
%% @doc returns the next free index. Indexes are never reused so a dangling
%%      mem_ref throws segmentation_fault instead of aliasing newer data.
%% @end
search_empty(Mem) ->
    #mem_ref{mem_id = erlang:max(1, array:size(Mem))}.