%%%---------------------------------------------------------------------------
%%% Copyright (C) 2021, Skulup All Rights Reserved
%%% Unauthorized copy of this file is through any medium strictly not allowed.
%%%
%%% Authors : Alpha Shaw <shawalpha5@gmail.com>
%%% Created :  21 Apr 2021  by <shawalpha5@gmail.com>
%%% Purpose :
%%%
%%%
%%% 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(mnesplit_lib).
-export([merge_only/3, last_version/3, last_modified/3]).

-spec merge_only
    (init, {atom(), atom(), list(atom()), list(any())}, atom()) ->
  {ok, atom()};
    (done, any(), atom()) ->
  ok;
    (any(), any(), atom()) ->
  {ok, list(mnesplit:action()), atom()} |
  {ok, mnesplit:action(), atom()} |
  {inconsistency, any(), atom()}.

merge_only(init, {_, set, _, _}, _) -> {ok, set};
merge_only(init, {_, bag, _, _}, _) -> {ok, bag};
merge_only(done, _, _) -> ok;
merge_only([A], [B], set) ->
  {inconsistency, {merge, A, B}, set};
merge_only(A, B, bag) when is_list(A), is_list(B) ->
  {Aonly, Bonly} = merge_bags(A, B),
  {ok, lists:append([lists:map(fun(Ae) -> {write_remote, Ae} end, Aonly),
    lists:map(fun(Be) -> {write_local, Be} end, Bonly)]), bag}.

-spec last_modified
    (init, {atom(), atom(), list(atom()), list(any())}, atom()) ->
  {ok, atom()};
    (done, any(), atom()) ->
  ok;
    (any(), any(), atom()) ->
  {ok, list(mnesplit:action()), atom()} |
  {ok, mnesplit:action(), atom()} |
  {inconsistency, any(), atom()}.

last_modified(init, {Table, Type, Attrs, Xargs}, Node) ->
  last_version(init, {Table, Type, Attrs, [modified | Xargs]}, Node);
last_modified(A, B, C) ->
  last_version(A, B, C).

-spec last_version
    (init, {atom(), atom(), list(atom()), list(any())}, atom()) ->
  {ok, atom()};
    (done, any(), atom()) ->
  ok;
    (any(), any(), atom()) ->
  {ok, list(mnesplit:action()), atom()} |
  {ok, mnesplit:action(), atom()} |
  {inconsistency, any(), atom()}.

last_version(init, {Table, set, Attrs, [VField | _]}, _Node) ->
  {ok, {set, pos(VField, Table, Attrs)}};
last_version(init, {Table, bag, Attrs, [VField, IField | _]}, _Node) ->
  {ok, {bag, pos(VField, Table, Attrs), pos(IField, Table, Attrs)}};
last_version(done, _State, _Node) ->
  ok;
last_version([A], [B], {set, Field} = Ms) when is_tuple(A), is_tuple(B) ->
  case element(Field, A) >= element(Field, B) of
    true -> {ok, {write_remote, A}, Ms};
    false -> {ok, {write_local, B}, Ms}
  end;
last_version(A, B, {bag, Vfield, Ifield} = State) ->
  Actions = merge_versioned(A, B, Vfield, Ifield, []),
  {ok, Actions, State}.

merge_bags(A, B) ->
  merge_bags(lists:sort(A), lists:sort(B), [], []).

merge_bags(A, [], Aonly, Bonly) ->
  {lists:append([A, Aonly]), Bonly};
merge_bags([], B, Aonly, Bonly) ->
  {Aonly, lists:append([B, Bonly])};
merge_bags([A | NextA], [A | NextB], Aonly, Bonly) ->
  merge_bags(NextA, NextB, Aonly, Bonly);
merge_bags([A | NextA], [B | NextB], Aonly, Bonly) ->
  case A < B of
    true ->
      merge_bags(NextA, [B | NextB], [A | Aonly], Bonly);
    false ->
      merge_bags([A | NextA], NextB, Aonly, [B | Bonly])
  end.

merge_versioned(A, [], _, _, Acts) ->
  % local elements only
  lists:append([Acts, lists:map(fun(E) -> {write_remote, E} end, A)]);
merge_versioned([], B, _, _, Acts) ->
  % remote elements only
  lists:append([Acts, lists:map(fun(E) -> {write_local, E} end, B)]);
merge_versioned([A | NextA], B, VF, IF, Acts) ->
  Index = element(IF, A),
  Avers = element(VF, A),
  case lists:keytake(Index, IF, B) of
    false ->
      % no corresponding element int set B at all, write remotely
      merge_versioned(NextA, B, VF, IF, [{write_remote, A} | Acts]);
    {value, A, NextB} ->
      % elements are equal, no action required
      merge_versioned(NextA, NextB, VF, IF, Acts);
    {value, Bb, NextB} ->
      % there is a corresponding element. check version
      Bvers = element(VF, Bb),
      case Avers < Bvers of
        true ->
          % remote element has better version, update local
          merge_versioned(NextA, NextB, VF, IF,
            [{write_local, Bb}, {delete_local, A} | Acts]);
        false ->
          % local element has better version, update remote side
          merge_versioned(NextA, NextB, VF, IF,
            [{write_remote, A}, {delete_remote, Bb} | Acts])
      end
  end.


pos(A, T, L) ->
  pos(A, T, L, 2).  % record tag is the 1st element in the tuple

pos(H, _, [H | _], P) ->
  P;
pos(H, Tab, [_ | T], P) ->
  pos(H, Tab, T, P + 1);
pos(A, Tab, [], _) ->
  mnesia:abort({missing_attribute, Tab, A}).