-module(logic_recurrence).
-include("records.hrl").

-export([parse_rrule/1, generate_occurrences/3]).
-export([validate_rrule/1]).

%% Типы частоты повторения
-define(FREQ_DAILY, <<"DAILY">>).
-define(FREQ_WEEKLY, <<"WEEKLY">>).
-define(FREQ_MONTHLY, <<"MONTHLY">>).

%% Парсинг RRULE из JSON
parse_rrule(RRuleMap) when is_map(RRuleMap) ->
  Freq = maps:get(<<"freq">>, RRuleMap, maps:get(freq, RRuleMap, undefined)),
  Interval = maps:get(<<"interval">>, RRuleMap, maps:get(interval, RRuleMap, undefined)),
  Until = maps:get(<<"until">>, RRuleMap, maps:get(until, RRuleMap, undefined)),
  Count = maps:get(<<"count">>, RRuleMap, maps:get(count, RRuleMap, undefined)),
  ByDay = maps:get(<<"byday">>, RRuleMap, maps:get(byday, RRuleMap, [])),

  {ok, #{
    freq => Freq,
    interval => Interval,
    until => Until,
    count => Count,
    byday => ByDay
  }}.

%% Валидация RRULE
validate_rrule(RRule) ->
  try
    % Поддерживаем и атомы, и бинарные ключи
    Freq = get_freq(RRule),
    Interval = get_interval(RRule),
    ValidFreq = (Freq =:= ?FREQ_DAILY orelse
      Freq =:= ?FREQ_WEEKLY orelse
      Freq =:= ?FREQ_MONTHLY orelse
      Freq =:= <<"DAILY">> orelse
      Freq =:= <<"WEEKLY">> orelse
      Freq =:= <<"MONTHLY">>),
    ValidInterval = is_integer(Interval) andalso Interval >= 1,
    ValidFreq andalso ValidInterval
  catch
    _:_ -> false
  end.

get_freq(#{freq := Freq}) -> Freq;
get_freq(#{<<"freq">> := Freq}) -> Freq.

get_interval(#{interval := Interval}) -> Interval;
get_interval(#{<<"interval">> := Interval}) -> Interval.

%% Генерация вхождений в заданном диапазоне
generate_occurrences(StartTime, RRule, RangeEnd) ->
  Freq = normalize_freq(get_freq(RRule)),
  Interval = get_interval(RRule),
  UntilRaw = maps:get(until, RRule, maps:get(<<"until">>, RRule, undefined)),
  Count = maps:get(count, RRule, maps:get(<<"count">>, RRule, undefined)),
  ByDay = maps:get(byday, RRule, maps:get(<<"byday">>, RRule, [])),

  EndBoundary = case UntilRaw of
                  undefined -> RangeEnd;
                  U when is_binary(U) ->
                    {ok, UntilDt} = parse_datetime_str(U),
                    erlang:min(UntilDt, RangeEnd)
                end,

  generate_by_freq(StartTime, Freq, Interval, ByDay, EndBoundary, Count, 0, []).

normalize_freq(<<"DAILY">>) -> ?FREQ_DAILY;
normalize_freq(<<"WEEKLY">>) -> ?FREQ_WEEKLY;
normalize_freq(<<"MONTHLY">>) -> ?FREQ_MONTHLY;
normalize_freq(Freq) when is_atom(Freq) -> Freq.

parse_datetime_str(Str) ->
  [DateStr, TimeStr] = string:split(Str, "T"),
  TimeStrNoZ = string:trim(TimeStr, trailing, "Z"),

  [YearStr, MonthStr, DayStr] = string:split(DateStr, "-", all),
  [HourStr, MinuteStr, SecondStr] = string:split(TimeStrNoZ, ":", all),

  Year = binary_to_integer(YearStr),
  Month = binary_to_integer(MonthStr),
  Day = binary_to_integer(DayStr),
  Hour = binary_to_integer(HourStr),
  Minute = binary_to_integer(MinuteStr),
  Second = binary_to_integer(SecondStr),

  {ok, {{Year, Month, Day}, {Hour, Minute, Second}}}.

minus(Dt1, Dt2) when Dt1 < Dt2 -> Dt1;
minus(_, Dt2) -> Dt2.

%% Генерация по частоте
generate_by_freq(Current, ?FREQ_DAILY, Interval, _ByDay, EndBoundary, MaxCount, Count, Acc) ->
  case should_stop(Current, EndBoundary, MaxCount, Count) of
    true -> lists:reverse(Acc);
    false ->
      generate_by_freq(
        add_days(Current, Interval),
        ?FREQ_DAILY, Interval, _ByDay, EndBoundary, MaxCount,
        Count + 1, [Current | Acc]
      )
  end;

generate_by_freq(Current, ?FREQ_WEEKLY, Interval, ByDay, EndBoundary, MaxCount, Count, Acc) ->
  case should_stop(Current, EndBoundary, MaxCount, Count) of
    true -> lists:reverse(Acc);
    false ->
      {NewOccurrences, Next} =
        case ByDay of
          [] ->
            {[Current], add_weeks(Current, Interval)};
          Days ->
            %% BYDAY: expand weekdays in Current's week (on/after Current),
            %% then jump Interval weeks from that week's Monday.
            Occs = [O || O <- filter_by_weekday(Current, Days), O =< EndBoundary],
            {Occs, add_weeks(week_start_monday(Current), Interval)}
        end,
      %% Prepend reversed batch so final lists:reverse/1 yields chronological order.
      generate_by_freq(
        Next,
        ?FREQ_WEEKLY, Interval, ByDay, EndBoundary, MaxCount,
        Count + 1, lists:reverse(NewOccurrences) ++ Acc
      )
  end;

generate_by_freq(Current, ?FREQ_MONTHLY, Interval, ByDay, EndBoundary, MaxCount, Count, Acc) ->
  case should_stop(Current, EndBoundary, MaxCount, Count) of
    true -> lists:reverse(Acc);
    false ->
      NewOccurrences = case ByDay of
                         [] -> [Current];
                         _ -> filter_by_month_day(Current, ByDay)
                       end,
      generate_by_freq(
        add_months(Current, Interval),
        ?FREQ_MONTHLY, Interval, ByDay, EndBoundary, MaxCount,
        Count + 1, NewOccurrences ++ Acc
      )
  end.

%% Вспомогательные функции
should_stop(Current, EndBoundary, MaxCount, Count) ->
  (Current > EndBoundary) orelse (MaxCount =/= undefined andalso Count >= MaxCount).

add_days({{Y, M, D}, Time}, N) ->
  Days = calendar:date_to_gregorian_days(Y, M, D) + N,
  {calendar:gregorian_days_to_date(Days), Time}.

add_weeks(DateTime, N) ->
  add_days(DateTime, N * 7).

add_months({{Y, M, D}, Time}, N) ->
  TotalMonths = Y * 12 + M - 1 + N,
  NewYear = TotalMonths div 12,
  NewMonth = TotalMonths rem 12 + 1,
  NewDay = minus(D, calendar:last_day_of_the_month(NewYear, NewMonth)),
  {{NewYear, NewMonth, NewDay}, Time}.

%% BYDAY codes (RFC 5545): MO..SU. calendar:day_of_the_week = 1=Mon .. 7=Sun.
week_start_monday({{Y, M, D}, Time}) ->
  Dow = calendar:day_of_the_week({Y, M, D}),
  add_days({{Y, M, D}, Time}, -(Dow - 1)).

byday_to_dow(<<"MO">>) -> 1;
byday_to_dow(<<"TU">>) -> 2;
byday_to_dow(<<"WE">>) -> 3;
byday_to_dow(<<"TH">>) -> 4;
byday_to_dow(<<"FR">>) -> 5;
byday_to_dow(<<"SA">>) -> 6;
byday_to_dow(<<"SU">>) -> 7;
byday_to_dow("MO") -> 1;
byday_to_dow("TU") -> 2;
byday_to_dow("WE") -> 3;
byday_to_dow("TH") -> 4;
byday_to_dow("FR") -> 5;
byday_to_dow("SA") -> 6;
byday_to_dow("SU") -> 7;
byday_to_dow(mo) -> 1;
byday_to_dow(tu) -> 2;
byday_to_dow(we) -> 3;
byday_to_dow(th) -> 4;
byday_to_dow(fr) -> 5;
byday_to_dow(sa) -> 6;
byday_to_dow(su) -> 7;
byday_to_dow(_) -> undefined.

%% Occurrences for BYDAY in the week of Current, same time-of-day.
%% Only dates on or after Current (so DTSTART mid-week does not emit earlier weekdays).
filter_by_weekday(Current, Days) when is_list(Days) ->
  Monday = week_start_monday(Current),
  Occs =
    lists:foldl(
      fun(DayCode, Acc) ->
        case byday_to_dow(DayCode) of
          undefined -> Acc;
          TargetDow ->
            Occ = add_days(Monday, TargetDow - 1),
            case Occ >= Current of
              true -> [Occ | Acc];
              false -> Acc
            end
        end
      end,
      [],
      Days
    ),
  lists:sort(Occs);
filter_by_weekday(DateTime, _) ->
  [DateTime].

filter_by_month_day(DateTime, _Days) ->
  [DateTime].