From 0a5e86c4a0a16a8d99164b68bf6ba9c67a1501b9 Mon Sep 17 00:00:00 2001
From: Aleksey Sabilin <aleksey@sabilin.com>
Date: Sat, 1 Aug 2026 18:17:23 +0300
Subject: [PATCH] fix(recurrence): honor BYDAY for weekly RRULE expansion

Expand weekdays in the DTSTART week and advance by Interval weeks from Monday;
add unit coverage for MO/WE/FR and mid-week starts.
---
 src/logic/logic_recurrence.erl       | 73 ++++++++++++++++++++++++----
 test/unit/logic_recurrence_tests.erl | 25 ++++++++++
 2 files changed, 88 insertions(+), 10 deletions(-)

diff --git a/src/logic/logic_recurrence.erl b/src/logic/logic_recurrence.erl
index b6ba352..2f1a0e9 100644
--- a/src/logic/logic_recurrence.erl
+++ b/src/logic/logic_recurrence.erl
@@ -106,15 +106,21 @@ generate_by_freq(Current, ?FREQ_WEEKLY, Interval, ByDay, EndBoundary, MaxCount,
   case should_stop(Current, EndBoundary, MaxCount, Count) of
     true -> lists:reverse(Acc);
     false ->
-      % Если указаны дни недели, генерируем только в эти дни
-      NewOccurrences = case ByDay of
-                         [] -> [Current];
-                         Days -> filter_by_weekday(Current, Days)
-                       end,
+      {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(
-        add_weeks(Current, Interval),
+        Next,
         ?FREQ_WEEKLY, Interval, ByDay, EndBoundary, MaxCount,
-        Count + 1, NewOccurrences ++ Acc
+        Count + 1, lists:reverse(NewOccurrences) ++ Acc
       )
   end;
 
@@ -151,9 +157,56 @@ add_months({{Y, M, D}, Time}, N) ->
   NewDay = minus(D, calendar:last_day_of_the_month(NewYear, NewMonth)),
   {{NewYear, NewMonth, NewDay}, Time}.
 
-filter_by_weekday(DateTime, _Days) ->
-  % Упрощённая версия — всегда возвращаем текущую дату
-  % В полной версии нужно проверять день недели
+%% 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) ->
diff --git a/test/unit/logic_recurrence_tests.erl b/test/unit/logic_recurrence_tests.erl
index b670364..bacc4d0 100644
--- a/test/unit/logic_recurrence_tests.erl
+++ b/test/unit/logic_recurrence_tests.erl
@@ -7,6 +7,7 @@ logic_recurrence_test_() ->
     {"Validate RRULE test", fun test_validate_rrule/0},
     {"Generate daily occurrences test", fun test_daily_occurrences/0},
     {"Generate weekly occurrences test", fun test_weekly_occurrences/0},
+    {"Generate weekly BYDAY occurrences test", fun test_weekly_byday_occurrences/0},
     {"Generate monthly occurrences test", fun test_monthly_occurrences/0},
     {"Generate with count limit test", fun test_count_limit/0},
     {"Generate with until limit test", fun test_until_limit/0}
@@ -71,6 +72,30 @@ test_weekly_occurrences() ->
   ?assertEqual({{2026, 4, 20}, {10, 0, 0}}, lists:nth(1, Occurrences)),
   ?assertEqual({{2026, 5, 11}, {10, 0, 0}}, lists:nth(4, Occurrences)).
 
+test_weekly_byday_occurrences() ->
+  %% Monday start, MO/WE/FR for two weeks → 6 slots
+  StartTime = {{2026, 4, 20}, {10, 0, 0}},
+  RRule = #{
+    freq => <<"WEEKLY">>,
+    interval => 1,
+    byday => [<<"MO">>, <<"WE">>, <<"FR">>]
+  },
+  RangeEnd = {{2026, 5, 1}, {10, 0, 0}},
+
+  Occurrences = logic_recurrence:generate_occurrences(StartTime, RRule, RangeEnd),
+  ?assertEqual(6, length(Occurrences)),
+  ?assertEqual({{2026, 4, 20}, {10, 0, 0}}, lists:nth(1, Occurrences)),
+  ?assertEqual({{2026, 4, 22}, {10, 0, 0}}, lists:nth(2, Occurrences)),
+  ?assertEqual({{2026, 4, 24}, {10, 0, 0}}, lists:nth(3, Occurrences)),
+  ?assertEqual({{2026, 4, 27}, {10, 0, 0}}, lists:nth(4, Occurrences)),
+  ?assertEqual({{2026, 4, 29}, {10, 0, 0}}, lists:nth(5, Occurrences)),
+  ?assertEqual({{2026, 5, 1}, {10, 0, 0}}, lists:nth(6, Occurrences)),
+
+  %% Mid-week DTSTART must not emit earlier weekdays of the same week
+  WedStart = {{2026, 4, 22}, {10, 0, 0}},
+  MidWeek = logic_recurrence:generate_occurrences(WedStart, RRule, {{2026, 4, 24}, {10, 0, 0}}),
+  ?assertEqual([{{2026, 4, 22}, {10, 0, 0}}, {{2026, 4, 24}, {10, 0, 0}}], MidWeek).
+
 test_monthly_occurrences() ->
   StartTime = {{2026, 4, 20}, {10, 0, 0}},
   RRule = #{