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Implement BYSETPOS support for monthly recurrence rules with BYMONTHDAY #985

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Implement BYSETPOS support for monthly recurrence rules with BYMONTHDAY #985

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163 changes: 128 additions & 35 deletions lib/ical/recur_iterator.js
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Original file line number Diff line number Diff line change
Expand Up @@ -353,26 +353,29 @@ class RecurIterator {
throw new Error("Malformed values in BYDAY part");
}
} else if (this.has_by_data("BYMONTHDAY")) {
// Change the day value so that normalisation won't change the month.
this.last.day = 1;

// Get a sorted list of days in the starting month that match the rule.
let normalized = this.normalizeByMonthDayRules(
this.last.year,
this.last.month,
this.rule.parts.BYMONTHDAY
).filter(d => d >= this.last.day);

if (normalized.length) {
// There's at least one valid day, use it.
this.last.day = normalized[0];
this.by_data.BYMONTHDAY = normalized;
if (this.has_by_data("BYSETPOS")) {
this._initMonthlyByMonthDaySetPos();
} else {
// There's no occurrence in this month, find the next valid month.
// The longest possible sequence of skipped months is February-April-June,
// so we might need to call next_month up to three times.
if (!this.next_month() && !this.next_month() && !this.next_month()) {
throw new InvalidRecurrenceRuleError();
// Change the day value so that normalisation won't change the month.
this.last.day = 1;
// Get a sorted list of days in the starting month that match the rule.
let normalized = this.normalizeByMonthDayRules(
this.last.year,
this.last.month,
this.rule.parts.BYMONTHDAY
).filter(d => d >= this.last.day);

if (normalized.length) {
// There's at least one valid day, use it.
this.last.day = normalized[0];
this.by_data.BYMONTHDAY = normalized;
} else {
// There's no occurrence in this month, find the next valid month.
// The longest possible sequence of skipped months is February-April-June,
// so we might need to call next_month up to three times.
if (!this.next_month() && !this.next_month() && !this.next_month()) {
throw new InvalidRecurrenceRuleError();
}
}
}
}
Expand Down Expand Up @@ -802,28 +805,32 @@ class RecurIterator {
}
}
} else if (this.has_by_data("BYMONTHDAY")) {
this.by_indices.BYMONTHDAY++;
if (this.has_by_data("BYSETPOS")) {
data_valid = this._nextMonthlyByMonthDaySetPos();
} else {
Comment on lines +808 to +810

@kewisch kewisch Jun 2, 2026

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Have you considered cases where we have BYMONTHDAY, BYSETPOS, and other rule parts? I think theoretically a BYDAY could appears as well.

this.by_indices.BYMONTHDAY++;

if (this.by_indices.BYMONTHDAY >= this.by_data.BYMONTHDAY.length) {
this.by_indices.BYMONTHDAY = 0;
this.increment_month();
if (this.by_indices.BYMONTHDAY >= this.by_data.BYMONTHDAY.length) {
return 0;
this.by_indices.BYMONTHDAY = 0;
this.increment_month();
if (this.by_indices.BYMONTHDAY >= this.by_data.BYMONTHDAY.length) {
return 0;
}
}
}

let daysInMonth = Time.daysInMonth(this.last.month, this.last.year);
let day = this.by_data.BYMONTHDAY[this.by_indices.BYMONTHDAY];
let daysInMonth = Time.daysInMonth(this.last.month, this.last.year);
let day = this.by_data.BYMONTHDAY[this.by_indices.BYMONTHDAY];

if (day < 0) {
day = daysInMonth + day + 1;
}
if (day < 0) {
day = daysInMonth + day + 1;
}

if (day > daysInMonth) {
this.last.day = 1;
data_valid = this.is_day_in_byday(this.last);
} else {
this.last.day = day;
if (day > daysInMonth) {
this.last.day = 1;
data_valid = this.is_day_in_byday(this.last);
} else {
this.last.day = day;
}
}
} else {
this.increment_month();
Expand Down Expand Up @@ -1359,6 +1366,92 @@ class RecurIterator {
return 0;
}

/**
* Find the first BYMONTHDAY in the current month at a BYSETPOS position
* on or after minDay.
*
* @private
* @param {Number} minDay
* @return {Number|null}
*/
_findByMonthDaySetPos(minDay) {
let monthDays = this.normalizeByMonthDayRules(
this.last.year,
this.last.month,
this.rule.parts.BYMONTHDAY
);

for (let spIndex = 0; spIndex < monthDays.length; spIndex++) {
let day = monthDays[spIndex];
if (day < minDay) {
continue;
}
if (this.check_set_position(spIndex + 1) ||
this.check_set_position(spIndex - monthDays.length)) {
return day;
}
}

return null;
}

/**
* Initialize MONTHLY iteration with BYMONTHDAY and BYSETPOS.
*
* @private
*/
_initMonthlyByMonthDaySetPos() {
let day = this._findByMonthDaySetPos(this.last.day);
let attempts = 0;

while (!day && attempts++ < 48) {
this.last.day = 1;
this.increment_month();
day = this._findByMonthDaySetPos(1);
}

if (!day) {
throw new InvalidRecurrenceRuleError();
}

this.last.day = day;
this.by_data.BYMONTHDAY = this.normalizeByMonthDayRules(
this.last.year,
this.last.month,
this.rule.parts.BYMONTHDAY
);
}

/**
* Advance MONTHLY iteration with BYMONTHDAY and BYSETPOS.
*
* @private
* @return {Number} 1 if a valid day was found, 0 otherwise
*/
_nextMonthlyByMonthDaySetPos() {
let day = this._findByMonthDaySetPos(this.last.day + 1);
let attempts = 0;

while (!day && attempts++ < 48) {

@kewisch kewisch Jun 2, 2026

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Could you add a comment on why 48 attempts, here and in the last function?

this.last.day = 1;
this.increment_month();
day = this._findByMonthDaySetPos(1);
}

if (!day) {
return 0;
}

this.last.day = day;
this.by_data.BYMONTHDAY = this.normalizeByMonthDayRules(
this.last.year,
this.last.month,
this.rule.parts.BYMONTHDAY
);

return 1;
}

/**
* Checks if given value is in BYSETPOS.
*
Expand Down
124 changes: 124 additions & 0 deletions test/recur_iterator_test.js
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Original file line number Diff line number Diff line change
Expand Up @@ -866,6 +866,130 @@ suite('recur_iterator', function() {
"2015-04-19T08:00:00Z"
]
});
// Choose the last valid day among a list of month-end candidates.
// This covers February and months with 30 or 31 days.
testRRULE('FREQ=MONTHLY;BYMONTHDAY=28,29,30,31;BYSETPOS=-1', {
dtStart: '2026-01-01',
dates: [
'2026-01-31',
'2026-02-28',
'2026-03-31',
'2026-04-30',
'2026-05-31',
'2026-06-30',
'2026-07-31',
'2026-08-31',
'2026-09-30',
'2026-10-31',
'2026-11-30',
'2026-12-31',
]
});
// Pick the first occurrence from multiple BYMONTHDAY values.
testRRULE('FREQ=MONTHLY;BYMONTHDAY=1,15;BYSETPOS=1;COUNT=3', {
dtStart: '2016-01-01',
byCount: true,
dates: [
'2016-01-01',
'2016-02-01',
'2016-03-01',
]
});
// Pick the second occurrence from the same BYMONTHDAY list.
testRRULE('FREQ=MONTHLY;BYMONTHDAY=1,15;BYSETPOS=2;COUNT=3', {
dtStart: '2016-01-01',
byCount: true,
dates: [
'2016-01-15',
'2016-02-15',
'2016-03-15',
]
});
// Pick the last occurrence from the same BYMONTHDAY list.
testRRULE('FREQ=MONTHLY;BYMONTHDAY=1,15;BYSETPOS=-1;COUNT=3', {
dtStart: '2016-01-01',
byCount: true,
dates: [
'2016-01-15',
'2016-02-15',
'2016-03-15',
]
});
// Pick multiple positions from the same sorted list, verifying order.
testRRULE('FREQ=MONTHLY;BYMONTHDAY=1,15;BYSETPOS=1,2;COUNT=4', {
dtStart: '2016-01-01',
byCount: true,
dates: [
'2016-01-01',
'2016-01-15',
'2016-02-01',
'2016-02-15',
]
});
// Combine negative BYMONTHDAY normalization with BYSETPOS selection.
testRRULE('FREQ=MONTHLY;BYMONTHDAY=-1;BYSETPOS=1;COUNT=3', {
dtStart: '2016-01-01',
byCount: true,
dates: [
'2016-01-31',
'2016-02-29',
'2016-03-31',
]
});
// Mixed positive and negative BYMONTHDAY values with BYSETPOS=-1.
testRRULE('FREQ=MONTHLY;BYMONTHDAY=-1,31;BYSETPOS=-1;COUNT=4', {
dtStart: '2016-01-01',
byCount: true,
dates: [
'2016-01-31',
'2016-02-29',
'2016-03-31',
'2016-04-30',
]
});
// Start mid-month and pick the second BYMONTHDAY occurrence each month.
testRRULE('FREQ=MONTHLY;BYMONTHDAY=5,15,25;BYSETPOS=2;COUNT=4', {
dtStart: '2016-01-16',
byCount: true,
dates: [
'2016-02-15',
'2016-03-15',
'2016-04-15',
'2016-05-15',
]
});
// Select the last valid candidate from a list that spans both 30- and 31-day months.
testRRULE('FREQ=MONTHLY;BYMONTHDAY=31,30;BYSETPOS=-1;COUNT=4', {
dtStart: '2016-01-01',
byCount: true,
dates: [
'2016-01-31',
'2016-03-31',
'2016-04-30',
'2016-05-31',
]
});
// 31st with BYSETPOS=1 across a long UNTIL range, ensuring months without
// a 31st are skipped and the recurrence continues correctly.
testRRULE('FREQ=MONTHLY;BYMONTHDAY=31;BYSETPOS=1;UNTIL=20161231', {
dtStart: '2016-01-01',
until: true,
dates: [
'2016-01-31',
'2016-03-31',
'2016-05-31',
'2016-07-31',
'2016-08-31',
'2016-10-31',
'2016-12-31',
]
});
// BYSETPOS is too large for the list of matching BYMONTHDAY values,
// so the rule produces no occurrences.
testRRULE('FREQ=MONTHLY;BYMONTHDAY=1,15;BYSETPOS=3', {
dtStart: '2016-01-01',
noInstance: true
});
});

suite('YEARLY', function() {
Expand Down
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