Custom function to loop through a 2D array used as a calendar - Google App Script

I'm trying to create a custom function that finds the last activity performed and returns the next activity in thesequence. Sample data, e.g. "Chess" is followed in by "Pilates" and then "Elevate". Then next activity is "Kundalini", etc. Once, the last activity is done, the cycle repeats.

Essentially, loop through a 2D array that's a calendar, find the last (most recent) activity and then return the next value from the alt_activity range. If the previous activity is the last value from the alt_activity range, start from the start of the alt_activity range once more.

/**
* Looks for the alternate workout in the LAST matching value within the calendar.
* @param {array} input The calendar array to be searched.
* @return The first match starting from the last row (WEEK) and column (DAY) of the @param.
* @customfunction
*/
function calender_search(calendar_range, alt_activities_range)
{
//Search by ROW(), COLUMN() think of ADDRESS function in spreadsheets
//rows are weeks within the calendar
//let week = Object.keys(calendar_range);

for (week = calendar_range.length - 1; i >= 0; i--)
{
for (day = calendar_range[week].length - 1; j >= 0; j--)
{
// look for the match from the alt_activities_range. IF -1 returned (no match) continue loop
IF (alt_activities_range.indexOf(calendar_range[week][day]) > -1)
{
IF (calendar_range[week][day].indexOf(alt_activities_range) = alt_activties_range.length - 1)
{
return alt_activities_range[0];
}
//Go to index of match PLUS 1 for NEXT value to be returned
return calendar_range[week][day].indexOf(alt_activities_range) + 1;
}

}
}
}

Table 1

Table 2

Created the custom function but the results are not what I was expecting from what I've been able to piece together.

The modulo operator % is used to wrap around to the beginning of alt_activities_range when the last activity is reached.

function calender_search(calendar_range, alt_activities_range) {
  for (let week = calendar_range.length - 1; week >= 0; week--) {
    for (let day = calendar_range[week].length - 1; day >= 0; day--) {
      if (alt_activities_range.indexOf(calendar_range[week][day]) > -1) {
        if (calendar_range[week][day] === alt_activities_range[alt_activities_range.length - 1]) {
          return alt_activities_range[0];
        }
        // Go to index of match PLUS 1 for NEXT value to be returned
        return alt_activities_range[(alt_activities_range.indexOf(calendar_range[week][day]) + 1) % alt_activities_range.length];
      }
    }
  }
}