const dayjs = require("dayjs");

const isToday = require("dayjs/plugin/isToday");
const utc = require("dayjs/plugin/utc");
const timezone = require("dayjs/plugin/timezone");

dayjs.extend(isToday);
dayjs.extend(utc);
dayjs.extend(timezone);

const getMinutesFromMidnight = (date) => {
  return date.hour() * 60 + date.minute();
};

const getSlots = ({
  calendarTimeZone,
  eventLength,
  selectedTimeZone,
  selectedDate,
  dayStartTime,
  dayEndTime
}) => {

  if(!selectedDate) return []
  
  const lowerBound = selectedDate.tz(selectedTimeZone).startOf("day");

  // Simple case, same timezone
  if (calendarTimeZone === selectedTimeZone) {
    const slots = [];
    const now = dayjs();
    for (
      let minutes = dayStartTime;
      minutes <= dayEndTime - eventLength;
      minutes += parseInt(eventLength, 10)
    ) {
      const slot = lowerBound.add(minutes, "minutes");
      if (slot > now) {
        slots.push(slot);
      }
    }
    return slots;
  }

  const upperBound = selectedDate.tz(selectedTimeZone).endOf("day");

  // We need to start generating slots from the start of the calendarTimeZone day
  const startDateTime = lowerBound
    .tz(calendarTimeZone)
    .startOf("day")
    .add(dayStartTime, "minutes");

  let phase = 0;
  if (startDateTime < lowerBound) {
    // Getting minutes of the first event in the day of the chooser
    const diff = lowerBound.diff(startDateTime, "minutes");

    // finding first event
    phase = diff + eventLength - (diff % eventLength);
  }

  // We can stop as soon as the selectedTimeZone day ends
  const endDateTime = upperBound
    .tz(calendarTimeZone)
    .subtract(eventLength, "minutes");

  const maxMinutes = endDateTime.diff(startDateTime, "minutes");

  const slots = [];
  const now = dayjs();
  for (
    let minutes = phase;
    minutes <= maxMinutes;
    minutes += parseInt(eventLength, 10)
  ) {
    const slot = startDateTime.add(minutes, "minutes");

    const minutesFromMidnight = getMinutesFromMidnight(slot);

    if (
      minutesFromMidnight < dayStartTime ||
      minutesFromMidnight > dayEndTime - eventLength ||
      slot < now
    ) {
      continue;
    }

    slots.push(slot.tz(selectedTimeZone));
  }

  return slots;
};

export default getSlots