homepage/src/components/nextTopic.js

const TUESDAY = 2
const THURSDAY = 4
const WEEK = 7

// for easier mocking in tests
var today = new Date()

export default function NextTopic() {
  // don't put the nextTopic date in the staticly generated html
  // because it would be outdated rather quickly
  const isSSR = typeof window === "undefined"
  if (isSSR) {
    test()
    return "unbekannt"
  }

  return formatDateInfo(getNextTopicDate())
}

// javascript dates are not nice

function getNextTopicDate() {
  // first thursday and third tuesday in month
  const nextTopic = zeroizeTime(today)

  // first thursday
  if (calculatePriorWeekdays(THURSDAY) === 0) {
    addDays(nextTopic, getDaysUntilNext(THURSDAY, nextTopic))
    return nextTopic
  }
  // third tuesday
  const priorTuesdays = calculatePriorWeekdays(TUESDAY)
  if (priorTuesdays <= 2) {
    addDays(nextTopic, getDaysUntilNext(TUESDAY, nextTopic))
    addDays(nextTopic, WEEK * (2 - priorTuesdays))
    return nextTopic
  }
  // first thursday next month
  const currentMonth = today.getMonth()
  addDays(nextTopic, getDaysUntilNext(THURSDAY, nextTopic))
  while (nextTopic.getMonth() === currentMonth) {
    addDays(nextTopic, WEEK)
  }
  return nextTopic
}

/**
 * calculate how many of the given weekday this month already had.
 * for example: how many tuesdays were in this month already
 */
function calculatePriorWeekdays(weekday) {
  const testDate = new Date(today)
  testDate.setDate(1)

  var priorWeekdays = 0
  while (testDate < today) {
    if (testDate.getDay() === weekday) {
      priorWeekdays++
    }
    testDate.setDate(testDate.getDate() + 1)
  }

  return priorWeekdays
}

/**
 * how many days are there until the next <weekday> starting from <date>
 */
function getDaysUntilNext(weekday, date) {
  return mod(weekday - date.getDay(), WEEK)
}

/**
 * just the modulo function, but always return the positive result
 */
function mod(n, m) {
  return ((n % m) + m) % m
}

/**
 * add <days> days to the <date>
 * but do it in a way that ignores daylight savings time
 */
function addDays(date, days) {
  date.setDate(date.getDate() + days)
  if (date.getHours() > 12) {
    date.setDate(date.getDate() + 1)
  } else if (date.getHours() !== 0) {
    date.setDate(date.getDate() - 1)
  }
}

/**
 * return a human readable representation of the date
 */
function formatDateInfo(date) {
  const dayNames = {
    "2": "Dienstag",
    "4": "Donnerstag",
  }

  const dayName = dayNames[date.getDay()]
  const isoDate = getISODateString(date)
  const weeks = weeksBetween(today, date)

  if (weeks === 0 && date.getDay() === today.getDay()) {
    return `Heute, ${isoDate}`
  } else if (weeks === 0) {
    return `Diese Woche ${dayName}, ${isoDate}`
  } else if (weeks === 1) {
    return `Nächste Woche ${dayName}, ${isoDate}`
  } else {
    return `${dayName} in ${weeks} Wochen, ${isoDate}`
  }
}

/**
 * how many sunday to monday transitions are between the two dates
 */
function weeksBetween(datetime1, datetime2) {
  const date1 = zeroizeTime(datetime1)
  const date2 = zeroizeTime(datetime2)
  const MILLISECONDS_IN_WEEK = 7 * 24 * 60 * 60 * 1000
  var weeks = Math.floor((date2 - date1) / MILLISECONDS_IN_WEEK)
  // if there is a sunday to monday transition between
  if (mod(date1.getDay() - 1, WEEK) > mod(date2.getDay() - 1, WEEK)) {
    weeks += 1
  }
  return weeks
}

function getISODateString(date) {
  const year = date.getFullYear()
  const month = date.getMonth() + 1
  const day = date.getDate()
  const monthPadded = (month < 10 ? "0" : "") + month
  const dayPadded = (day < 10 ? "0" : "") + day
  return `${year}-${monthPadded}-${dayPadded}`
}

function zeroizeTime(date) {
  const copy = new Date(date)
  copy.setHours(0)
  copy.setMinutes(0)
  copy.setSeconds(0)
  copy.setMilliseconds(0)
  return copy
}

// test, because this is complicated

function test() {
  testLateSunday()
  testYear2020()

  // reset to correct value
  today = new Date()
}

function testLateSunday() {
  today = new Date("2020-01-19T23:59:59+01:00")
  const result = formatDateInfo(getNextTopicDate())
  console.assert(
    result === "Nächste Woche Dienstag, 2020-01-21",
    `starting at ${getISODateString(
      today
    )}: was ${result}, expected "Nächste Woche Dienstag, 2020-01-21"`
  )
}

function testYear2020() {
  const topicsIn2020 = [
    "2020-01-02",
    "2020-01-21",
    "2020-02-06",
    "2020-02-18",
    "2020-03-05",
    "2020-03-17",
    "2020-04-02",
    "2020-04-21",
    "2020-05-07",
    "2020-05-19",
    "2020-06-04",
    "2020-06-16",
    "2020-07-02",
    "2020-07-21",
    "2020-08-06",
    "2020-08-18",
    "2020-09-03",
    "2020-09-15",
    "2020-10-01",
    "2020-10-20",
    "2020-11-05",
    "2020-11-17",
    "2020-12-03",
    "2020-12-15",
  ]
  today = zeroizeTime(new Date("2020-01-01"))
  let currentIndex = 0

  while (today <= new Date("2020-12-15")) {
    const result = getISODateString(getNextTopicDate())
    const expect = topicsIn2020[currentIndex]
    console.assert(
      result === expect,
      `starting at ${getISODateString(
        today
      )}: was ${result}, expected ${expect}`
    )
    if (getISODateString(today) === result) {
      currentIndex++
    }
    addDays(today, 1)
  }
}