介绍

给定 mm/yyyy 的月份，打印次月份的月历。

实现

JavaScript

/*
*  This algorithm accepts a month in the format mm/yyyy.
*  And prints out the month's calendar.
*  It uses an epoch of 1/1/1900, Monday.
*/

class Month {
  constructor () {
    this.Days = ['M', 'T', 'W', 'Th', 'F', 'S', 'Su']
    this.BDays = ['M', 'Su', 'S', 'F', 'Th', 'W', 'T']
    this.epoch = { month: 1, year: 1900 }
    this.monthDays = [31, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
    this.monthDaysLeap = [31, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
  }

  printCal (days, startDay, output = value => console.log(value)) {
    output('M   T   W   Th  F   S   Su')
    const dates = []; let i;
    for (i = 1; i <= days; i++) {
      dates.push(i)
    }
    for (i = 0; i < this.Days.indexOf(startDay); i++) {
      dates.unshift(' ')
    }
    while (true) {
      let row = ''
      for (i = 0; (i < 7) && (dates.length !== 0); i++) {
        row += dates.shift()
        while ((row.length % 4) !== 0) {
          row += ' '
        }
      }
      output(row)
      if (dates.length === 0) break
    }
  }

  parseDate (date) {
    const dateAr = []; let block = ''; let i
    for (i = 0; i < date.length; i++) {
      if (date[i] === '/') {
        dateAr.push(parseInt(block))
        block = ''
        continue
      }
      block += date[i]
    }
    dateAr.push(parseInt(block))
    if (dateAr.length !== 2) throw new Error('Improper string encoding')
    const dateOb = { month: dateAr[0], year: dateAr[1] }
    return dateOb
  }

  isLeapYear (year) {
    if (((year % 400) === 0) || (((year % 100) !== 0) && ((year % 4) === 0))) return true
    return false
  }

  isGreater (startDate, endDate) {
    if (startDate.year > endDate.year) {
      return true
    } else if (startDate.year < endDate.year) {
      return false
    } else if (startDate.month > endDate.month) {
      return true
    } else if (startDate.month < endDate.month) {
      return false
    }
    return true
  }

  getDayDiff (startDate, endDate) {
    if (this.isGreater(startDate, endDate) === null) {
      return 0
    } else if ((this.isGreater(startDate, endDate) === true)) {
      const midDate = startDate
      startDate = endDate
      endDate = midDate
    }
    let diff = 0
    while (startDate.year !== endDate.year) {
      diff += (this.isLeapYear(startDate.year)) ? 366 : 365
      startDate.year = startDate.year + 1
    }
    while (startDate.month !== endDate.month) {
      if (startDate.month < endDate.month) {
        if (this.isLeapYear(startDate.year)) diff += this.monthDaysLeap[startDate.month]
        else diff += this.monthDays[startDate.month]
        startDate.month = startDate.month + 1
      } else {
        if (this.isLeapYear(startDate.year)) diff -= this.monthDaysLeap[startDate.month - 1]
        else diff -= this.monthDays[startDate.month - 1]
        startDate.month = startDate.month - 1
      }
    }
    return diff
  }

  generateMonthCal (date) {
    const Month = this.parseDate(date); let day = ''
    let difference = this.getDayDiff(this.epoch, Month)
    difference = difference % 7
    let Month2 = this.parseDate(date)
    day = (this.isGreater(Month2, this.epoch)) ? this.Days[difference] : this.BDays[difference]
    Month2 = this.parseDate(date)
    if (this.isLeapYear(Month2.year)) this.printCal(this.monthDaysLeap[Month2.month], day)
    else this.printCal(this.monthDays[Month2.month], day)
  }
}

// const x = new Month()
// x.generateMonthCal('1/2021')

扩展

通过年月计算月中天数

const getMonthDays = (monthNumber, year) => {
  const the31DaysMonths = [1, 3, 5, 7, 8, 10, 12]
  const the30DaysMonths = [4, 6, 9, 11]

  if (!the31DaysMonths.includes(monthNumber) && !the30DaysMonths.includes(monthNumber) && (monthNumber !== 2)) {
    throw new TypeError('Invalid Month Number.')
  }

  if (the31DaysMonths.includes(monthNumber)) return 31 
  if (the30DaysMonths.includes(monthNumber)) return 30 

  // Check for Leap year
  // (year % 400) === 0) || (((year % 100) !== 0) && ((year % 4) === 0))
  if (year % 4 === 0) {
    if ((year % 100 !== 0) || (year % 100 === 0 && year % 400 === 0)) {
      return 29
    }
  }

  return 28
}

计算两日相差天数

/*
    DateDayDifference Method
    ------------------------
    DateDayDifference method calculates the number of days between two dates.

    Algorithm & Explanation : https://ncalculators.com/time-date/date-difference-calculator.htm
*/

// Internal method for make calculations easier
const isLeap = (year) => {
  if (year % 400 === 0) return true
  else if (year % 100 === 0) return false
  else if (year % 4 === 0) return true
  else return false
}
const DateToDay = (dd, mm, yyyy) => {
  return Math.floor((365 * (yyyy - 1)) + ((yyyy - 1) / 4) - ((yyyy - 1) / 100) + ((yyyy - 1) / 400) + dd + (((367 * mm) - 362) / 12) + (mm <= 2 ? 0 : isLeap(yyyy) ? -1 : -2))
}

const DateDayDifference = (date1, date2) => {
  // firstly, check that both input are string or not.
  if (typeof date1 !== 'string' && typeof date2 !== 'string') {
    return new TypeError('Argument is not a string.')
  }
  // extract the first date
  const [firstDateDay, firstDateMonth, firstDateYear] = date1.split('/').map((ele) => Number(ele))
  // extract the second date
  const [secondDateDay, secondDateMonth, secondDateYear] = date2.split('/').map((ele) => Number(ele))
  // check the both data are valid or not.
  if (firstDateDay < 0 || firstDateDay > 31 ||
        firstDateMonth > 12 || firstDateMonth < 0 ||
        secondDateDay < 0 || secondDateDay > 31 ||
        secondDateMonth > 12 || secondDateMonth < 0) {
    return new TypeError('Date is not valid.')
  }
  return Math.abs(DateToDay(secondDateDay, secondDateMonth, secondDateYear) - DateToDay(firstDateDay, firstDateMonth, firstDateYear))
}

// Example : DateDayDifference('17/08/2002', '10/10/2020') => 6630

计算某日周几

/*
                    DateToDay Method
                    ----------------
    The DateToDay method takes a date in string format and
    returns the name of a day. The approach behind this method
    is very simple, we first take a string date and check
    whether their date is valid or not, if the date is valid
    then we do this But apply the algorithm shown below. The
    algorithm shown below gives us the number of the day and
    finally converts it to the name of the day.

    Algorithm & Explanation : https://cs.uwaterloo.ca/~alopez-o/math-faq/node73.html
*/

// March is taken as the first month of the year.
const calcMonthList = {
  1: 11,
  2: 12,
  3: 1,
  4: 2,
  5: 3,
  6: 4,
  7: 5,
  8: 6,
  9: 7,
  10: 8,
  11: 9,
  12: 10
}

// show the week day in a number : Sunday - Saturday => 0 - 6
const daysNameList = { // weeks-day
  0: 'Sunday',
  1: 'Monday',
  2: 'Tuesday',
  3: 'Wednesday',
  4: 'Thursday',
  5: 'Friday',
  6: 'Saturday'
}

const DateToDay = (date) => {
  // firstly, check that input is a string or not.
  if (typeof date !== 'string') {
    return new TypeError('Argument is not a string.')
  }
  // extract the date
  const [day, month, year] = date.split('/').map((x) => Number(x))
  // check the data are valid or not.
  if (day < 0 || day > 31 || month > 12 || month < 0) {
    return new TypeError('Date is not valid.')
  }
  // divide year to century and yearDigit value.
  const yearDigit = (year % 100)
  const century = Math.floor(year / 100)
  // Apply the algorithm shown above
  const weekDay = Math.abs((day + Math.floor((2.6 * calcMonthList[month]) - 0.2) - (2 * century) + yearDigit + Math.floor(yearDigit / 4) + Math.floor(century / 4)) % 7)
  // return the weekDay name.
  return daysNameList[weekDay]
}

// Example : DateToDay("18/12/2020") => Friday