FindMonthCalendar.js

/*
 * 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.
 */
import { isLeapYear } from '../Maths/LeapYear'

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
 }

 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 += isLeapYear(startDate.year) ? 366 : 365
 startDate.year = startDate.year + 1
 }
 while (startDate.month !== endDate.month) {
 if (startDate.month < endDate.month) {
 if (isLeapYear(startDate.year))
 diff += this.monthDaysLeap[startDate.month]
 else diff += this.monthDays[startDate.month]
 startDate.month = startDate.month + 1
 } else {
 if (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 (isLeapYear(Month2.year))
 this.printCal(this.monthDaysLeap[Month2.month], day)
 else this.printCal(this.monthDays[Month2.month], day)
 }
}

export { Month }

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