Find Month Calendar

E
T
R
```/*
*  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)
}
}

export { Month }

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