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/**

* Problem 19 - Counting Sundays

* @see {@link https://projecteuler.net/problem=19}

* You are given the following information, but you may prefer to do some research for yourself.

* 1 Jan 1900 was a Monday.

* Thirty days has September,

* April, June and November.

* All the rest have thirty-one,

* Saving February alone,

* Which has twenty-eight, rain or shine.

* And on leap years, twenty-nine.

* A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.

* How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?

* @author ddaniel27

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

function problem19() {

let sundaysCount = 0 // Count of Sundays

let dayOfWeek = 2 // 1st Jan 1900 was a Monday, so 1st Jan 1901 was a Tuesday

for (let year = 1901; year <= 2000; year++) {

for (let month = 1; month <= 12; month++) {

if (dayOfWeek === 0) {

// If it's a Sunday (0 is Sunday, 1 is Monday, ..., 6 is Saturday)

sundaysCount++

}

let daysInMonth = 31

if (month === 4 || month === 6 || month === 9 || month === 11) {

// April, June, September, November

daysInMonth = 30

} else if (month === 2) {

// February

daysInMonth = isLeapYear(year) ? 29 : 28

}

dayOfWeek = (dayOfWeek + daysInMonth) % 7 // Calculate the day of the week

}

return sundaysCount

}

export { problem19 }