Function problem19

Project-Euler/Problem019.js:21–46

()

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 }