euler_19.js

#!/usr/bin/js

/*
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)?
*/

var lengths = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
var day_counter = 2; // Jan. 1, 1901 --> Tuesday
var hit_counter = 0;
var year = 1901;

while (year <= 2000) {

    if (year % 4 == 0) {
        lengths[1] = 29;
        // For other centuries:
        //if (year % 100 == 0) and (year % 400 != 0) {
        //    lengths[1] = 28
        //}
    } else {
        lengths[1] = 28;
    }

    for (var i = 0; i < 12; i++) {
        if (day_counter % 7 == 0) {
            hit_counter += 1;
        }
        day_counter += lengths[i];
    }

    year += 1;
}
console.log(hit_counter);