19.rb

#!/usr/bin/ruby -w
#
# Problem 19
# 14 June 2002
#
# 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)?

require 'date'

d = Date.civil(1901, 1, 1)
sundays = 0
end_of_century = Date.civil(2000, 12, 31)

while d < end_of_century
  sundays += 1 if d.wday == 0
  d = d >> 1
end

puts sundays