ruby_stuff.rb

# Nicholas Fine
# nicholas.fine@gmail.com
# This is just my general collection of useful methods, algorithms, etc.

##############################################################################
# Get the date for the nth weekday of a given month and year.  Call date_for_nth_weekday({2009,3,:monday,3}) to
# get the third monday of april in 2009
 def date_for_nth_weekday(h={:year, :month, :week_day, :nth})
    days_of_week = {:sunday => 0, :monday => 1, :tuesday => 2, :wednesday => 3, :thursday => 4, :friday => 5, :saturday => 6}
    twd = days_of_week[h[:week_day]]
    nth_wday_of_the_month(h[:year], h[:month], twd, h[:nth])
  end 

  def nth_wday_of_the_month(year, month, twd, n)
    day_check = Date.new(year, month, 1)
    while day_check.wday != twd
      day_check += 1.day
    end
    (n - 1).times do
      day_check += 7.days
    end
    return day_check
  end

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# Get the date for Easter sunday given a year.
  # Found easter algorithm at http://snippets.dzone.com/posts/show/7109 under a Creative Commons 3.0 license
  def easter(some_year)
    golden_number = (some_year % 19) + 1 

    if some_year <= 1752 then
      # Julian calendar
      dominical_number = (some_year + (some_year / 4) + 5) % 7 
      paschal_full_moon = (3 - (11 * golden_number) - 7) % 30
    else
      # Gregorian calendar
      dominical_number = (some_year + (some_year / 4) - (some_year / 100) + (some_year / 400)) % 7 
      solar_correction = (some_year - 1600) / 100 - (some_year - 1600) / 400 
      lunar_correction = (((some_year - 1400) / 100) * 8) / 25
      paschal_full_moon = (3 - 11 * golden_number + solar_correction - lunar_correction) % 30
    end 

    dominical_number += 7 until dominical_number > 0 

    paschal_full_moon += 30 until paschal_full_moon > 0 
    paschal_full_moon -= 1 if paschal_full_moon == 29 or (paschal_full_moon == 28 and golden_number > 11) 

    difference = (4 - paschal_full_moon - dominical_number) % 7 
    difference += 7 if difference < 0 

    day_easter = paschal_full_moon + difference + 1 

    if day_easter < 11 then
      # Easter occurs in March.
      return Date.new(y=some_year, m=3, d=day_easter + 21) 
    else
      #Easter occurs in April.
      return Date.new(y=some_year, m=4, d=day_easter - 10) 
    end 

  end
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