"Special" day formats

gistfile1.rb

# http://www.timeanddate.com/date/special-dates.html
patterns = [
  "%Y-%m-%d",   # 2001-01-31
  "%y-%m-%d",   # 01-01-31
  "%Y-%-m-%-d", # 2001-1-31
  "%y-%-m-%-d", # 01-1-31

  "%m-%d-%Y",   # 01-31-2001
  "%m-%d-%y",   # 01-31-01
  "%-m-%-d-%Y", # 1-31-2001
  "%-m-%-d-%y", # 1-31-01

  "%d-%m-%Y",   # 31-01-2001
  "%d-%m-%y",   # 31-01-01
  "%-d-%-m-%Y", # 31-1-2001
  "%-d-%-m-%y", # 31-1-01
]

special = []

# http://www.ssa.gov/planners/lifeexpectancy.htm
days  = (82.1 * 365).to_i
start = Date.parse("1990-08-20")

days.times do |idx|
  date = start + idx
  special_date = false

  patterns.each do |pattern|
    formatted = date.strftime(pattern)
    split_formatted = formatted.gsub(/-/, '').split('')
    is_special = false

    # Palindrome
    palin = true
    split_formatted.each_with_index do |char, i|
      palin &= char == split_formatted[split_formatted.length - 1 - i]
    end
    is_special ||= palin

    # Sequential
    sections = formatted.split('-').map(&:to_i)
    is_special ||= (sections[0] == sections[1] + 1) && (sections[0] == sections[2] + 2)
    is_special ||= (sections[0] == sections[1] - 1) && (sections[0] == sections[2] - 2)

    # Repeating sequences
    (1..4).each do |length|
      groups = split_formatted.each_slice(length).to_a
      is_special ||= (groups.size > 1 && groups.uniq.size == 1)
    end

    if is_special
      puts formatted
      special_date = true
    end
  end

  if special_date
    special << date
  end
end

puts
puts "There are at least #{special.size} days in your life with special formatting (out of #{days} total)."
puts "That comes out to #{special.size.to_f / days}% of the time, or on average once every #{1 / (special.size.to_f / days)} days."