class TZInfo::TZDataDayOfMonth

def initialize(spec)
  raise "Invalid on: #{spec}" if spec !~ /^([0-9]+)|(last([A-z]+))|(([A-z]+)([<>]=)([0-9]+))$/
  
  if $1
    @type = :absolute
    @day_of_month = $1.to_i
  elsif $3
    @type = :last
    @day_of_week = parse_day_of_week($3)
  else
    @type = :comparison
    @day_of_week = parse_day_of_week($5)
    @operator = parse_operator($6)
    @day_of_month = $7.to_i
  end
end

def parse_day_of_week(day_of_week)
  lower = day_of_week.downcase
  if lower =~ /^mon/
    1
  elsif lower =~ /^tue/
    2
  elsif lower =~ /^wed/
    3
  elsif lower =~ /^thu/
    4
  elsif lower =~ /^fri/
    5
  elsif lower =~ /^sat/
    6
  elsif lower =~ /^sun/
    0
  else
    raise "Invalid day of week: #{day_of_week}"
  end
end

def parse_operator(operator)
  if operator == '>='
    :greater_equal
  elsif operator == '<='
    :less_equal
  else
    raise "Invalid operator: #{operator}"
  end
end

def to_absolute(year, month)
  case @type
    when :last
      last_day_in_month = (Date.new(year, month, 1) >> 1) - 1          
      offset = last_day_in_month.wday - @day_of_week
      offset = offset + 7 if offset < 0
      last_day_in_month - offset
    when :comparison
      pivot = Date.new(year, month, @day_of_month)         
      offset = @day_of_week - pivot.wday
      offset = -offset if @operator == :less_equal
      offset = offset + 7 if offset < 0
      offset = -offset if @operator == :less_equal          
      pivot + offset
    else #absolute
      Date.new(year, month, @day_of_month)
  end
end