require 'rational'

class DateTime
  class << self
    # DateTimes aren't aware of DST rules, so use a consistent non-DST offset when creating a DateTime with an offset in the local zone
    def local_offset
      ::Time.local(2007).utc_offset.to_r / 86400
    end

    def current
      ::Time.zone_default ? ::Time.zone.now.to_datetime : ::Time.now.to_datetime
    end
  end

  # Tells whether the DateTime object's datetime lies in the past
  def past?
    self < ::DateTime.current
  end

  # Tells whether the DateTime object's datetime lies in the future
  def future?
    self > ::DateTime.current
  end

  # Seconds since midnight: DateTime.now.seconds_since_midnight
  def seconds_since_midnight
    sec + (min * 60) + (hour * 3600)
  end

  # Returns a new DateTime where one or more of the elements have been changed according to the +options+ parameter. The time options
  # (hour, minute, sec) reset cascadingly, so if only the hour is passed, then minute and sec is set to 0. If the hour and
  # minute is passed, then sec is set to 0.
  def change(options)
    ::DateTime.civil(
      options[:year]  || year,
      options[:month] || month,
      options[:day]   || day,
      options[:hour]  || hour,
      options[:min]   || (options[:hour] ? 0 : min),
      options[:sec]   || ((options[:hour] || options[:min]) ? 0 : sec),
      options[:offset]  || offset,
      options[:start]  || start
    )
  end

  # Uses Date to provide precise Time calculations for years, months, and days.
  # The +options+ parameter takes a hash with any of these keys: <tt>:years</tt>,
  # <tt>:months</tt>, <tt>:weeks</tt>, <tt>:days</tt>, <tt>:hours</tt>,
  # <tt>:minutes</tt>, <tt>:seconds</tt>.
  def advance(options)
    d = to_date.advance(options)
    datetime_advanced_by_date = change(:year => d.year, :month => d.month, :day => d.day)
    seconds_to_advance = (options[:seconds] || 0) + (options[:minutes] || 0) * 60 + (options[:hours] || 0) * 3600
    seconds_to_advance == 0 ? datetime_advanced_by_date : datetime_advanced_by_date.since(seconds_to_advance)
  end

  # Returns a new DateTime representing the time a number of seconds ago
  # Do not use this method in combination with x.months, use months_ago instead!
  def ago(seconds)
    since(-seconds)
  end

  # Returns a new DateTime representing the time a number of seconds since the instance time
  # Do not use this method in combination with x.months, use months_since instead!
  def since(seconds)
    self + Rational(seconds.round, 86400)
  end
  alias :in :since

  # Returns a new DateTime representing the start of the day (0:00)
  def beginning_of_day
    change(:hour => 0)
  end
  alias :midnight :beginning_of_day
  alias :at_midnight :beginning_of_day
  alias :at_beginning_of_day :beginning_of_day

  # Returns a new DateTime representing the end of the day (23:59:59)
  def end_of_day
    change(:hour => 23, :min => 59, :sec => 59)
  end

  # Adjusts DateTime to UTC by adding its offset value; offset is set to 0
  #
  # Example:
  #
  #   DateTime.civil(2005, 2, 21, 10, 11, 12, Rational(-6, 24))       # => Mon, 21 Feb 2005 10:11:12 -0600
  #   DateTime.civil(2005, 2, 21, 10, 11, 12, Rational(-6, 24)).utc   # => Mon, 21 Feb 2005 16:11:12 +0000
  def utc
    new_offset(0)
  end
  alias_method :getutc, :utc

  # Returns true if offset == 0
  def utc?
    offset == 0
  end

  # Returns the offset value in seconds
  def utc_offset
    (offset * 86400).to_i
  end

  # Layers additional behavior on DateTime#<=> so that Time and ActiveSupport::TimeWithZone instances can be compared with a DateTime
  def compare_with_coercion(other)
    other = other.comparable_time if other.respond_to?(:comparable_time)
    other = other.to_datetime unless other.acts_like?(:date)
    compare_without_coercion(other)
  end
  alias_method :compare_without_coercion, :<=>
  alias_method :<=>, :compare_with_coercion
end