Meetup

require 'date'

class Meetup
  DAYS_OF_WEEK = {
    sunday: 0,
    monday: 1,
    tuesday: 2,
    wednesday: 3,
    thursday: 4,
    friday: 5,
    saturday: 6,
  }.freeze

  START_DAYS = {
    first: 1,
    second: 8,
    teenth: 13,
    third: 15,
    fourth: 22,
    #
    # We don't know when last starts. Last day of the month
    # could be one of 28, 29, 30, or 31. We use `last_start_day`
    # to help figure this out.
    #
  }.freeze

  def initialize(month, year)
    @month = month
    @year = year
  end

  ##
  # @param day_of_week [Symbol]
  # @param nth [Integer]
  #
  def day(day_of_week, which)
    date = Date.new(@year, @month, start_day(which))

    6.times do
      break if date.wday == DAYS_OF_WEEK[day_of_week]

      date = date.next_day
    end

    date
  end

  private

  def last_start_day
    Date.new(@year, @month).next_month - 7
  end

  def start_day(which)
    return last_start_day.mday if which == :last

    START_DAYS[which]
  end
end

require 'date'

class Meetup
  VERSION = 3

  DAYS_OF_WEEK = {
    0 => :sunday,
    1 => :monday,
    2 => :tuesday,
    3 => :wednesday,
    4 => :thursday,
    5 => :friday,
    6 => :saturday,
  }.freeze

  def initialize(month, year)
    @month = month
    @year = year
    @days_in_month = (Date.new(year, 12, 31) << (12 - month)).day

    @calendar = {
      first: [*1..7],
      second: [*8..14],
      third: [*15..21],
      fourth: [*22..28],
      teenth: [*13..19],
      last: [*(@days_in_month - 6)..@days_in_month],
    }
  end

  def day(weekday, schedule)
    section_of_month = @calendar[schedule]

    is_weekday = lambda do |n|
      DAYS_OF_WEEK[Date.new(@year, @month, n).wday] == weekday
    end

    Date.new(@year, @month, section_of_month.find(&is_weekday))
  end
end

require 'date'

class Meetup
  ##
  # Map the symbols to the week numbers.
  #
  DAYS_OF_WEEK = %i[
    sunday monday tuesday wednesday thursday friday saturday
  ].map.with_index { |d, i| [d, i] }.to_h

  def initialize(month, year)
    @month = month
    @year = year
    days_in_month = (Date.new(year, month).next_month - 1).day

    @schedules = {
      first: 1,
      second: 8,
      third: 15,
      fourth: 22,
      teenth: 13,
      last: days_in_month - 6,
    }
  end

  def day(weekday, schedule)
    start = @schedules[schedule]
    wday = DAYS_OF_WEEK[weekday]

    d = Date.new(@year, @month, start)

    d + (wday - d.wday) + (wday < d.wday ? 7 : 0)
  end
end

require 'date'

class Meetup
  VERSION = 5

  CALENDAR_SCHEDULES = {
    first: 1,
    second: 8,
    third: 15,
    fourth: 22,
    teenth: 13,
  }.freeze

  DAYS_OF_WEEK = %i[
    sunday monday tuesday wednesday thursday friday saturday
  ].map.with_index { |d, i| [d, i] }.to_h

  def initialize(month, year)
    @month = month
    @year = year

    ##
    # The last schedule is month-specific.
    #
    days_in_month = (Date.new(year, month).next_month - 1).day
    @last_schedule = days_in_month - 6
  end

  # 2024, 2, :second
  # 2024, 2,
  def day(weekday, schedule)
    date_schedule_starts = Date.new(@year, @month, schedule_start(schedule))
    wday = DAYS_OF_WEEK[weekday]

    (1)
    wday += 7 if wday < date_schedule_starts.wday
    date_schedule_starts + (wday - date_schedule_starts.wday)
  end

  def schedule_start(schedule)
    return @last_schedule if schedule == :last

    CALENDAR_SCHEDULES[schedule] or
      raise "Unknown schedule: “#{schedule.inspect}”"
  end
end