Ruby implementation of Cal (printing calendar)

cal.rb

# This was a fun exercise to recreate the bash tool `cal`, which prints a calendar of the current month in your terminal

require 'minitest/autorun'
require 'date'

class Cal
  def print_calendar(date = Date.today)
    puts calendar(date)
  end

  def calendar(date)
    [
      calendar_title(date),
      'Su Mo Tu We Th Fr Sa',
      format_days_of_month(date).each_slice(7).map { |slice| slice.join(' ') },
      ''
    ].join("\n")
  end

  private

  def format_days_of_month(date)
    days_in_month = Date.new(date.year, date.month, -1).day
    first_day_day_of_week = Date.new(date.year, date.month, 1).wday
    padding = Array.new(first_day_day_of_week, '')
    month_with_padding = padding + (1..days_in_month).to_a
    month_with_padding.map do |day|
      day.to_s.rjust(2)
    end
  end

  def calendar_title(date)
    date.strftime("%B %Y").center(20)
  end
end

# Running the file will print today's date
Cal.new.print_calendar(Date.today)


# Running the file will also automatically run the tests
class TestCal < Minitest::Test
  describe '#calendar' do

    it 'prints April 2020 correctly' do
      # Test data has hardcoded `\s` instead of spaces because many text editors auto-trim whitespace
      april_calendar =  <<-CALENDAR
     April 2020\s\s\s\s\s
Su Mo Tu We Th Fr Sa
          1  2  3  4
 5  6  7  8  9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29 30
CALENDAR
      assert_equal(
        april_calendar,
        Cal.new.calendar(Date.parse('2020-04-01'))
      )
    end
  end
end