Week bitmask / bitarray

week.rb

require 'set'

class Week
  DAYS = %w[Mon Tue Wed Thu Fri Sat Sun]

  REVERTED_DAYS = DAYS.reverse

  attr_reader :day_bitmask

  def self.new(*days)
    from_days(*days)
  end

  def self.from_days(*days)
    instance = allocate
    instance.instance_eval { @day_bitmask = days_to_bitmask(*days) }
    instance
  end

  def self.from_bitmask(int)
    instance = allocate
    instance.instance_eval { @day_bitmask = int }
    instance
  end

  def to_bitmask
    @day_bitmask
  end

  # An integer acts like a bitarray.
  # This first transforms the bitmask to a bitarray, and then takes the
  # days whose corresponsing bit is set to 1.
  def to_a
    DAYS.select.with_index { |_, i| day_bitmask[i] == 1 }
  end

  def days
    to_a
  end

  private

  def days_to_bitmask(*days)
    bitarray = days_to_bitarray(*days)
    bitarray_to_bitmask(bitarray)
  end

  def days_to_bitarray(*days)
    # This is probably super yagni
    days = Set.new(days)
    REVERTED_DAYS.map { |day| days.include?(day) ? 1 : 0 }
  end

  def bitarray_to_bitmask(ba)
    ba.join.to_i(2)
  end
end

Week.from_days('Mon', 'Thu', 'Fri', 'Sat').to_bitmask # => 57
Week.from_days('Mon', 'Thu', 'Sun').to_bitmask # => 73
Week.from_days('Mon', 'Thu', 'Sat').to_bitmask # => 41
Week.from_days('Fri', 'Sat', 'Sun').to_bitmask # => 112
Week.from_days('Mon', 'Thu').to_bitmask # => 9
Week.from_days('Thu').to_bitmask # => 8

week = Week.from_bitmask(112) # => #<Week:0x007f7c7db5e428 @day_bitmask=112>
week.to_a # => ["Fri", "Sat", "Sun"]

# Has all? => & operator
# Yes => value & target == value
41 & 57  # => 41
9 & 57  # => 9
8 & 57  # => 8
# No => value & target != value
41 & 73 # => 9
9 & 112 # => 0

# Has any?
# Yes => value & target > 0
9 & 57  # => 9
112 & 57 # => 48
# No => value & target == 0
9 & 112 # => 0
8 & 112 # => 0

#
# Old version
# def to_a
#   day_bitmask
#     .to_s(2)
#     .rjust(7, '0')
#     .reverse
#     .each_char
#     .with_index
#     .with_object([]) { |(bit, i), result|
#       result << DAYS[i] if bit == '1'
#     }
# end

# Old version
# DAYS_WITH_INDEX = Hash[DAYS.map.with_index { |day, idx| [day, idx] }]
# def days_to_bitmask(*days)
#   # compact because sadly values_at(missing_key) returns [nil]
#   DAYS_WITH_INDEX.values_at(*days).compact.reduce(0) do |result, day_idx|
#     result + 2**day_idx
#   end
# end