Bitwise operation to get the weekday name

Question

The function days_name receive a val (string) corresponding to weekdays in hexadecimal.

def days_name(val)
    value = val.to_i(16)
    week = Array.new

    if value.eql? 127
      week.push 'All'
    elsif value.eql? 0
      week.push 'No'
    else
      week.push 'mon' if (value & 1) > 0
      week.push 'tue' if (value & 2) > 0
      week.push 'wed' if (value & 4) > 0
      week.push 'thu' if (value & 8) > 0
      week.push 'fri' if (value & 16) > 0
      week.push 'sar' if (value & 32) > 0
      week.push 'sun' if (value & 64) > 0
    end
    week.join(' ')
end

Answer 1

• days_name? Shouldn't that rather be day_names? Actually, given its function it should probably be weekdays

• Shouldn't "sar" be "sat" for Saturday? Never seen it abbreviated "sar"

• The opposite of "all" is "none" - not "no"

• Day names are capitalized in English, so "Mon", "Tue", "Wed", etc.

• You rarely need to create an array and then push items to it. It's easier to simply filter a complete array. But if you do need to create an empty array, just use [] - no need for Array.new

Sticking close to your current code, I'd do this

def weekdays(hex)
  value = hex.to_i(16)
  case value
  when 127
    'All'
  when 0
    'None'
  else
    days = %w(Mon Tue Wed Thu Fri Sat Sun)
    days.each_with_index.select { |_, i| value & 2**i > 0 }.map(&:first).join(" ")
  end
end

However, I'd probably prefer a method that simply returns an array of day names, rather than a string. Then let other code check and join the array as needed.

Something like this

def weekdays(hex)
  value = hex.to_i(16)
  days = %w(Mon Tue Wed Thu Fri Sat Sun)
  days.each_with_index.select { |_, i| value & 2**i > 0 }.map(&:first)
end

---

Update: As tokland points out in the comments, it'd be cleaner to use map.with_index plus compact, rather than each_with_index.select plus map. I.e.

def weekdays(hex)
  value = hex.to_i(16)
  days = %w(Mon Tue Wed Thu Fri Sat Sun)
  days.map.with_index { |day, i| day if value & 2**i > 0 }.compact
end

or, for the first code block

def weekdays(hex)
  value = hex.to_i(16)
  case value
  when 127
    'All'
  when 0
    'None'
  else
    days = %w(Mon Tue Wed Thu Fri Sat Sun)
    days.map.with_index { |day, i| day if value & 2**i > 0 }.compact.join(" ")
  end
end

---

As mentioned in the comments, I happened to make a gem for weekday bitmasks a while back, which you might find useful. The major difference is that the gem expects Sunday to be zero, Monday to be 1 and so forth (like Date#wday and most other date APIs).