Reverse number algorithm in pure Ruby. No string and array methods which are related to the ordering is allowed.

reverse_number.rb

# reverse_number(12345)
# => 54321
def reverse_number(number)
  raise 'Input must be number' unless number.is_a? Numeric
  
  reversed_numbers = []
  # do the operation until all the numbers are modulused and divided
  loop do
    # modulus by 10 to get the remaining and add them to the reverse_numbers array
    reversed_numbers << number % 10
    # divide the numbers by 10 to pull out the number which is already added to the reverse_numbers
    # when numbers / 10 equals to 0 means all the number are accessed
    break if (number = number / 10) == 0
  end

  sum    = 0 
  digit  = 1
  length = reversed_numbers.length - 1

  # reversly loop through the array
  (length).downto(0) do |i|
    # multiple digit by 10 based on the current number position
    digit = digit * 10 if i < length 
    # sum up all the values and multiply by the digit
    sum = sum + (reversed_numbers[i] * digit)
  end

  sum
end