1-peasant_multiplication_method.rb

  def multiply(number1, number2)
    # convert number1 to binary and reverse it
    # so that the most significant bit is last
    n1_binary_str = number1.to_s(2).reverse
    binary_array  = n1_binary_str.split '' # convert string to array
    
    add_amount = nil
    total = 0
    binary_array.each_with_index do |char, index| 
      # double the add_amount, or set it to number2 if it is nil
      add_amount = add_amount ? (add_amount + add_amount) : number2 
 
      # add add_amount to the total if the binary for
      # number2 has a binary 1 in given place
      total += add_amount if char == '1'
    end
    return total
  end

2-example_usage.rb

multiply 1, 2
#=> 2
multiply 400, 2
#=> 800
multiply 400, 20
#=> 8000
multiply 400, 200
#=> 80000