Peasant multiplication algorithm implemented in ruby. You can check this out in action at http://repl.it/onr

1-peasant_multiplication.rb

class PeasantMultiplication
  ## multiply using the Peasant Multiplication Algorithm
  def self.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 = number2

    # traverses the binary_array and calculates the total
    return binary_array.reduce(0) do |total, char|
      # add add_amount to the total if the binary for
      # number2 has a 1 in given place
      total += add_amount if char == '1'

      # double the add_amount each time
      add_amount = add_amount + add_amount
      total
    end

  end
end

## This class verifies the correctness of a given multiplication algorithm
class CheckCorrectness
  MultCase = Struct.new(:n1, :n2, :ans)

  ## initialize array of test cases for multiplication with a range of n numbers
  def initialize(n)
    @check_algorithm = []
    (0..n).each do |n1|
      (0..n).each do |n2|
        @check_algorithm << MultCase.new(n1, n2, n1*n2)
      end
    end
  end

  def check_correctness(multiplier)
    @check_algorithm.each { |mult|
      result = multiplier.multiply(mult.n1, mult.n2)
      if (result != mult.ans)
        raise "ERROR: #{mult.n1} * #{mult.n2} calculated to be #{result}, expected #{mult.ans}"
        end
    }
    puts "All examples passed!"
  end
end

ckc = CheckCorrectness.new 250 # check multiplication with numbers 1 to 100
ckc.check_correctness PeasantMultiplication # this will output "All examples passed!"

2-example_usage.rb

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