5/20/15 - Peasant Multiplication

Today in math, we learned about a method of multiplication called Peasant Multiplication that works for positive whole numbers, and is pretty cool. Here's a rundown on how it works, although it's kind of hard to understand and I'm not sure how well the website explains it.

I implemented this in ruby!

	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

view raw 1-peasant_multiplication_method.rb hosted with ❤ by GitHub

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

view raw 2-example_usage.rb hosted with ❤ by GitHub

View it in action here by pressing the run button

The full code I wrote is here, which is extensible in case I get around to testing the efficiency of this algorithm compared to other methods.