Feb 1, 2010 
Russian Peasant Multiplication: a very simple and elegant way to multiple.
Read more: School-boy example of how and why it works and about Ancient Egyptian Multiplication on wikipedia
So here’s my simple implementation: (or as a gist on github)
module RussianPeasantMultiplication
def russian_peasant_multiply(b)
numbers_to_add = []
a, b = [self, b].sort #So we have the smaller number as the first
negative_operands = [a, b].select { |n| n < 0 }
result_should_be_negative = negative_operands.size.odd? # or negative_operands.size == 1
# Now get the absoultes
a, b = [a, b].map { |n| n.abs }
while( a > 1 )
a = a >> 1 # halv it
b = b < < 1 # double it
if a.odd? # or (a % 2 == 0 )
numbers_to_add << b
else
end
end
result = numbers_to_add.inject(0) { |sum, n| sum += n }
result = result * -1 if result_should_be_negative
result
end
end
class Integer
include RussianPeasantMultiplication
end
# Tests
require "test/unit"
class TestInteger < Test::Unit::TestCase
def test_russian_peasant_multiply
assert_equal(22 * 70, 22.russian_peasant_multiply(70))
end
def test_russian_peasant_multiply_for_negative_numbers
assert_equal(-22 * 70, -22.russian_peasant_multiply(70))
end
def test_russian_peasant_multiply_for_negative_arguments
assert_equal(22 * -70, 22.russian_peasant_multiply(-70))
end
def test_russian_peasant_multiply_for_negative_numbers_and_arguments
assert_equal(-22 * -70, -22.russian_peasant_multiply(-70))
end
def test_russian_peasant_multiply_for_zero
assert_equal(0 * -70, 0.russian_peasant_multiply(-70))
end
def test_russian_peasant_multiply_for_zero_arguments
assert_equal(-22 * 0, -22.russian_peasant_multiply(0))
end
def test_russian_peasant_multiply_for_zero_numbers_and_arguments
assert_equal(0 * 0, 0.russian_peasant_multiply(0))
end
end
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for a birthday gift! Thanks Ujwala!