Created
January 29, 2010 22:31
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Simple implementation of Russian Peasant Multiplication with tests
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# http://mathforum.org/dr.math/faq/faq.peasant.html | |
# http://en.wikipedia.org/wiki/Ancient_Egyptian_multiplication | |
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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