Peasant Multiplication (https://www.cut-the-knot.org/Curriculum/Algebra/PeasantMultiplication.shtml)

peasant_multiplication.rb

def multiplication(a,b)
    m = {}
    while a >= 1
        m[a] = b
        a = a / 2
        b = b * 2
    end
    m.select { |k, _| k % 2 != 0 }.values.sum
end

peasant_multiplication_spec.rb

describe 'RPM' do
    it 'multiplication 11 * 11' do
        expect(main(11, 11)).to eq(121)
    end
end