Emulate array insertion but allowing repeated numbers

insersection.rb

def intersection(a, b)
  hash = {}
  c = []

  a.each do |x|
    hash[x] ? hash[x] += 1 : hash[x] = 1
  end

  b.each do |x|
    c << x && hash[x] -= 1 if hash[x] && hash[x] > 0
  end

  c
end

describe '#insertion' do
  context 'positive numbers' do
    let(:a){ [3,6,5,12,9,4,6] }
    let(:b){ [6,8,6,3,1,1] }

    it 'gets the insertion' do
      expect(insertion(a,b)).to eq [6,6,3]
    end
  end

  context 'negative numbers' do
    let(:a){ [3,-6,-5,12,9,4,6,-5] }
    let(:b){ [-6,-5,6,3,1,1,-5] }

    it 'gets the insertion' do
      expect(insertion(a,b)).to eq [-6, -5, 6, 3, -5]
    end
  end
end