Codility demo test (Ruby)

codility_demo_test.rb

def equi(a)
  lower_elements_sum = 0 
  higher_elements_sum = a.inject(:+)
  
  a.each_with_index do |el, i|
    lower_elements_sum += a[i - 1] if i > 0
    higher_elements_sum -= el
    
    return i if lower_elements_sum == higher_elements_sum
  end 

  return -1
end

describe "equi" do
  it "returns equilibrium index" do
    equi([-7, 1, 5, 2, -4, 3, 0]).should satisfy { |n| [3,6].include?(n) }
    equi([3, 2, 1, -6, 0]).should eql(4)
    equi([0, 3, 2, 1, -6]).should eql(0)
  end 
  it "returns -1 if no equilibrium index exists" do
    equi([1, 2, 3, 4, 5, 6]).should eql(-1)
  end 
end