Ruby equilibrium index - odd usage of inject

I spent a bit of time today tackling the equilibrium index problem (described here)

After writing my own solution (which performed badly with large numbers), I decided to find one that would be a perfect score. I found this (which Codility scores as 100/100):

  def equi(a)
    left, right = 0, a.inject(0, &:+)
    indices = []
    a.each_with_index do |val, i|
      right -= val

      indices << i if right == left

      left += val
    end
    indices
  end

What I don't understand is the piece of parallel assignment and use of inject at the top of the method. Is anyone able to describe what this is doing?

Many thanks! Stu

It assigns 0 to left and the sum of a's elements to right. The 0 argument is there because otherwise an empty array would return nil. The shorthand used for summing is Symbol#to_proc and is unnecessary because inject directly takes a symbol as its argument - inject(0, :+).