100% scorer for Ruby on Codility's 'equi' test

equi.rb

# http://codility.com/ for info

def equi(a)
  pres, posts = 0, a.reduce(:+)

  0.upto(a.length - 1) do |pivot|
    pres += (pivot > 0 ? a[pivot-1] : 0)
    posts -= a[pivot]
    return pivot if pres == posts
  end
  
  -1
end

#puts equi([-7, 1, 5, 2, -4, 3, 0])

Cool; thanks! My first attempt got 43% as it failed on a couple of edge cases, and all of the performance tests :-(
Good to learn about the better solution.

def equi(a)
  left, right = 0, a.inject{|sum, k| sum += k}

  (0...a.length).each do |j|
    # deduct right part
    right = right - a[j]
    # skip first iterate for left
    left = left + a[j-1] if j > 0

    return j if left == right
  end

  -1
end

@mikhailov a.inject{|sum, k| sum += k} is long form of a.reduce(:+) or a.reduce(&:+)