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Ruby Puzzle the Equilibrium Index

A zero-indexed array A consisting of N integers is given. An equilibrium index of this array is any integer P such that 0 ≤ P < N and the sum of elements of lower indices is equal to the sum of elements of higher indices, i.e.

A[0] + A[1] + ... + A[P−1] = A[P+1] + ... + A[N−2] + A[N−1].

Sum of zero elements is assumed to be equal to 0. This can happen if P = 0 or if P = N−1.
For example, consider the following array A consisting of N = 8 elements:

A[0] = -1
A[1] = 3
A[2] = -4
A[3] = 5
A[4] = 1
A[5] = -6
A[6] = 2
A[7] = 1

P = 1 is an equilibrium index of this array, because:
A[0] = −1 = A[2] + A[3] + A[4] + A[5] + A[6] + A[7]

P = 3 is an equilibrium index of this array, because:
A[0] + A[1] + A[2] = −2 = A[4] + A[5] + A[6] + A[7]

P = 7 is also an equilibrium index, because:
A[0] + A[1] + A[2] + A[3] + A[4] + A[5] + A[6] = 0

and there are no elements with indices greater than 7.

P = 8 is not an equilibrium index, because it does not fulfill the condition 0 ≤ P < N.

Write a function:
def solution(a)

that, given a zero-indexed array A consisting of N integers, returns any of its equilibrium indices. The function should return −1 if no equilibrium index exists.

For example, given array A shown above, the function may return 1, 3 or 7, as explained above.

Assume that:
N is an integer within the range [0..100,000];

each element of array A is an integer within the range [−2,147,483,648..2,147,483,647].

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SOLUTION

Puzzle wrote not a programmer obviously, because the numbers in programming languages are not separated by commas if they are integer.
so I wrote a decision based what is a set of elements.
and in the beginning of the code there is test class for Range.

#ruby2.2.3
def solution(a)
  eqi = []
  i = a.index{|x| x.class == Range}
  a[i] = (a[i].first).downto(a[i].last).map {|g| g} if a[i].first > a[i].last
  a[i] = a[i].to_a if a[i].first < a[i].last
  a.flatten!

  a.each_with_index do |e,index|
    a1 = a.slice(0,index + 1)
    a2 = a.slice(index,a.size)

    r1 = a1.inject(0) { |sum,x| sum + x }
    r2 = a2.inject(0) { |sum,x| sum + x }

    if r1 == r2
      eqi << true
    else
      eqi << false
    end
  end

  eqi.count(true) #count of the equilibrium index
end

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