Find partial consecutive sum

partial_consecutive_sum.rb

def find_partial_sum(array, goal)
  istart, iend, sum = 0, 0, 0
  while iend < array.length
    while iend < array.length && sum < goal
      sum += array[iend]
      iend += 1
    end

    while istart < iend && sum > goal
      sum -= array[istart]
      istart += 1
    end

    if sum == goal
      return array[istart...iend]
    end
  end

  return nil
end

puts find_partial_sum([2, 1, 3, 2, 1], 5).inspect