max_difference.rb

require 'pry'

def maxDifference(a)
  min = 0
  max_diff = 0

  a.each_with_index do |value, index|
    if value < a[min]
      min = index
    end
    diff = value - a[min]

    if diff > max_diff
      max_diff = diff
    end
  end

  # If max_diff is 0 then we had a strictly decreasing-in-size list of elements.
  max_diff == 0 ? -1 : max_diff
end

require 'minitest/autorun'

class MaxDifferenceTest < Minitest::Test
  def test_max_difference_is_positive
    diff = maxDifference([2, 3, 10, 2, 4, 8, 1])
    assert diff == 8
  end

  def test_no_max_difference
    diff = maxDifference([5, 4, 3, 2, 1])
    assert diff == -1
  end
end