My take on http://qandwhat.apps.runkite.com/i-failed-a-twitter-interview/

water_and_blocks.rb

def count_blocks_with_water blocks
  blocks.max.downto(0).reduce(0){|sum, level|
    sum +
      blocks.each_with_index.select{|block, idx|
        block >= level
      }.map{|_, idx|
        idx
      }.each_cons(2).reduce(0){|accu, tuple|
        accu + tuple[1] - tuple[0] - 1
      }
  }
end


require 'minitest/unit'

class CountBlocks < MiniTest::Unit::TestCase

  #  X
  # XXX

  def test_case1
    assert_equal 0, count_blocks_with_water([1, 2, 1])
  end

  # XXX
  # XXX

  def test_case2
    assert_equal 0, count_blocks_with_water([2, 2, 2])
  end


  # X X
  # XXX

  def test_case3
    assert_equal 1, count_blocks_with_water([2, 1, 2])
  end


  # X X
  # X X

  def test_case4
    assert_equal 2, count_blocks_with_water([2, 0, 2])
  end


  #            X
  #            X
  #      X     X
  #      XX  X X
  #  X   XX  XXX
  # XXX  XXXXXXX

  def test_case5
    assert_equal 15, count_blocks_with_water([1, 2, 1, 0, 0, 4, 3, 1, 1, 3, 2, 6])
  end


  def test_case6
    # Stolen from https://gist.github.com/igor47/7228586
    [
      [[1,0,1],                  1],
      [[5,0,5],                  5],
      [[0,1,0,1,0],              1],
      [[1,0,1,0],                1],
      [[1,0,1,2,0,2],            3],
      [[2,5,1,2,3,4,7,7,6],     10],
      [[5,1,0,1],                1],
      [[2,5,1,2,3,4,7,7,6,3,5], 12]
    ].each do |blocks, result|

      assert_equal result, count_blocks_with_water(blocks)

    end
  end

end

MiniTest::Unit.autorun