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

water_and_blocks.ex

defmodule Rain do
  def count_blocks_with_water(blocks) do
    Enum.map(Enum.max(blocks)..0, &(&1)) |>
      Enum.reduce 0, fn(level, sum) ->

        sum +
          (Enum.with_index(blocks) |>
          Enum.filter(fn({block, _}) ->
            block >= level
          end) |>
          Enum.map(fn({_, idx}) ->
            idx
          end) |>
          each_cons |>
          Enum.reduce 0, fn({from, to}, accu) ->
            accu + to - from - 1
          end)

      end
  end


  def each_cons(list) do
    {cons, _} = List.foldl list, {[], nil}, fn
      (e, {result, nil})      -> {result, e}
      (e, {result, previous}) -> {[{previous, e} | result], e}
    end

    Enum.reverse cons
  end

end

ExUnit.start

defmodule Tests do
  use ExUnit.Case

  #  X
  # XXX


  test "case 1" do
    assert Rain.count_blocks_with_water([1, 2, 1]) == 0
  end

  # XXX
  # XXX


  test "case 2" do
    assert Rain.count_blocks_with_water([2, 2, 2]) == 0
  end

  # X X
  # XXX


  test "case 3" do
    assert Rain.count_blocks_with_water([2, 1, 2]) == 1
  end

  # X X
  # X X


  test "case 4" do
    assert Rain.count_blocks_with_water([2, 0, 2]) == 2
  end

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


  test "case 5" do
    assert Rain.count_blocks_with_water([1, 2, 1, 0, 0, 4, 3, 1, 1, 3, 2, 6]) == 15
  end

  test "case 6" do
    [
      {[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}
    ] |> Enum.each fn({result, blocks}) ->
          assert Rain.count_blocks_with_water(result) == blocks
         end

  end

end