defmodule TutorialD do
#  @spec (list +union+ term) :: maybe_improper_list
  ##
  ## row is Dict(columnname, value)
  ## table is HashSet(row)
  def union(left,right) do
    Set.union(left, right)
  end
  def minus(left, right) do
    Set.difference(left, right)
  end
  def intersect(left, right) do
    Set.intersection(left, right)
  end
  def where(left, f) do
    Stream.filter(left, f) |> Enum.into(HashSet.new)
  end
  def project(left, attributes) when is_list(attributes) do
    Stream.map(left, fn(x) ->
                       Dict.take(x, attributes)
               end) |> Enum.into(HashSet.new)
  end
  def fields(left) do
    Enum.at(left, 0) |> Map.keys 
  end
  def fnjoin(left, right) do
    lkset = fields(left) |> Enum.into(HashSet.new)
    rkset = fields(right) |> Enum.into(HashSet.new)
    ks = Set.intersection(lkset, rkset)
    Enum.reduce(left, 
                [],
                fn(x, acc) ->
                  z = Enum.filter_map(right,
                                      fn(y) ->
                                        Dict.equal?(Map.take(y, ks),
                                                    Map.take(x, ks))
                                      end,
                                      fn(y) ->
                                          Map.merge(x, y)
                                      end)
                    case z do
                      [] -> acc
                      [e] -> 
                        [e|acc]
                    end
                end)
  end
  defp product(left, right) do
    Enum.reduce(left, [], 
                fn(x, acc) -> 
                  Enum.map(right, &(Dict.merge(x, &1))) |> 
                    Enum.into(acc)
                end)
  end
  def join(left, right) do
    if (Enum.count(left) == 0 or Enum.count(right) == 0) do
      HashSet.new()
    else
      fnjoin(left, right) |> Enum.into(HashSet.new)
    end
  end
  def matching(left, right) do
    if (Enum.count(left) == 0 or Enum.count(right) == 0) do
      HashSet.new()
    else
      lkset = fields(left)
      fnjoin(left, right) |> project(lkset)
    end
  end
  def divideby(left, right) do
    fl = fields(left)
    fr = fields(right)
    fd = fl -- fr 
    pleft = project(left, fd)
    pr = Enum.into(product(pleft, right), HashSet.new)
    minus(pleft, 
          project(minus(pr, left), fd))
  end
  def rename(left, from, to) do
    fl = fields(left)
    Enum.reduce(left, HashSet.new, 
                fn(x, a) ->
                  v = Dict.get(x, from)
                  y = Dict.delete(Dict.put(x, to, v), from)
                  Set.put(a, y)
                end)
  end
  def extend_add(left, fun, x) do
    Enum.reduce(left, HashSet.new,
                fn(e, a) ->
                  Set.put(a, Dict.put(e, x, fun.(e)))
                end)
  end
  def update(left, right) do
    minus(right, left) |> write!()
  end
  def write!(left) do
    IO.inspect [write: left]
    left
  end
end