defmodule ContentIndexer.Services.Similarity do

  @moduledoc """
  ** Summary **
   This module accepts a list of tuples which contain the document id and a hash of terms
   and and their TF_IDF weights, it also accepts query terms in the form of a hash of terms and
   weights, same format as in the tuple above.

   [
     { 1, %{ "abc" => 0.001, "term1" => 0.123, "term2" => 0.934, "term3" => 0.945 } },
     { 1, %{ "abc" => 0.001, "term1" => 0.123, "term2" => 0.934, "term3" => 0.945 } }…
   ]

   The module will compute the similarity of all the provided documents to the
   query terms. It will then return an ordered set of terms and their corresponding
   weights
  """

  @doc """
    Compares a nested list of documents representing individual index items against a set of query terms

    ## Parameters

      - document_list: List of tuples containing the file_name & a list of tokens and their respective weights in the index
      - query: List of tuples containing the query term as String and it's respective weight

    ## Example

      iex> ContentIndexer.Services.Similarity.compare(
            [
              {"test1.md", [{"great", 0.0066469689853797444}, {"how", 0.01994090695613923}]},
              {"test2.md", [{"silent", 0.0066469689853797444}, {"instrument", 0.01994090695613923}]}
            ],
            [
              {"great", -0.6931471805599453}
            ])
            ["test1.md"]
      """
  def compare(document_list, query_terms) do
    document_list
    |> get_similarity(query_terms)
    |> get_filenames()
  end

  @doc """
    See the compare function as this one does the same just omitting the filenames
  """
  def get_similarity(document_list, query_terms) do
    val = document_list
    |> Enum.map(fn(doc) ->
      {elem(doc, 0), compare_doc(elem(doc, 1), query_terms)}
    end)
    |> order_docs
    Enum.into(val, %{})
  end

  @doc """
    retrives a list of filenames for the similarity_map - see the compare function
  """
  def get_filenames(similarity_map) do
    similarity_map
    |> sort_similarity_map()
    |> Enum.filter(fn(r) ->
      val = elem(r, 1)
      val != 0.0
    end)
    |> Enum.map(fn(r) ->
      elem(r, 0)
    end)
  end

  # private functions

  defp sort_similarity_map(similarity_map) do
    similarity_map
    |> Enum.sort(&(elem(&1, 1) <= elem(&2, 1)))
  end

  # return a list of documents as well as their cosime similarity to the term
  defp compare_doc(document, query) do
    d1_weights = get_relevant_weights(document, query)
    query_vals = Keyword.values query
    dot_prod = dot_product(Enum.zip(d1_weights, query_vals))

    d1_magnitude = magnitude(d1_weights)
    d2_magnitude = magnitude(query_vals)

    if d1_magnitude == 0 || d2_magnitude == 0 do
      0.0
    else
      abs(dot_prod / (d1_magnitude * d2_magnitude))
    end
  end

  defp dot_product(value_array) do
    value_array
    |> Enum.reduce(0, fn(x, acc) ->
      (elem(x, 0) * elem(x, 1)) + acc
     end)
  end

  defp magnitude(values) do
    # No math library wtf using erlang instead
    :math.sqrt(Enum.reduce(values, 0, fn(x, acc) ->
      (x * x) + acc
    end))
  end

  defp get_relevant_weights(document, query) do
    # get the query keys corresponding weights from the document
    # weight is zero if the key is not in the document
    query
    |> Enum.map(fn(k) ->
      key = elem(k, 0)
      weight = document
      |> Enum.filter(fn(f) -> elem(f, 0) == key end)
      |> List.first

      case weight do
        nil ->
          {key, 0.0}
        _ ->
          {key, elem(weight, 1)}
      end

    end)
    |> Enum.into(%{})
    |> Map.values
  end

  defp order_docs(x) do
    y = length x
    if y < 2 do
      x
    else
      halfway = round(Float.floor(y / 2))
      front_half = Enum.slice(x, 0, halfway)
      back_half = Enum.slice(x, halfway, y)
      merge(order_docs(front_half), order_docs(back_half))
    end
  end

  defp merge([], list) do
    list
  end

  defp merge(list, []) do
    list
  end

  defp merge(list1, list2) do
    [h1 | t1] = list1
    [h2 | t2] = list2
    {_, w1} = h1
    {_, w2} = h2

    if w1 > w2 do
      [h1 | merge(t1, list2)]
    else
      [h2 | merge(list1, t2)]
    end
  end
end