Nsryjdtyk

I wrote this Paginator module that's part of a small application that displays Elixir Github repos in a table.

I'm new to Elixir and looking for code improvements with as much as possible. Best practices. Commenting. Optimizations. The usage of structs for input output. Etc.

#paginator.ex

defmodule Paginator do



  @moduledoc """

  A module that implements functions for performing simple pagination functionality



  Invoke it using the module's `call` function that takes a struct as parameter.



  ## Struct key values



    - total: The total amount of records (mandatory)

    - page: The page currently on (default: 1)

    - per_page: The number of records per page (default: 10)

    - max_display: The number of pages displayed in the pagination menu (default: 10)

    - max_results: Optional argument that limits the total number of records it can paginate



  ## Examples



    iex> Paginator.call %Paginator(total: 1000)



    %Paginator.Output{

      first: nil,

      last: 100,

      next: 2,

      page: 1,

      pages: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10],

      previous: nil

    }



  """



  # The struct with the pagination info that gets returned

  defmodule Output do

    defstruct [:page, :first, :last, :previous, :next, :pages]

  end



  @doc """

  # Struct that's passed to the module used to calculate the pagination data

  """

  @enforce_keys [:total]

  defstruct page: 1, per_page: 10, total: nil, max_display: 10, max_results: nil



  @doc """

  Invokes the module. Takes a struct with the input and returns a struct with the pagination data

  """

  def call(data) do



    data = data

      |> Map.put(:max_pages, max_pages(data))

      |> Map.put(:half, half(data))



    %Output{

      page: data.page,

      first: first(data),

      last: last(data),

      pages: pages(data),

      previous: previous(data),

      next: next(data)

    }

  end



  """

  Returns the maximum pages.

  @TODO: this must be a bug. add test that has total less than max_results

  """

  defp max_pages(data) do

    cond do

      data.total <= data.per_page ->

        0

      data.max_results !== nil and data.max_results < data.total ->

        Kernel.trunc(data.max_results / data.per_page)

      true ->

        Kernel.trunc(data.total / data.per_page)

    end

  end



  # Returns the first page

  defp first(data) do

    if data.total >= data.per_page and data.page !== 1, do: 1, else: nil

  end



  # Returns the last page

  defp last(data) do

    if data.page < data.max_pages, do: data.max_pages, else: nil

  end



  # Returns the half value of `max_display` rounded down

  defp half(data) do

    Kernel.trunc(data.max_display / 2)

  end



  # Returns the `pages` list. The number of list items depends on the number specified in `max_display`

  defp pages(data) do

    if data.total === nil or data.total === 0 or data.max_pages === 0 do

      

    else

      Enum.to_list begin_pages(data)..end_pages(data)

    end

  end



  # Returns the page that the `pages` list starts on

  defp begin_pages(data) do

    cond do

      data.page + data.half >= data.max_pages ->

        # when reaching the end

        data.max_pages - (data.max_display - 1)

      data.page > data.half ->

        # odd vs even pages

        if rem(data.max_display, 2) === 0 do

          data.page - (data.half - 1)

        else

          data.page - data.half

        end

      true ->

        1

    end

  end



  # Returns the page that the `pages` list ends on

  defp end_pages(data) do

    end_page = data.page + data.half

    cond do

      end_page >= data.max_pages ->

        # when reaching the end

        data.max_pages

      data.page <= data.half ->

        data.max_display

      true ->

        end_page

    end

  end



  # Returns the page number that is prior than the current page.

  # If the current page is 1 it returns nil

  defp previous(data) do

    if data.page > 1, do: data.page - 1, else: nil

  end



  # Returns the page number that is latter than the current page.

  # If the current page is equal to the last it returns nil

  defp next(data) do

    if data.page < data.max_pages, do: data.page + 1, else: nil

  end

end

#paginator_test.exs

defmodule PaginatorTest do

  use ConnCase

  alias Paginator



  describe "Test Paginator" do



    test "page 1 of 15" do

      res = Paginator.call %Paginator{total: 150}



      assert res.first == nil

      assert res.previous == nil

      assert res.next == 2

      assert res.last == 15

      assert res.pages == [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

    end



    test "page 5 of 15" do

      res = Paginator.call %Paginator{page: 5, total: 150}



      assert res.first == 1

      assert res.previous == 4

      assert res.next == 6

      assert res.last == 15

      assert res.pages == [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

    end



    test "page 6 of 15" do

      res = Paginator.call %Paginator{page: 6, total: 150}



      assert res.first == 1

      assert res.previous == 5

      assert res.next == 7

      assert res.last == 15

      assert res.pages == [2, 3, 4, 5, 6, 7, 8, 9, 10, 11]

    end



    test "page 7 of 15" do

      res = Paginator.call %Paginator{page: 7, total: 150}



      assert res.pages == [3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

    end



    test "page 13 of 15" do

      res = Paginator.call %Paginator{page: 13, total: 150}



      assert res.pages == [6, 7, 8, 9, 10, 11, 12, 13, 14, 15]



    end



    test "per page 50" do

      res = Paginator.call %Paginator{page: 25, per_page: 50, total: 2000}



      assert res.first == 1

      assert res.previous == 24

      assert res.next == 26

      assert res.last == 40

      assert res.pages == [21, 22, 23, 24, 25, 26, 27, 28, 29, 30]



    end



    test "last page" do

      res = Paginator.call %Paginator{page: 50, total: 500}



      assert res.first == 1

      assert res.previous == 49

      assert res.next == nil

      assert res.last == nil

    end



    test "max display" do

      res = Paginator.call %Paginator{page: 8, max_display: 5, total: 2000}



      assert res.first == 1

      assert res.previous == 7

      assert res.next == 9

      assert res.pages == [6, 7, 8, 9, 10]



      res = Paginator.call %Paginator{page: 9, max_display: 5, total: 2000}

      assert res.pages == [7, 8, 9, 10, 11]



    end



    test "max results - total more than max" do

      res = Paginator.call %Paginator{page: 96, total: 2000, max_results: 1000}



      assert res.last == 100

      assert res.pages == [91, 92, 93, 94, 95, 96, 97, 98, 99, 100]



    end



    test "max results - max more than total" do

      res = Paginator.call %Paginator{page: 96, total: 2000, max_results: 1000}



      assert res.last == 100

      assert res.pages == [91, 92, 93, 94, 95, 96, 97, 98, 99, 100]

    end



    test "no pages - zero total" do

      res = Paginator.call %Paginator{total: 0}



      assert res.first == nil

      assert res.previous == nil

      assert res.next == nil

      assert res.pages == 

    end



    test "no pages - low total" do

      res = Paginator.call %Paginator{total: 5}



      assert res.first == nil

      assert res.previous == nil

      assert res.next == nil

      assert res.pages == 

    end

  end

end