vatchex_greece

defmodule VatchexGreece.Validate do
  # SPDX-FileCopyrightText: 2023 Isaak Tsalicoglou <isaak@waseigo.com>
  # SPDX-License-Identifier: Apache-2.0

  @moduledoc """
  Module with functions used to validate Greek VAT IDs.
  """
  @moduledoc since: "0.5.0"

  @valid_chars Enum.map(0..9, fn x -> Kernel.to_string(x) end)

  @doc """
  Remove the "EL" or "GR" prefixes if present, remove any whitespace (leading,
  trailing, or internal), and return the VAT ID's minimal valid representation,
  including adding a leading "0" in case the provided ID had 8 digits (old format).
  """
  def minimize(vat_id) do
    vat_id
    |> String.graphemes()
    |> Enum.map(fn x -> if x in @valid_chars, do: x end)
    |> Enum.reject(&is_nil/1)
    |> Enum.join()
    |> String.pad_leading(9, "0")
  end

  defp checksum(_, acc \\ 0)

  defp checksum([head | tail], acc) do
    acc = acc * 2 + head
    checksum(tail, acc)
  end

  defp checksum([], acc), do: acc

  defp calculate_check_digit(number) do
    String.graphemes(number)
    |> Enum.map(&String.to_integer/1)
    |> checksum()
    |> Kernel.*(2)
    |> Integer.mod(11)
    |> Integer.mod(10)
  end

  @doc """
  Check that the passed VAT ID only contains digits.
  """
  def check_only_digits(vat_id) do
    c =
      vat_id
      |> Integer.parse()
      |> Kernel.!=(:error)

    if c do
      {:ok, vat_id}
    else
      {:error, "Error: VAT ID #{vat_id} does not contain only digits."}
    end
  end

  @doc """
  Unsafe check that the passed VAT ID only contains digits.
  """
  def check_only_digits!(vat_id) do
    case check_only_digits(vat_id) do
      {:ok, vat_id} -> vat_id
      {:error, message} -> raise RuntimeError, message: message
    end
  end

  @doc """
  Boolean check that the passed VAT ID only contains digits.
  """
  def only_digits?(vat_id) do
    case check_only_digits(vat_id) do
      {:ok, _} -> true
      _ -> false
    end
  end

  @doc """
  Check that the passed VAT ID has the proper length.
  """
  def check_proper_length(vat_id) do
    c =
      vat_id
      |> String.length()
      |> Kernel.==(9)

    if c do
      {:ok, vat_id}
    else
      {:error,
       "Error: VAT ID #{vat_id} has an incorrect length (not 9 digits)."}
    end
  end

  @doc """
  Unsafe check that the passed VAT ID has the proper length.
  """
  def check_proper_length!(vat_id) do
    case check_proper_length(vat_id) do
      {:ok, vat_id} -> vat_id
      {:error, message} -> raise RuntimeError, message: message
    end
  end

  @doc """
  Boolean check that the passed VAT ID has the proper length.
  """
  def proper_length?(vat_id) do
    case check_proper_length(vat_id) do
      {:ok, _} -> true
      _ -> false
    end
  end

  @doc """
  Check that the passed VAT ID contains the correct checksum digit.
  """
  def check_correct_checksum(vat_id) do
    last_digit = String.slice(vat_id, -1..-1)

    c =
      vat_id
      |> minimize()
      |> String.slice(0, 9 - 1)
      |> calculate_check_digit()
      |> to_string()
      |> Kernel.==(last_digit)

    if c do
      {:ok, vat_id}
    else
      {:error, "Error: VAT ID #{vat_id} checksum mismatch."}
    end
  end

  @doc """
  Unsafe check that the passed VAT ID contains the correct checksum digit.
  """
  def check_correct_checksum!(vat_id) do
    case check_correct_checksum(vat_id) do
      {:ok, vat_id} -> vat_id
      {:error, message} -> raise RuntimeError, message: message
    end
  end

  @doc """
  Boolean check that the passed VAT ID contains the correct checksum digit.
  """
  def correct_checksum?(vat_id) do
    case check_correct_checksum(vat_id) do
      {:ok, _} -> true
      _ -> false
    end
  end

  @doc """
  Check whether the VAT ID passed (as string) is valid, i.e. it has the correct
  length (9 total, and only digits), where the last digit is equal to the checksum
  calculate from the first 8 digits.
  """
  def valid?(vat_id) do
    checks = [
      &only_digits?/1,
      &proper_length?/1,
      &correct_checksum?/1
    ]

    false not in Enum.map(checks, fn func -> func.(vat_id) end)
  end
end