isbn-verifier

elixir/isbn-verifier/lib/isbn_verifier.ex

@@ -0,0 +1,41 @@
+defmodule IsbnVerifier do
+  @doc """
+    Checks if a string is a valid ISBN-10 identifier
+
+    ## Examples
+
+      iex> IsbnVerifier.isbn?("3-598-21507-X")
+      true
+
+      iex> IsbnVerifier.isbn?("3-598-2K507-0")
+      false
+
+  """
+
+  @regex ~r/^(?<digits>\d{9})(?<check_digit>[0-9X])$/
+
+  @spec isbn?(String.t()) :: boolean
+  def isbn?(isbn) do
+    with isbn_without_dashes <- String.replace(isbn, "-", ""),
+         %{"digits" => digits, "check_digit" => check_digit} <-
+           Regex.named_captures(@regex, isbn_without_dashes) do
+      rem(check_sum(digits) + check_digit(check_digit), 11) == 0
+    else
+      _ -> false
+    end
+  end
+
+  defp check_sum(digits) do
+    {1, check_sum} =
+      digits
+      |> String.graphemes()
+      |> Enum.reduce({10, 0}, fn digit, {index, acc} ->
+        {index - 1, acc + String.to_integer(digit) * index}
+      end)
+
+    check_sum
+  end
+
+  defp check_digit("X"), do: 10
+  defp check_digit(string), do: String.to_integer(string)
+end