string_calculator.ex

defmodule StringCalculator do
  def has_delimiter?(str) do
    String.starts_with?(str, "//")
  end

  def extract_delimiter(str) do
    {String.at(str, 2), String.slice(str, 3..-1)}
  end

  def replace_whitespace_with_separator(str, separator) do
    String.replace(str, "\n", separator, global: true)
  end

  def string_to_list(str, separator) do
    str
    |> StringCalculator.replace_whitespace_with_separator(separator)
    |> String.split(separator)
  end

  def parse_string_list([head | tail]) do
    [String.to_integer(head) | StringCalculator.parse_string_list(tail)]
  end

  def parse_string_list([]) do
    []
  end

  def sum_list([head | tail]) do
    head + sum_list(tail)
  end

  def sum_list([]) do
    0
  end

  def calculate(rawString) do
    {separator, str} = if StringCalculator.has_delimiter?(rawString) do
      StringCalculator.extract_delimiter(rawString)
    else
      {",", rawString}
    end

    str
    |> StringCalculator.string_to_list(separator)
    |> StringCalculator.parse_string_list
    |> StringCalculator.sum_list
  end
end