defmodule PlagiarismCheck do
  def plagiarism_check(code1, code2) do
    c1 = Enum.join(code1, " ")
    c2 = Enum.join(code2, " ")

    if c1 == c2 do
      false
    else
      words1 = ~r/\w+/ |> Regex.scan(c1) |> List.flatten()
      words2 = ~r/\w+/ |> Regex.scan(c2) |> List.flatten()

      candidates =
        words1
        |> Enum.zip(words2)
        |> Enum.reduce(%{}, fn {w1, w2}, acc ->
          if w1 != w2 and not numeric?(w1), do: Map.put(acc, w1, w2), else: acc
        end)

      c1_placeheld =
        Enum.reduce(candidates, c1, fn {orig, _repl}, acc ->
          replace_word(acc, orig, "PLACEHOLDER" <> orig)
        end)

      c1_final =
        Enum.reduce(candidates, c1_placeheld, fn {orig, repl}, acc ->
          replace_word(acc, "PLACEHOLDER" <> orig, repl)
        end)

      c1_final == c2
    end
  end

  defp numeric?(w) do
    case Integer.parse(w) do
      {_n, ""} -> true
      _ -> false
    end
  end

  defp replace_word(text, target, replacement) do
    escaped = Regex.escape(target)

    text
    |> String.replace(~r/(\W)#{escaped}(\W)/, "\\1#{replacement}\\2")
    |> String.replace(~r/(\W)#{escaped}/, "\\1#{replacement}")
  end
end