A really shitty parser that gets a list of string expressions from a Postgres index creation definition

parse_expressions.ex

  # CREATE INDEX users_lower_email_idx ON public.users USING btree (lower((email)::text))
  # CREATE INDEX unique_email_com3 ON public.users USING btree (email, id) WHERE (email ~~ '%.com'::citext)
  defp parse_columns_from_index_def(string, using) do
    string
    |> String.trim_leading("CREATE ")
    |> String.trim_leading("UNIQUE ")
    |> String.trim_leading("INDEX ")
    |> String.replace(~r/^[a-zA-Z0-9_\.]+\s/, "")
    |> String.trim_leading("ON ")
    |> String.replace(~r/^[\S]+/, "")
    |> String.trim_leading()
    |> String.trim_leading("USING #{using} ")
    |> do_parse_columns()
    |> then(&{:ok, &1})
  catch
    :error -> :error
  end

  def parse_columns(char) do
    do_parse_columns(char)
  end

  defp do_parse_columns(char, state \\ [], field \\ "", acc \\ [])

  defp do_parse_columns("(" <> rest, [], field, acc) do
    do_parse_columns(rest, [:outer], field, acc)
  end

  defp do_parse_columns(")" <> _rest, [:outer], field, acc) do
    if field == "" do
      Enum.reverse(acc)
    else
      Enum.reverse([field | acc])
    end
  end

  defp do_parse_columns("(" <> rest, [:outer], field, acc) do
    do_parse_columns(rest, [:in_paren, :in_field, :outer], field, acc)
  end

  defp do_parse_columns(", " <> rest, [:in_field, :outer], field, acc) do
    do_parse_columns(rest, [:in_field, :outer], "", [field | acc])
  end

  defp do_parse_columns(<<str::binary-size(1)>> <> rest, [:outer], field, acc) do
    do_parse_columns(rest, [:in_field, :outer], field <> str, acc)
  end

  defp do_parse_columns("''" <> rest, [:in_quote | stack], field, acc) do
    do_parse_columns(rest, [:in_quote | stack], field <> "'", acc)
  end

  defp do_parse_columns("'" <> rest, [:in_quote | stack], field, acc) do
    do_parse_columns(rest, stack, field <> "'", acc)
  end

  defp do_parse_columns(<<str::binary-size(1)>> <> rest, [:in_quote | stack], field, acc) do
    do_parse_columns(rest, [:in_quote | stack], field <> str, acc)
  end

  defp do_parse_columns("'" <> rest, stack, field, acc) do
    do_parse_columns(rest, [:in_quote | stack], field <> "'", acc)
  end

  defp do_parse_columns("(" <> rest, stack, field, acc) do
    do_parse_columns(rest, [:in_paren | stack], field <> "(", acc)
  end

  defp do_parse_columns(")" <> rest, [:in_paren | stack], field, acc) do
    do_parse_columns(rest, stack, field <> ")", acc)
  end

  defp do_parse_columns("), " <> rest, [:in_field | stack], field, acc) do
    do_parse_columns(rest, [:in_field | stack], "", [field | acc])
  end

  defp do_parse_columns(")" <> _rest, [:in_field | _stack], field, acc) do
    Enum.reverse([field | acc])
  end

  defp do_parse_columns(<<str::binary-size(1)>> <> rest, [:in_paren | stack], field, acc) do
    do_parse_columns(rest, [:in_paren | stack], field <> str, acc)
  end

  defp do_parse_columns(<<str::binary-size(1)>> <> rest, [:outer], field, acc) do
    do_parse_columns(rest, [:in_field, :outer], field <> str, acc)
  end

  defp do_parse_columns(<<str::binary-size(1)>> <> rest, [:in_field | stack], field, acc) do
    do_parse_columns(rest, [:in_field | stack], field <> str, acc)
  end

  defp do_parse_columns(", " <> rest, [:in_field | stack], field, acc) do
    do_parse_columns(rest, stack, "", [field | acc])
  end

  defp do_parse_columns(")" <> _rest, [:outer], field, acc) do
    Enum.reverse([field | acc])
  end

  defp do_parse_columns("", [:in_field | _stack], field, acc) do
    Enum.reverse([field | acc])
  end

  defp do_parse_columns("", [:outer], field, acc) do
    if field == "" do
      Enum.reverse(acc)
    else
      Enum.reverse([field | acc])
    end
  end

  defp do_parse_columns(other, stack, field, acc) do
    raise "Unexpected character: #{inspect(other)} at #{inspect(stack)} with #{inspect(field)} - #{inspect(acc)}"
  end