# Copyright 2019 Hubert Chathi <hubert@uhoreg.ca>
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

defmodule Polyjuice.Util do
  @doc ~S"""
  Escape a user ID localpart so that it only uses the allowable character set.

  The method used is defined in
  https://matrix.org/docs/spec/appendices#mapping-from-other-character-sets

  By default, it will escape upper-case letters.  If you want upper-case
  letters to simply be lower-cased, use the `fold_case: true` option.

  Examples:

    iex> Polyjuice.Util.escape_localpart("abc\x01 !\",-./09:;<=>?@AZ[\\]^_`az{|}~á")
    "abc=01=20=21=22=2c-./09=3a=3b=3c=3d=3e=3f=40_a_z=5b=5c=5d=5e__=60az=7b=7c=7d=7e=c3=a1"

    iex> Polyjuice.Util.escape_localpart("abc\x01 !\",-./09:;<=>?@AZ[\\]^_`az{|}~á", fold_case: true)
    "abc=01=20=21=22=2c-./09=3a=3b=3c=3d=3e=3f=40az=5b=5c=5d=5e_=60az=7b=7c=7d=7e=c3=a1"
  """
  def escape_localpart(localpart, opts \\ []) when is_binary(localpart) and is_list(opts) do
    fold_case = Keyword.get(opts, :fold_case, false)
    do_escape(localpart, fold_case) |> IO.iodata_to_binary()
  end

  defp do_escape("", _), do: ""

  defp do_escape(<<c>> <> rest, fold_case)
       when c < ?- or (c > ?9 and c < ?A) or (c > ?Z and c != ?_ and c < ?a) or c > ?z do
    if c < 0xF do
      ["=0", Integer.to_string(c, 16) |> String.downcase() | do_escape(rest, fold_case)]
    else
      [?=, Integer.to_string(c, 16) |> String.downcase() | do_escape(rest, fold_case)]
    end
  end

  defp do_escape(<<c>> <> rest, fold_case) when c >= ?A and c <= ?Z do
    if fold_case do
      [c - ?A + ?a | do_escape(rest, fold_case)]
    else
      [?_, c - ?A + ?a | do_escape(rest, fold_case)]
    end
  end

  defp do_escape(<<?_>> <> rest, false) do
    ["__" | do_escape(rest, false)]
  end

  defp do_escape(string, fold_case) do
    size = chunk_size(string, fold_case, 0)
    <<chunk::binary-size(size), rest::binary>> = string
    [chunk | do_escape(rest, fold_case)]
  end

  defp chunk_size("", _, acc), do: acc

  defp chunk_size(<<c>> <> _, _, acc)
       when c < ?- or (c > ?9 and c != ?= and c < ?A) or (c > ?Z and c != ?_ and c < ?a) or c > ?z do
    acc
  end

  defp chunk_size(<<c>> <> _, false, acc) when (c >= ?A and c <= ?Z) or c == ?_ do
    acc
  end

  defp chunk_size(<<_char>> <> rest, fold_case, acc) do
    chunk_size(rest, fold_case, acc + 1)
  end
end