defmodule Aprs.EncodingUtils do
  @moduledoc """
  Utilities for handling encoding issues in APRS packet data.

  APRS packets can contain arbitrary bytes that may not be valid UTF-8,
  which causes issues when trying to JSON encode the data for transmission
  to web clients.
  """

  @doc """
  Sanitizes a binary to ensure it can be safely JSON encoded.

  If the binary is valid UTF-8, returns it as-is.
  If it contains invalid UTF-8 sequences, replaces them with the Unicode
  replacement character (�) or removes them entirely.

  ## Examples

      iex> Aprs.EncodingUtils.sanitize_string("Hello World")
      "Hello World"

      iex> Aprs.EncodingUtils.sanitize_string(<<72, 101, 108, 108, 111, 211, 87, 111, 114, 108, 100>>)
      "HelloWorld"
  """
  def sanitize_string(binary) when is_binary(binary) do
    if String.valid?(binary) do
      binary
    else
      # Replace invalid UTF-8 sequences with replacement character or remove them
      scrub_invalid_utf8(binary)
    end
  end

  def sanitize_string(nil), do: nil
  def sanitize_string(other), do: other

  @doc """
  Sanitizes all string fields in an APRS packet to ensure safe JSON encoding.
  """
  def sanitize_packet(%Aprs.Packet{} = packet) do
    %{
      packet
      | information_field: sanitize_string(packet.information_field),
        data_extended: sanitize_data_extended(packet.data_extended)
    }
  end

  @doc """
  Sanitizes string fields in the data_extended structure.
  """
  def sanitize_data_extended(nil), do: nil

  def sanitize_data_extended(%{comment: comment} = data_extended) when is_map(data_extended) do
    %{data_extended | comment: sanitize_string(comment)}
  end

  def sanitize_data_extended(%Parser.Types.MicE{message: message} = mic_e) do
    %{mic_e | message: sanitize_string(message)}
  end

  def sanitize_data_extended(data_extended), do: data_extended

  # Private helper functions

  defp scrub_invalid_utf8(binary) do
    binary
    |> :binary.bin_to_list()
    |> Enum.filter(&valid_byte?/1)
    |> Enum.map(&scrub_byte/1)
    |> :binary.list_to_bin()
    |> String.trim()
  end

  # Check if byte should be kept
  defp valid_byte?(byte) when byte >= 32 and byte <= 126, do: true
  defp valid_byte?(byte) when byte in [9, 10, 13], do: true
  defp valid_byte?(_byte), do: false

  # Keep valid bytes as-is
  defp scrub_byte(byte), do: byte

  @doc """
  Converts a binary to a hex string representation for debugging.

  ## Examples

      iex> Aprs.EncodingUtils.to_hex(<<72, 101, 108, 108, 111>>)
      "48656C6C6F"
  """
  def to_hex(binary) when is_binary(binary) do
    binary
    |> :binary.bin_to_list()
    |> Enum.map(&Integer.to_string(&1, 16))
    |> Enum.map_join("", &String.pad_leading(&1, 2, "0"))
  end

  @doc """
  Returns information about a binary's encoding validity.

  ## Examples

      iex> Aprs.EncodingUtils.encoding_info("Hello")
      %{valid_utf8: true, byte_count: 5, char_count: 5}

      iex> Aprs.EncodingUtils.encoding_info(<<72, 101, 211, 108, 111>>)
      %{valid_utf8: false, byte_count: 5, invalid_at: 2}
  """
  def encoding_info(binary) when is_binary(binary) do
    valid = String.valid?(binary)
    byte_count = byte_size(binary)

    base_info = %{
      valid_utf8: valid,
      byte_count: byte_count
    }

    if valid do
      Map.put(base_info, :char_count, String.length(binary))
    else
      # Try to find where the invalid sequence starts
      invalid_at = find_invalid_byte_position(binary, 0)
      Map.put(base_info, :invalid_at, invalid_at)
    end
  end

  defp find_invalid_byte_position(<<>>, _pos), do: nil

  defp find_invalid_byte_position(binary, pos) do
    case binary do
      <<head::binary-size(1), tail::binary>> ->
        if String.valid?(head) do
          find_invalid_byte_position(tail, pos + 1)
        else
          pos
        end

      _ ->
        pos
    end
  end
end