defmodule ConfigParser do
  @moduledoc """
    This library implements a parser for config files in the style of Windows INI, 
    as parsed by the Python [configparser](https://docs.python.org/3/library/configparser.html) library.
  
    The `ConfigParser` module includes routines that can parse a file, the contents of a string, or from a stream of lines.

    To parse the content of a config file call the `parse_file` function and pass the file's path:

    
      {:ok, parse_result} = ConfigParser.parse_file("/path/to/file")
    

    To parse config information out of a string, call the `parse_string` method:

      {:ok, parse_result} = ConfigParser.parse_string(\"\"\"
        [interesting_config]
        config_key = some interesting value
        \"\"\")
    
    Given a stream whose elements represent the successive lines of a config file, the library can parse the content of the stream:

      fake_stream = ["[section]", "key1 = value2", "key2:value2"] |> Stream.map(&(&1))
      {:ok, parse_result} = ConfigParser.parse_stream(fake_stream)
    

    As mentioned previously the result of doing the parsing is a tuple.  If successful, the first element of the tupe is `:ok` and the second element is the parsed result.

    If the parser encounters an error, then the first part of the tuple will be the atom `:error` and the second element will be a string describing the error that was encountered:

      {:error, "Syntax Error on line 3"}
    
  """

  defmodule ParseState do
    defstruct line_number: 1,         # What line of the "file" are we parsing
          current_section: nil,       # Section that definitions go into
              last_indent: 0,         # The amount of whitespace on the last line
            continuation?: false,     # Could the line being parsed be a coninuation
                 last_key: nil,       # If this is a continuation, which key would it continue
                   result: {:ok, %{}} # The result as it is being built.

    def begin_section(parse_state, new_section) do
      # Create a new result, based on the old, with the new section added
      {:ok, section_map} = parse_state.result

      # Only add a new section if it's not already there
      section_key = String.strip(new_section)
      unless Map.has_key?(section_map, section_key) do
        new_result = {:ok, Map.put(section_map, section_key, %{}) }
      end

      # next line cannot be a continuation 
      %{parse_state | current_section: section_key, 
                               result: new_result, 
                        continuation?: false,
                             last_key: nil}
    end

    def define_config(parse_state, key, value) do
      {:ok, section_map} = parse_state.result

      if parse_state.current_section != nil do
        # pull the values out for the section that's currently being built
        value_map = section_map[parse_state.current_section]

        # create a new set of values by adding the key/value pair passed in
        if value == nil do
          new_values = Map.put(value_map, String.strip(key), nil)
        else
          new_values = Map.put(value_map, String.strip(key), String.strip(value))
        end

        # create a new result replacing the current section with thenew values
        new_result = {:ok, Map.put(section_map, parse_state.current_section, new_values)}

        # The next line could be a continuation of this value so set continuation to true
        # and store the key that we're defining now.
        %{parse_state | result: new_result, 
                 continuation?: true,
                      last_key: String.strip(key)}
      else
        new_result = {:error, "A configuration section must be defined before defining configuration values in line #{parse_state.line_number}"}
        %{parse_state | result: new_result}
      end
    end

    def append_continuation(parse_state, continuation_value) do
      {:ok, section_map} = parse_state.result

      # pull the values out for the section that's currently being built
      value_map = section_map[parse_state.current_section]

      # create a new set of values by adding the key/value pair passed in
      new_value = "#{value_map[parse_state.last_key]} #{continuation_value}"

      define_config(parse_state, parse_state.last_key, new_value)
    end
  end

  @section_regex ~r{\[([^\]]+)\]}
  @equals_definition_regex ~r{([^=]+)=(.*)}
  @colon_definition_regex ~r{([^:]+):(.*)}
  @value_like_regex ~r{\s*(\S.*)}

  # If the parse state indicates an error we simply skip over lines and propogate
  # the error.
  defp parse_line(_line, parse_state = %ParseState{result: {:error, _error_string}}) do
    parse_state
  end

  # Parse a line while the parse state indicates we're in a good state
  defp parse_line(line, parse_state = %ParseState{result: {:ok, _}}) do
    line = strip_inline_comments(line)

    # find out how many whitespace characters are on the front of the line
    indent_level = indent_level(line)

    if parse_state.continuation? 
       && indent_level > parse_state.last_indent
       && (match = Regex.run(@value_like_regex, line)) do

      # note that we do not increase the "last indent"
      %{ParseState.append_continuation(parse_state, String.strip(line)) | line_number: parse_state.line_number + 1, continuation?: true}
    else
      cond do
        # if we can skip this line (it's empty or a comment) then simply advance the line number
        # and note that the next line can't be a continuation
        can_skip_line(line) ->
          %{parse_state | line_number: parse_state.line_number + 1, continuation?: false, last_indent: indent_level}

        # match a line that begins a new section like "[new_section]"
        match = Regex.run(@section_regex, line) ->
          [_, new_section] = match
          %{ParseState.begin_section(parse_state, new_section) | line_number: parse_state.line_number + 1 , last_indent: indent_level}

        # match a line that defines a value "key = value"
        match = Regex.run(@equals_definition_regex, line) ->
          [_, key, value] = match
          %{ParseState.define_config(parse_state, key, value) | line_number: parse_state.line_number + 1, last_indent: indent_level}

        # match a line that defines a value "key : value"
        match = Regex.run(@colon_definition_regex, line) ->
          [_, key, value] = match
          %{ParseState.define_config(parse_state, key, value) | line_number: parse_state.line_number + 1, last_indent: indent_level}

        # when there's a value-ish line that on a line by itself, but which is not a continuation
        # then it actually represents a key that has no associated value (or a value of nil)
        match = Regex.run(@value_like_regex, line) ->
          [_, key] = match
          %{ParseState.define_config(parse_state, key, nil) | continuation?: false, line_number: parse_state.line_number + 1, last_indent: indent_level}

        # Any non-matching lines result in a syntax error
        true ->
          %{parse_state | result: {:error, "Syntax Error on line #{parse_state.line_number}"}}

      end # cond
    end # continuation if
  end

  @doc """
    Accepts `config_file_path`, a file system path to a config file.
    Attempts to opens and parses the contents of that file.
  """
  def parse_file(config_file_path) do
    file_stream = File.stream!(config_file_path, [], :line)
    parse_stream(file_stream)
  end

  @doc """
    Parse a string as if it was the content of a config file.
  """
  def parse_string(config_string) do
    {:ok, pid} = StringIO.open(config_string)
    line_stream = IO.stream(pid, :line)
    parse_stream(line_stream)
  end

  @doc """
    Parses a stream whose elements should be strings representing the
    individual lines of a config file.
  """
  def parse_stream(line_stream) do
    %ParseState{result: result} = Enum.reduce(line_stream, %ParseState{}, &parse_line/2)
    result
  end

  # Calulate how much whitespace is at the front of the given
  # line.
  defp indent_level(line) do
    [_whole, spaces | _rest] = Regex.run(~r{(\s*).*}, line)
    spaces = String.replace(spaces, "\t", "  ")
    String.length(spaces)
  end

  # Returns true if the parser can ignore the line passed in.
  # this is done if the line is a comment just whitespace
  defp can_skip_line(line) do
    is_comment(line) || is_empty(line)
  end

  # Returns true if the line appears to be a comment
  @hash_comment_regex ~r{#.*}
  @semicolon_comment_regex ~r{;.*}

  defp is_comment(line) do
    String.strip(line) =~ @hash_comment_regex || String.strip(line) =~ @semicolon_comment_regex
  end

  # returns true if the line contains only whitespace
  defp is_empty(line) do
    String.strip(line) == ""
  end

  # semicolons on a line define the start of a comment.
  # this removes the semicolon and anything following it.
  defp strip_inline_comments(line) do
    line_list = String.split(line, ";")
    List.first(line_list)
  end
end