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## -*- texinfo -*-
## @deftypefn  {} {@var{text} =} get_first_help_sentence (@var{name})
## @deftypefnx {} {@var{text} =} get_first_help_sentence (@var{name}, @var{max_len})
## @deftypefnx {} {[@var{text}, @var{status}] =} get_first_help_sentence (@dots{})
## Return the first sentence of a function's help text.
##
## The first sentence is defined as the text after the function declaration
## until either the first period (".") or the first appearance of two
## consecutive newlines ("\n\n").  The text is truncated to a maximum length of
## @var{max_len}, which defaults to 80.  If the text must be truncated the last
## three characters of the text are replaced with @qcode{"..."} to indicate
## that more text was available.
##
## The optional output argument @var{status} returns the status reported by
## @code{makeinfo}.  If only one output argument is requested, and @var{status}
## is nonzero, a warning is displayed.
##
## As an example, the first sentence of this help text is
##
## @example
## @group
## get_first_help_sentence ("get_first_help_sentence")
## @print{} ans = Return the first sentence of a function's help text.
## @end group
## @end example
## @end deftypefn

function [text, status] = get_first_help_sentence (name, max_len = 80)

  if (nargin < 1)
    print_usage ();
  endif

  if (! ischar (name))
    error ("get_first_help_sentence: NAME must be a string");
  endif

  if (! isnumeric (max_len) || max_len <= 0 || max_len != fix (max_len))
    error ("get_first_help_sentence: MAX_LEN must be positive integer");
  endif

  ## First, we get the raw help text
  [help_text, format] = get_help_text (name);

  ## Then, we take action depending on the format
  switch (lower (format))
    case "plain text"
      [text, status] = first_sentence_plain_text (help_text, max_len);
    case "texinfo"
      [text, status] = first_sentence_texinfo (help_text, max_len);
    case "html"
      [text, status] = first_sentence_html (help_text, max_len);
    case "not documented"
      error ("get_first_help_sentence: '%s' is not documented\n", name);
    case "not found"
      error ("get_first_help_sentence: '%s' not found\n", name);
    otherwise
      error ("get_first_help_sentence: internal error: unsupported help text format: '%s'\n", format);
  endswitch

  if (nargout <= 1 && status != 0)
    warning ("get_first_help_sentence: couldn't run makeinfo on '%s'", name);
  endif

endfunction

## This function extracts the first sentence from a plain text help text
function [text, status] = first_sentence_plain_text (help_text, max_len)

  ## Extract first line by searching for a period followed by whitespace
  ## followed by a capital letter (Nearly the same rule as Texinfo).
  period_idx = regexp (help_text, '\.\s+(?:[A-Z]|\n)', "once");
  ## ... or a double end-of-line (we subtract 1 because we are not interested
  ## in capturing the first newline).
  line_end_idx = regexp (help_text, "\n\n", "once") - 1;
  help_len = length (help_text);
  min_idx = min ([period_idx, line_end_idx, help_len]);
  if (min_idx < max_len)
    text = help_text(1:min_idx);
  else
    if (max_len > 3)
      text = help_text(1:(max_len-3));
      text = [text, "..."];
    else
      text = help_text(1:max_len);
    endif
  endif

  status = 0;

endfunction

## This function extracts the first sentence from a Texinfo help text.
## The function works by removing @def* from the texinfo text.  After this, we
## render the text to plain text using makeinfo, and then extract the first
## line.
function [text, status] = first_sentence_texinfo (help_text, max_len)

  ## Lines ending with "@\n" are continuation lines, so they should be
  ## concatenated with the following line.
  help_text = strrep (help_text, "@\n", " ");

  ## Find, and remove, lines that start with @def.  This should remove things
  ## such as @deftypefn, @deftypefnx, @defvar, etc.
  keep = true (size (help_text));
  def_idx = strfind (help_text, "@def");
  if (! isempty (def_idx))
    endl_idx = find (help_text == "\n");
    for k = 1:length (def_idx)
      endl = endl_idx(find (endl_idx > def_idx(k), 1));
      if (isempty (endl))
        endl = numel (keep);
      endif
      keep(def_idx(k):endl) = false;
    endfor

    ## Remove the @end ... that corresponds to the @def we removed above
    def1 = def_idx(1);
    space_idx = find (help_text == " ");
    space_idx = space_idx(find (space_idx > def1, 1));
    bracket_idx = find (help_text == "{" | help_text == "}");
    bracket_idx = bracket_idx(find (bracket_idx > def1, 1));
    if (isempty (space_idx) && isempty (bracket_idx))
      error ("get_first_help_sentence: couldn't parse texinfo");
    endif
    sep_idx = min (space_idx, bracket_idx);
    def_type = help_text(def1+1:sep_idx-1);

    end_idx = strfind (help_text, sprintf ("@end %s", def_type));
    if (isempty (end_idx))
      error ("get_first_help_sentence: couldn't parse texinfo");
    endif
    keep(end_idx(1):end) = false;

    help_text = help_text(keep);
  endif

  ## Run makeinfo to generate plain text
  [help_text, status] = __makeinfo__ (help_text, "plain text");

  ## Extract first line with plain text method.
  text = first_sentence_plain_text (help_text, max_len);

endfunction

## This function extracts the first sentence from a html help text.
## The function simply removes the tags and treats the text as plain text.
function [text, status] = first_sentence_html (help_text, max_len)

  ## Strip tags
  [help_text, status] = strip_html_tags (help_text);

  ## Extract first line with plain text method.
  text = first_sentence_plain_text (help_text, max_len);

endfunction


%!assert (regexp (get_first_help_sentence ('get_first_help_sentence'), ...
%!                "Return the first sentence of a function.s help text\."))

%!assert (get_first_help_sentence ('get_first_help_sentence', 28), ...
%!        "Return the first sentence...")

## Test input validation
%!error <Invalid call> get_first_help_sentence ()
%!error <NAME must be a string> get_first_help_sentence (1)
%!error <MAX_LEN must be positive integer> get_first_help_sentence ("ls", "a")
%!error <MAX_LEN must be positive integer> get_first_help_sentence ("ls", 0)
%!error <MAX_LEN must be positive integer> get_first_help_sentence ("ls", 80.1)
