########################################################################
##
## Copyright (C) 2009-2022 The Octave Project Developers
##
## See the file COPYRIGHT.md in the top-level directory of this
## distribution or <https://octave.org/copyright/>.
##
## This file is part of Octave.
##
## Octave is free software: you can redistribute it and/or modify it
## under the terms of the GNU General Public License as published by
## the Free Software Foundation, either version 3 of the License, or
## (at your option) any later version.
##
## Octave is distributed in the hope that it will be useful, but
## WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
## GNU General Public License for more details.
##
## You should have received a copy of the GNU General Public License
## along with Octave; see the file COPYING.  If not, see
## <https://www.gnu.org/licenses/>.
##
########################################################################

## -*- texinfo -*-
## @deftypefn  {} {} help @var{name}
## @deftypefnx {} {} help --list
## @deftypefnx {} {} help .
## @deftypefnx {} {} help
## Display the help text for @var{name}.
##
## For example, the command @kbd{help help} prints a short message describing
## the @code{help} command.
##
## Given the single argument @code{--list}, list all operators, keywords,
## built-in functions, and loadable functions available in the current session
## of Octave.
##
## Given the single argument @code{.}, list all operators available in the
## current session of Octave.
##
## If invoked without any arguments, @code{help} displays instructions on how
## to access help from the command line.
##
## The help command can provide information about most operators, but
## @var{name} must be enclosed by single or double quotes to prevent
## the Octave interpreter from acting on @var{name}.  For example,
## @code{help "+"} displays help on the addition operator.
## @seealso{doc, lookfor, which, info}
## @end deftypefn

function retval = help (name)

  if (nargin == 0)

    text = "\n\
  For help with individual commands and functions type\n\
\n\
    help NAME\n\
\n\
  (replace NAME with the name of the command or function you would\n\
  like to learn more about; for an operator, enclose \"NAME\" in quotes).\n\
\n\
  For a more detailed introduction to GNU Octave, consult the manual.\n\
  The manual may be read from the prompt by typing\n\
\n\
    doc\n\
\n\
  GNU Octave is supported and developed by its user community.\n\
  For more information visit https://www.octave.org.\n\n";

    if (nargout == 0)
      puts (text);
    else
      retval = text;
    endif

  elseif (nargin == 1 && ischar (name))

    if (strcmp (name, "--list"))
      list = do_list_functions ();
      if (nargout == 0)
        printf ("%s", list);
      else
        retval = list;
      endif
      return;
    endif

    if (strcmp (name, "."))
      list = do_list_operators ();
      if (nargout == 0)
        printf ("%s", list);
      else
        retval = list;
      endif
      return;
    endif

    ## Get help text
    [text, format] = get_help_text (name);

    ## Take action depending on help text format
    switch (lower (format))
      case "plain text"
        status = 0;
      case "texinfo"
        [text, status] = __makeinfo__ (text, "plain text");
      case "html"
        [text, status] = strip_html_tags (text);
      case "not documented"
        error ("help: '%s' is not documented\n", name);
      case "not found"
        do_contents (name);
        return;
      otherwise
        error ("help: internal error: unsupported help text format: '%s'\n", format);
    endswitch

    ## Print text
    if (status != 0)
      warning ("help: Texinfo formatting filter exited abnormally; raw Texinfo source of help text follows...\n");
    endif

    if (nargout == 0)
      if (! any (strcmp (name, {'./', '.\', '...'})))
        evalin ("caller", ['which ("' undo_string_escapes(name) '")']);
      endif
      printf ("\n%s\n%s", text, __additional_help_message__ ());
    else
      retval = text;
    endif

  else
    error ("help: invalid input\n");
  endif

endfunction

function retval = do_list_operators ()
  retval = sprintf ("*** operators:\n\n%s\n%s\n\n",
                    list_in_columns (__operators__ ()),
                    ["To obtain help on an operator, type\n"...
                     "      help \"NAME\"   or   help 'NAME'"]);
endfunction

function retval = do_list_functions ()

  operators = do_list_operators ();

  keywords = sprintf ("*** keywords:\n\n%s\n\n",
                      list_in_columns (__keywords__ ()));

  builtins = sprintf ("*** builtins:\n\n%s\n\n",
                      list_in_columns (__builtins__ ()));

  dirs = ostrsplit (path, pathsep);
  flist = "";
  for i = 2:numel (dirs)
    files = sort ({dir(fullfile (dirs{i}, "*.m")).name, ...
                   dir(fullfile (dirs{i}, "*.oct")).name, ...
                   dir(fullfile (dirs{i}, "*.mex")).name});

    if (! isempty (files))
      flist = sprintf ("%s*** functions in %s:\n\n%s\n\n",
                       flist, dirs{i}, list_in_columns (files));
    endif
  endfor

  retval = [operators, keywords, builtins, flist];

endfunction

function do_contents (name)

  found = false;

  dlist = dir_in_loadpath (name, "all");

  for i = 1:numel (dlist)
    fname = make_absolute_filename (fullfile (dlist{i}, "Contents.m"));

    [text, format] = get_help_text_from_file (fname);

    ## Take action depending on help text format
    switch (lower (format))
      case "plain text"
        status = 0;
      case "texinfo"
        [text, status] = __makeinfo__ (text, "plain text");
      case "html"
        [text, status] = strip_html_tags (text);
    endswitch

    if (! isempty (text))
      found = true;
      ## Print text.
      if (status != 0)
        warning ("help: Texinfo formatting filter exited abnormally; raw Texinfo source of help text follows...\n");
      endif
      printf ("%s:\n\n%s\n", fname, text);
    endif

  endfor

  if (found)
    puts (__additional_help_message__ ());
  else
    msg = feval (missing_function_hook, name);

    if (isempty (msg))
      msg = sprintf ("'%s' not found", name);
    endif

    error ("help: %s\n", msg);
  endif

endfunction


%!assert (! isempty (strfind (help ("ls"), "List directory contents")))
%!assert (! isempty (strfind (help ("."), "||")))

## Test input validation
%!error <invalid input> help (42)
%!error <called with too many inputs> help ("abc", "def")
%!error <'_! UNLIKELY_FCN! _' not found> help ("_! UNLIKELY_FCN! _")
