#+TITLE: Helm Gtags layer

* Table of Contents                                         :TOC_4_gh:noexport:
 - [[#description][Description]]
 - [[#features][Features]]
 - [[#install][Install]]
   - [[#gnu-global][GNU Global]]
 - [[#usage][Usage]]
   - [[#eldoc-integration][Eldoc integration]]
 - [[#key-bindings][Key bindings]]

* Description
=helm-gtags= and =ggtags= are clients for GNU Global. GNU Global is a source
code tagging system that allows querying symbol locations in source code, such
as definitions or references.

* Features

- Select any tag in a project retrieves by gtags
- Resume previous helm-gtags session
- Jump to a location based on context
- Find definitions
- Find references
- Present tags in current function only
- Create a tag database
- Jump to definitions in file
- Show stack of visited locations
- Manually update tag database
- Jump to next location in context stack
- Jump to previous location in context stack
- Jump to a file in tag database

* Install
** GNU Global

You can install =helm-gtags= from the software repository of your OS. For example, in Ubuntu:

#+BEGIN_SRC sh
  sudo apt-get install global
#+END_SRC

To use =helm-gtags=, you first have to install [[https://www.gnu.org/software/global/download.html][GNU Global]].

Download the latest tar.gz archive, then run these commands:

#+BEGIN_SRC sh
  tar xvf global-6.4.tar.gz
  cd global-6.4
  ./configure
  make
  sudo make install
#+END_SRC

To be able to use =ctags= and other backends, you need to copy the sample
=gtags.conf= either to =/etc/gtags.conf= or =$HOME/.globalrc=. For example:

#+begin_src sh
  cp gtags.conf ~/.globalrc
#+end_src

To use this configuration layer, add it to your =~/.spacemacs=. You will need to
add =gtags= to the existing =dotspacemacs-configuration-layers= list in this
file.

* Usage

Before using the helm-gtags, remember to create a GTAGS database by the following methods:

- From within Emacs, runs the command =helm-gtags-create-tags=, which is bound
  to ~SPC m g c~. If the language is not directly supported by GNU Global, you
  can choose =ctags= or =pygment= as a backend to generate tag database.

- From inside terminal, runs gtags at your project root in terminal:

#+BEGIN_SRC sh
  cd /path/to/project/root
  gtags
#+END_SRC

If the language is not directly supported by =gtags= but =ctags=, use this command instead:

#+BEGIN_SRC sh
  gtags --gtagslabel=ctags
#+END_SRC

** Eldoc integration

This layer also integrates =ggtags= for its Eldoc feature. That means, when
writing code, you can look at the minibuffer (at the bottom) and see variable
and function definition of the symbol the cursor is on. However, this feature is
only activated for programming modes that are not one of these languages:

- C mode
- C++ mode
- Common Lisp
- Emacs Lisp
- Python
- Ruby-mode

In addition, if output from =compile= (bound to ~SPC c C~), =shell-command=
(bound to ~SPC !~ and ~M-!~) or =async-shell-command= (bound to ~M-&~) commands
contains symbol in your project, you move cursor on such symbol and use any of
the gtags commands.

Since these modes have better Eldoc integration already.

* Key bindings

| Key Binding | Description                                               |
|-------------+-----------------------------------------------------------|
| ~SPC m g c~ | create a tag database                                     |
| ~SPC m g f~ | jump to a file in tag database                            |
| ~SPC m g g~ | jump to a location based on context                       |
| ~SPC m g G~ | jump to a location based on context (open another window) |
| ~SPC m g d~ | find definitions                                          |
| ~SPC m g D~ | find definitions (open another window)                    |
| ~SPC m g i~ | present tags in current function only                     |
| ~SPC m g l~ | jump to definitions in file                               |
| ~SPC m g n~ | jump to next location in context stack                    |
| ~SPC m g p~ | jump to previous location in context stack                |
| ~SPC m g r~ | find references                                           |
| ~SPC m g R~ | resume previous helm-gtags session                        |
| ~SPC m g s~ | select any tag in a project retrieved by gtags            |
| ~SPC m g S~ | show stack of visited locations                           |
| ~SPC m g u~ | manually update tag database                              |
