Fix how to create link ID for a table of contents

[tkyobc]

I fixed the problem that different IDs are appended to a table of contents and headlines. If this fix is applied, links in a table of contents become to work well.

Please check it.

[jmromer]

@larstvei friendly ping -- it'd be great to see this in master 😄

[larstvei]

Thanks @jkrmr. I didn't really understand what this did the last time I looked at it, so forgot about it. Please provide a minimal example with pull requests @tkyobc, that makes them easier to accept.

I think I get the basic issue now, but have one question: Does ox-md.el suffer from the same problem?

My main concern is that this is basically a large copy-paste from ox-md.el, which ox-gfm.el is derived from, where just one function call is changed. Wouldn't it be better to achieve this by changing ox-md.el? I'm guessing it has more users and is better maintained, so it would be good for ox-gfm.el to piggyback future enhancements to ox-md.el.

[jmromer]

@larstvei Sorry, I should have taken a look at the implementation. I think this could be fixed with a few characters.

The plugin generates a TOC with IDs as follows:

- [Motivation](#sec-1)
- [Scenarios](#sec-2)
- [Prior art](#sec-3)
- [Tasks](#sec-4)
- [Implementation notes](#sec-5)

# Motivation<a id="orgheadline1"></a>

What we'd want in order for them to work as expected is

- [Motivation](#orgheadline1)
- [Scenarios](#orgheadline2)
- [Prior art](#orgheadline3)
- [Tasks](#orgheadline4)
- [Implementation notes](#orgheadline5)

# Motivation<a id="orgheadline1"></a>

(Or, alternatively, for all IDs to be normalized on the sec- pattern instead of the orgheadline one, although that approach seems like it would take more work. I believe that's the approach this PR takes.)

I think the following would suffice for the less invasive approach:

;; layers/org/extensions/ox-gfm/ox-gfm.el L90-L101

(defun org-gfm-format-toc (headline)
  "Return an appropriate table of contents entry for HEADLINE. INFO is a
plist used as a communication channel."
  (let* ((title (org-export-data
                 (org-export-get-alt-title headline info) info))
         (level (1- (org-element-property :level headline)))
         (indent (concat (make-string (* level 2) ? )))
         (anchor (or (org-element-property :custom_id headline)
-                    (concat "sec-" (mapconcat 'number-to-string
+                    (concat "orgheadline" (mapconcat 'number-to-string
                                               (org-export-get-headline-number
                                                headline info) "-")))))
    (concat indent "- [" title "]" "(#" anchor ")")))

Unless I'm misunderstanding the issue?

[larstvei]

Good idea, but it it doesn't seem to work with nested structures.

For instance, this:

* h1
** test1
* h2
*** test2
* h3

exports to this:

- [h1](#sec-1)
  - [test1](#sec-1-1)
- [h2](#sec-2)
    - [test2](#sec-2-0-1)
- [h3](#sec-3)

# h1<a id="orgheadline2"></a>

## test1<a id="orgheadline1"></a>

# h2<a id="orgheadline4"></a>

### test2<a id="orgheadline3"></a>

# h3<a id="orgheadline5"></a>

In addition, i think newer versions of org mode enumerates the id's completely different.

I'll see if I can find a way to simplify the implementation, if not I'll include the change from @tkyobc .

[larstvei]

As far as I can tell commit 32dd362 fixes this. Tested it a couple of versions of org, both work fine.

The example above exports to:

- [h1](#orgheadline2)
  - [test1](#orgheadline1)
- [h2](#orgheadline4)
    - [test2](#orgheadline3)
- [h3](#orgheadline5)

# h1<a id="orgheadline2"></a>

## test1<a id="orgheadline1"></a>

# h2<a id="orgheadline4"></a>

### test2<a id="orgheadline3"></a>

# h3<a id="orgheadline5"></a>