notes/_includes/fix_linenos.html

{%- comment -%}
This file can be used to fix the HTML produced by Jekyll for highlighted
code with line numbers.

It works with `{% highlight some_language linenos %}...{% endhighlight %}`
and with the Kramdown option to add line numbers to fenced code.

The implementation was derived from the workaround provided by 
Dmitry Hrabrov (DeXP) at
https://github.com/penibelst/jekyll-compress-html/issues/71#issuecomment-188144901

EXPLANATION

The HTML produced by Rouge highlighting with lie numbers is of the form
`code table`. Jekyll (<= 4.1.1) always wraps the highlighted HTML
with `pre`. This wrapping is not only unnecessary, but also transforms
the conforming HTML produced by Rouge to non-conforming HTML, which
results in HTML validation error reports. 

The fix removes the outer `pre` tags whenever they contain the pattern
`<table class="rouge-table">`.
  
Apart from avoiding HTML validation errors, the fix allows the use of
the [Jekyll layout for compressing HTML](http://jch.penibelst.de),
which relies on `pre` tags not being nested, according to
https://github.com/penibelst/jekyll-compress-html/issues/71#issuecomment-172069842 

USAGE

(Any names can be used for `some_var` and `some_language`.)

{% capture some_var %}
{% highlight some_language linenos %}
Some code
{% endhighlight %}
{% endcapture %}
{% include fix_linenos.html code=some_var %}

For code fences:

{% capture some_var %}
```some_language
Some code
```
{% endcapture %}
{% assign some_var = some_var | markdownify %}
{% include fix_linenos.html code=some_var %}

CAVEATS

The above does not work when `Some code` happens to contain the matched string 
`<table class="rouge-table">`.

The use of this file overwrites the variable `fix_linenos_code` with `nil`.

{%- endcomment -%}

{% assign fix_linenos_code = include.code %}
{% if fix_linenos_code contains '<table class="rouge-table">' %}
  {% assign fix_linenos_code = fix_linenos_code | replace: '<pre class="highlight">', '<pre>' %}
  {% assign fix_linenos_code = fix_linenos_code | replace: "<pre><code", "<code" %}
  {% assign fix_linenos_code = fix_linenos_code | replace: "</code></pre>", "</code>" %}
{% endif %}
{{ fix_linenos_code }}
{% assign fix_linenos_code = nil %}