kjelsrud.dev/layouts/partials/toc.html

{{- $headers := findRE "<h[1-6].*?>(.|\n])+?</h[1-6]>" .Content -}}
{{- $has_headers := ge (len $headers) 1 -}}
{{- if $has_headers -}}
<aside id="toc-container" class="toc-container wide">
    <div class="toc">
        <details {{if (.Param "TocOpen") }} open{{ end }}>
            <summary accesskey="c" title="(Alt + C)">
                <span class="details">{{- i18n "toc" | default "Table of Contents" }}</span>
            </summary>

            <div class="inner">
                {{- $largest := 6 -}}
                {{- range $headers -}}
                {{- $headerLevel := index (findRE "[1-6]" . 1) 0 -}}
                {{- $headerLevel := len (seq $headerLevel) -}}
                {{- if lt $headerLevel $largest -}}
                {{- $largest = $headerLevel -}}
                {{- end -}}
                {{- end -}}

                {{- $firstHeaderLevel := len (seq (index (findRE "[1-6]" (index $headers 0) 1) 0)) -}}

                {{- $.Scratch.Set "bareul" slice -}}
                <ul>
                    {{- range seq (sub $firstHeaderLevel $largest) -}}
                    <ul>
                        {{- $.Scratch.Add "bareul" (sub (add $largest .) 1) -}}
                        {{- end -}}
                        {{- range $i, $header := $headers -}}
                        {{- $headerLevel := index (findRE "[1-6]" . 1) 0 -}}
                        {{- $headerLevel := len (seq $headerLevel) -}}

                        {{/* get id="xyz" */}}
                        {{- $id := index (findRE "(id=\"(.*?)\")" $header 9) 0 }}

                        {{- /* strip id="" to leave xyz, no way to get regex capturing groups in hugo */ -}}
                        {{- $cleanedID := replace (replace $id "id=\"" "") "\"" "" }}
                        {{- $header := replaceRE "<h[1-6].*?>((.|\n])+?)</h[1-6]>" "$1" $header -}}

                        {{- if ne $i 0 -}}
                        {{- $prevHeaderLevel := index (findRE "[1-6]" (index $headers (sub $i 1)) 1) 0 -}}
                        {{- $prevHeaderLevel := len (seq $prevHeaderLevel) -}}
                        {{- if gt $headerLevel $prevHeaderLevel -}}
                        {{- range seq $prevHeaderLevel (sub $headerLevel 1) -}}
                        <ul>
                            {{/* the first should not be recorded */}}
                            {{- if ne $prevHeaderLevel . -}}
                            {{- $.Scratch.Add "bareul" . -}}
                            {{- end -}}
                            {{- end -}}
                            {{- else -}}
                            </li>
                            {{- if lt $headerLevel $prevHeaderLevel -}}
                            {{- range seq (sub $prevHeaderLevel 1) -1 $headerLevel -}}
                            {{- if in ($.Scratch.Get "bareul") . -}}
                        </ul>
                        {{/* manually do pop item */}}
                        {{- $tmp := $.Scratch.Get "bareul" -}}
                        {{- $.Scratch.Delete "bareul" -}}
                        {{- $.Scratch.Set "bareul" slice}}
                        {{- range seq (sub (len $tmp) 1) -}}
                        {{- $.Scratch.Add "bareul" (index $tmp (sub . 1)) -}}
                        {{- end -}}
                        {{- else -}}
                    </ul>
                    </li>
                    {{- end -}}
                    {{- end -}}
                    {{- end -}}
                    {{- end }}
                    <li>
                        <a href="#{{- $cleanedID -}}" aria-label="{{- $header | plainify -}}">{{- $header | safeHTML -}}</a>
                        {{- else }}
                    <li>
                        <a href="#{{- $cleanedID -}}" aria-label="{{- $header | plainify -}}">{{- $header | safeHTML -}}</a>
                        {{- end -}}
                        {{- end -}}
                        <!-- {{- $firstHeaderLevel := len (seq (index (findRE "[1-6]" (index $headers 0) 1) 0)) -}} -->
                        {{- $firstHeaderLevel := $largest }}
                        {{- $lastHeaderLevel := len (seq (index (findRE "[1-6]" (index $headers (sub (len $headers) 1)) 1) 0)) }}
                    </li>
                    {{- range seq (sub $lastHeaderLevel $firstHeaderLevel) -}}
                    {{- if in ($.Scratch.Get "bareul") (add . $firstHeaderLevel) }}
                </ul>
                {{- else }}
                </ul>
                </li>
                {{- end -}}
                {{- end }}
                </ul>
            </div>
        </details>
    </div>
</aside>
<script>
    let activeElement;
    let elements;
    window.addEventListener('DOMContentLoaded', function (event) {
        checkTocPosition();

        elements = document.querySelectorAll('h1[id],h2[id],h3[id],h4[id],h5[id],h6[id]');
         // Make the first header active
         activeElement = elements[0];
         const id = encodeURI(activeElement.getAttribute('id')).toLowerCase();
         document.querySelector(`.inner ul li a[href="#${id}"]`).classList.add('active');
     }, false);

    window.addEventListener('resize', function(event) {
        checkTocPosition();
    }, false);

    window.addEventListener('scroll', () => {
        // Check if there is an object in the top half of the screen or keep the last item active
        activeElement = Array.from(elements).find((element) => {
            if ((getOffsetTop(element) - window.pageYOffset) > 0 && 
                (getOffsetTop(element) - window.pageYOffset) < window.innerHeight/2) {
                return element;
            }
        }) || activeElement

        elements.forEach(element => {
             const id = encodeURI(element.getAttribute('id')).toLowerCase();
             if (element === activeElement){
                 document.querySelector(`.inner ul li a[href="#${id}"]`).classList.add('active');
             } else {
                 document.querySelector(`.inner ul li a[href="#${id}"]`).classList.remove('active');
             }
         })
     }, false);

    const main = parseInt(getComputedStyle(document.body).getPropertyValue('--article-width'), 10);
    const toc = parseInt(getComputedStyle(document.body).getPropertyValue('--toc-width'), 10);
    const gap = parseInt(getComputedStyle(document.body).getPropertyValue('--gap'), 10);

    function checkTocPosition() {
        const width = document.body.scrollWidth;

        if (width - main - (toc * 2) - (gap * 4) > 0) {
            document.getElementById("toc-container").classList.add("wide");
        } else {
            document.getElementById("toc-container").classList.remove("wide");
        }
    }

    function getOffsetTop(element) {
        if (!element.getClientRects().length) {
            return 0;
        }
        let rect = element.getBoundingClientRect();
        let win = element.ownerDocument.defaultView;
        return rect.top + win.pageYOffset;   
    }
</script>
{{- end }}