hugo-PaperMod/layouts/partials/toc.html

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

            {{- $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>
            {{- end }}
        </div>
    </details>
</div>