hugo-PaperMod/layouts/partials/toc.html

{{- $headers := findRE "<h[1-4].*?>(.|\n])+?</h[1-4]>" .Content -}}
{{- $has_headers := ge (len $headers) 1 -}}
{{- if $has_headers -}}

{{- $largest := 6 -}}
{{- range $headers -}}
{{- $headerLevel := index (findRE "[1-4]" . 1) 0 -}}
{{- $headerLevel := len (seq $headerLevel) -}}
{{- if lt $headerLevel $largest -}}
{{- $largest = $headerLevel -}}
{{- end -}}
{{- end -}}

{{- $firstHeaderLevel := len (seq (index (findRE "[1-4]" (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 "[1-4]" . 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[1-4].*?>((.|\n])+?)</h[1-4]>" "$1" $header -}}

        {{- if ne $i 0 -}}
        {{- $prevHeaderLevel := index (findRE "[1-4]" (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  -}}">{{- $header | safeHTML -}}</a>
        {{- else -}}
    <li>
        <a href="#{{- $cleanedID -}}">{{- $header | safeHTML -}}</a>
        {{- end -}}
        {{- end -}}
        <!-- {{- $firstHeaderLevel := len (seq (index (findRE "[1-4]" (index $headers 0) 1) 0)) -}} -->
        {{- $firstHeaderLevel := $largest }}
        {{- $lastHeaderLevel := len (seq (index (findRE "[1-4]" (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 -}}