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

    <div class="inner">
        {{- if (.Param "UseHugoToc") }}
        {{- .TableOfContents -}}
        {{- else }}
        {{- $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)) -}}

        {{- $bareul := slice -}}
        <ul>
            {{- range seq (sub $firstHeaderLevel $largest) -}}
            <ul>
                {{- $bareul = $bareul | append (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 | strings.TrimSpace -}}

                {{- 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 . -}}
                    {{- $bareul = $bareul | append . -}}
                    {{- end -}}
                    {{- end -}}
                    {{- else -}}
                    </li>
                    {{- if lt $headerLevel $prevHeaderLevel -}}
                    {{- range seq (sub $prevHeaderLevel 1) -1 $headerLevel -}}
                    {{- if in $bareul . -}}
                </ul>
                {{/* manually do pop item */}}
                {{- $bareul = first (sub (len $bareul) 1) $bareul -}}
                {{- else -}}
            </ul>
            </li>
            {{- end -}}
            {{- end -}}
            {{- end -}}
            {{- end }}
            <li>
                <a href="#{{- $cleanedID -}}" aria-label="{{- $header | plainify | safeHTML -}}">{{- $header | plainify | safeHTML -}}</a>
                {{- else }}
            <li>
                <a href="#{{- $cleanedID -}}" aria-label="{{- $header | plainify | safeHTML -}}">{{- $header | plainify | 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 $bareul (add . $firstHeaderLevel) }}
        </ul>
        {{- else }}
        </ul>
        </li>
        {{- end -}}
        {{- end }}
        </ul>
        {{- end }}
    </div>
</details>
{{- end }}