tocgen

#!/usr/bin/env perl -n -pi

our $prevn;

BEGIN {
    $prevn = 0;
    print STDERR "<h1>Contents</h1>\n<ul>\n";
}

if (m{
    <h([123])\s+id="           # 1. hn
       ([^"]+)                 # 3. id
    ">
    (                          # 4. header
        (<code>[^(]+)?.+?      # 5. label
    )
    </h\1>
}x) {
    my ($hn, $id, $val, $label) = ($1, $2, $3, $4);

    if ($prevn) {
        if ($hn != $prevn) {
            print STDERR $hn > $prevn
                ? $hn - $prevn > 1
                ? "<ul><li><ul>" : "<ul>\n" : $prevn - $hn > 1
                ? "</li></ul></li></ul></li>\n" : "</li></ul></li>\n";
            $prevn = $hn;
        } else {
            print STDERR "</li>\n"
        }
    } else {
        $prevn = $hn;
    }
    print STDERR qq{<li><a href="#$id">$val</a>};

    $_ = qq{<h$hn id="$id">$val</h$hn>\n};
}

END { print STDERR "</li>\n</ul>\n" }