my %is_approved_element = map { $_ => 1 } qw(a b big blockquote br center cite
					     code dd div dl dt em font hr i img 
					     i ol p pre samp small span strong
					     sub sup table td th tr tt u ul);

my %is_block_element = map { $_ => 1 } qw(blockquote center div dl ol pre table ul);

my %is_raw_element = map { $_ => 1 } qw(dl ol pre table ul);

my %is_empty_element = map { $_ => 1} qw(br img hr);

sub render_text {
    my $str = shift;
    my $no_margin = shift;
    my $buf;
    my @tag_stack;
    my $raw;
    my $nl;
    my $in_p;

    $str =~ s/&(?!\#?[a-zA-Z0-9]+;)/&/g;
    $str =~ s{<person>([^<>]+)</person>} {sprintf(qq|<a href="%s?%s">%s</a>|, $script_name, user_id($1), $1)}gei;

    while ($str) {
	my $out;
	my $need_p = !$in_p && !$raw;

	if ($str =~ s/^([^<>\n]+)//) {
	    $out = $1;

	    for ($out) {
		my $in_a = grep { $_ eq 'a' } @tag_stack;

		unless ($in_a) {
		    s{((?:http|https|ftp|mailto):\S+[a-zA-Z0-9/])} {<a href="$1">$1</a>}g;
		}
	    }
	} elsif ($str =~ s/^<//) {
	    if ($str =~ s/^(\/?)([a-zA-Z][a-zA-Z0-9\.-]*)(.*?)>//) {
		my $close = $1;
		my $name = lc $2;
		my $attributes = $3;

		if ($is_approved_element{$name}) {
		    if ($is_block_element{$name}) {
			# A <p> tag produced by \n\n should go after a <blockquote> tag,
			# not before it.
			$need_p = 0;

			if ($close && $in_p) {
			    $buf .= '</p>';
			    $in_p = 0;
			}

			if ($is_raw_element{$name}) {
			    if ($close) {
				$raw--;
			    } else {
				$raw++;
			    }
			}
		    }

		    if ($close) {
			# Catch a pending \n so it doesn't get translated to <br />.
			if ($nl) {
			    $out .= "\n";
			    $nl = 0;
			}

			if ($tag_stack[0] eq $name) {
			    $out .= "</$name>";
			    shift(@tag_stack);
			}
		    } else {
			$out .= "<$name$attributes>";

			unless ($is_empty_element{$name}) {
			    unshift(@tag_stack, $name);

			    $str =~ s/^(\n+)//;
			    $out .= $1;
			}
		    }
		} else {
		    $out = "&lt;$close$name$attributes&gt;";
		}
	    } else {
		$out = '&lt;';
	    }
	} elsif ($str =~ s/^>//) {
	    $out = '&gt;';
	} elsif ($str =~ s/^(\n{2,})//) {
	    if ($in_p) {
		$buf .= '</p>';
		$in_p = 0;
	    }
	    $buf .= $1;
	} elsif ($str =~ s/^\n//) {
	    if ($raw) {
		$buf .= "\n";
	    } else {
		$nl = 1;
	    }
	}

	if ($out) {
	    if ($nl) {
		$buf .= "<br />\n";
		$nl = 0;
	    }

	    if ($need_p) {
		$buf .= '<p>';
		$in_p = 1;
	    }

	    $buf .= $out;
	}
    }

    for my $name (@tag_stack) {
	$buf .= "</$name>";
    }

    if ($in_p) {
	$buf .= '</p>';
    }

    if ($no_margin) {
	my $first_p = index($buf, '<p>');
	my $last_p = rindex($buf, '<p>');

	if ($first_p == $last_p) {
	    substr($buf, $first_p + 2, 0) = q| style="margin: 0"|;
	} else {
	    if ($last_p >= 0) {
		substr($buf, $last_p + 2, 0) = q| style="margin-bottom: 0"|;
	    }

	    if ($first_p >= 0) {
		substr($buf, $first_p + 2, 0) = q| style="margin-top: 0"|;
	    }
	}
    }

    return $buf;
}