Markdown.pl: add anchors and definitions for headers

Long documents often need to link within themselves in order to
provide a convenient table of contents section.

To facilitate this, all setext-style and atx-style headers defined
at the top-level (i.e. they start at the left margin) now have
automatic anchors added to them and link definitions added for
them provided there is not already a link definition with the
same id present.

These can be easily targeted using the "implicit link name"
shortcut (e.g. [Foo][]).

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>

Markdown.pl

@@ -27,7 +27,7 @@ All rights reserved.
 }
 
 require Exporter;
-use Digest::MD5 qw(md5);
+use Digest::MD5 qw(md5 md5_hex);
 use File::Basename qw(basename);
 use Scalar::Util qw(refaddr looks_like_number);
 use Pod::Usage;
@@ -64,6 +64,7 @@ my %g_perm_block_ids;
 # Global hashes, used by various utility routines
 my %g_urls;
 my %g_titles;
+my %g_anchors;
 my %g_block_ids;
 my %g_html_blocks;
 my %g_code_blocks;
@@ -343,6 +344,7 @@ sub Markdown {
     # articles):
     %g_urls = ();
     %g_titles = ();
+    %g_anchors = ();
     %g_block_ids = ();
     %g_html_blocks = ();
     %g_code_blocks = ();
@@ -373,7 +375,7 @@ sub Markdown {
     # Strip link definitions, store in hashes.
     $text = _StripLinkDefinitions($text);
 
-    $text = _RunBlockGamut("\n".$text);
+    $text = _RunBlockGamut("\n".$text, 1);
 
     # Unhashify code blocks
     $text =~ s/(\005\d+\006)/$g_code_blocks{$1}/g;
@@ -575,9 +577,9 @@ sub _RunBlockGamut {
 # These are all the transformations that form block-level
 # tags like paragraphs, headers, and list items.
 #
-    my $text = shift;
+    my ($text, $anchors) = @_;
 
-    $text = _DoHeaders($text);
+    $text = _DoHeaders($text, $anchors);
 
     # Do Horizontal Rules:
     $text =~ s{^ {0,3}\*(?: {0,2}\*){2,}[ \t]*$}{\n<hr$opt{empty_element_suffix}\n}gm;
@@ -694,8 +696,9 @@ sub _DoAnchors {
 	    $link_id = lc $link_text;	  # for shortcut links like [this][].
 	}
 
-	if (defined $g_urls{$link_id}) {
+	if (defined($g_urls{$link_id}) || defined($g_anchors{$link_id})) {
-	    my $url = _PrefixURL($g_urls{$link_id});
+	    my $url = $g_urls{$link_id};
+	    $url = defined($url) ? _PrefixURL($url) : $g_anchors{$link_id};
 	    # We've got to encode these to avoid conflicting
 	    # with italics, bold and strike through.
 	    $url =~ s!([*_~])!$g_escape_table{$1}!g;
@@ -866,8 +869,31 @@ sub _DoImages {
 }
 
 
+sub _MakeAnchorId {
+    use bytes;
+    my $link = lc(shift);
+    $link =~ s/^\s+//;
+    $link =~ s/\s+$//;
+    $link =~ s/\s+/ /g;
+    $link =~ tr/-a-z0-9_/_/cs;
+    return '' unless $link ne '';
+    $link = md5_hex($link) if length($link) > 64;
+    "_".$link."_";
+}
+
+
+sub _GetNewAnchorId {
+    my $link = lc(shift);
+    return '' if defined($g_anchors{$link});
+    my $id = _MakeAnchorId($link);
+    return '' unless $id;
+    $g_anchors{$link} = '#'.$id;
+    $id;
+}
+
+
 sub _DoHeaders {
-    my $text = shift;
+    my ($text, $anchors) = @_;
 
     # Setext-style headers:
     #     Header 1
@@ -880,15 +906,24 @@ sub _DoHeaders {
     #     ~~~~~~~~
     #
     $text =~ s{ ^(?:=+[ \t]*\n)?(.+)[ \t]*\n=+[ \t]*\n+ }{
-	"<h1>" . _RunSpanGamut($1) . "</h1>\n\n";
+	my $h = $1;
+	my $id = _GetNewAnchorId($h);
+	$id = " id=\"$id\"" if $id ne "";
+	"<h1$id>" . _RunSpanGamut($h) . "</h1>\n\n";
     }egmx;
 
     $text =~ s{ ^(?:-+[ \t]*\n)?(.+)[ \t]*\n-+[ \t]*\n+ }{
-	"<h2>" . _RunSpanGamut($1) . "</h2>\n\n";
+	my $h = $1;
+	my $id = _GetNewAnchorId($h);
+	$id = " id=\"$id\"" if $id ne "";
+	"<h2$id>" . _RunSpanGamut($h) . "</h2>\n\n";
     }egmx;
 
     $text =~ s{ ^(?:~+[ \t]*\n)?(.+)[ \t]*\n~+[ \t]*\n+ }{
-	"<h3>" . _RunSpanGamut($1) . "</h3>\n\n";
+	my $h = $1;
+	my $id = _GetNewAnchorId($h);
+	$id = " id=\"$id\"" if $id ne "";
+	"<h3$id>" . _RunSpanGamut($h) . "</h3>\n\n";
     }egmx;
 
 
@@ -907,8 +942,11 @@ sub _DoHeaders {
 	    \#*		# optional closing #'s (not counted)
 	    \n+
 	}{
+	    my $h = $2;
 	    my $h_level = length($1);
-	    "<h$h_level>" . _RunSpanGamut($2) . "</h$h_level>\n\n";
+	    my $id = $h_level <= 3 ? _GetNewAnchorId($h) : '';
+	    $id = " id=\"$id\"" if $id ne "";
+	    "<h$h_level$id>" . _RunSpanGamut($h) . "</h$h_level>\n\n";
 	}egmx;
 
     return $text;

syntax.md

@@ -687,6 +687,14 @@ tend to put them immediately after each paragraph in which they're
 used, but if you want, you can put them all at the end of your
 document, sort of like footnotes.
 
+All first, second and third level headers defined at the top-level
+(in other words they are not in lists and start at the left margin)
+using either the setext-style or atx-style automatically have an
+anchor id and link definition added for them provided there is not
+already a previous definition with the same id.  You can use this
+to place a table-of-contents at the top of the document that links
+to subsections later in the document.
+
 Here's an example of reference links in action:
 
       I get 10 times more traffic from [Google] [1] than from