Use Markdown::Perl for GitHub-style fenced code blocks (#57)

CLAUDE.md

@@ -57,7 +57,7 @@ All publication writes are atomic: `_publish_template_to_file`/`_atomically_writ
 
 - **`Plerd::Post`** (`Post.pm`) — one post per Markdown source file. The real work happens in `_process_source_file`, fired by a Moose **trigger** the moment `source_file` is set (not lazily). That method:
   - parses the leading `key: value` metadata block (terminated by a blank line) into `attributes`, then the Markdown body;
-  - runs the body and title through `Text::MultiMarkdown::markdown` then `Plerd::SmartyPants`;
+  - runs the body and title through `Markdown::Perl` (a shared converter in GitHub mode, so fenced code blocks, tables, and the rest of GFM work) then `Plerd::SmartyPants`. The converter drops `"` from its HTML-escaped characters so SmartyPants can curl literal double quotes into typographic ones; quotes inside code are still escaped via `html_escaped_code_characters`;
   - **mutates the source file in place** — if `time`, `published_filename`, or `guid` are missing, it computes them and rewrites the file with the full metadata block. This write-back is core behavior, not a side effect to "fix." Date logic: explicit W3C `time:` wins; else a `YYYY-MM-DD` filename prefix sets midnight of that date (or now, if the date is today); else now.
   - resolves tags (comma-separated `tags:` header) into shared `Plerd::Tag` objects via `$plerd->tag_named`.
   - `publish` renders `post.tt`. `send_webmentions` walks the post's links via `Web::Mention`.

cpanfile

@@ -1,3 +1,6 @@
+# Markdown::Perl (our Markdown engine) requires Perl 5.26 or later.
+requires 'perl', '5.026';
+
 requires 'Moose';
 requires 'Template';
 requires 'Path::Class';
@@ -7,7 +10,7 @@ requires 'JSON';
 requires 'DateTime' => '1.54';
 requires 'DateTime::Format::W3CDTF';
 requires 'URI';
-requires 'Text::MultiMarkdown';
+requires 'Markdown::Perl';
 requires 'YAML';
 requires 'FindBin';
 requires 'File::ChangeNotify';
@@ -21,3 +24,4 @@ requires 'Readonly';
 requires 'Web::Mention' => '0.703'; 
 requires 'MooseX::Types::URI';
 requires 'Test::Warn';
+requires 'Capture::Tiny'; # Used by t/init.t.

lib/Plerd/Post.pm

@@ -3,7 +3,7 @@ package Plerd::Post;
 use Moose;
 use DateTime;
 use DateTime::Format::W3CDTF;
-use Text::MultiMarkdown qw( markdown );
+use Markdown::Perl;
 use URI;
 use HTML::Strip;
 use Data::GUID;
@@ -18,6 +18,13 @@ use Web::Mention;
 use Readonly;
 Readonly my $WPM => 200; # The words-per-minute reading speed to assume
 
+my $markdown = Markdown::Perl->new(
+    mode                    => 'github',
+    # Drop '"' so literal quotes survive for Plerd::SmartyPants to curl.
+    html_escaped_characters => '&<>',
+);
+sub markdown { $markdown->convert( $_[0] ) }
+
 has 'plerd' => (
     is => 'ro',
     required => 1,
@@ -444,10 +451,11 @@ sub _process_source_file {
     }
     $self->body( $body );
 
-    foreach ( qw( title body ) ) {
-        if ( defined( $self->$_ ) ) {
-            $self->$_( Plerd::SmartyPants::process( markdown( $self->$_ ) ) );
-        }
+    foreach my $field ( qw( title body ) ) {
+        next unless defined $self->$field;
+        $self->$field(
+            Plerd::SmartyPants::process( markdown( $self->$field ) )
+        );
     }
 
     # Strip unnecessary <p> tags that the markdown processor just added to the title.

t/code_blocks.t

@@ -0,0 +1,75 @@
+use warnings;
+use strict;
+use Test::More;
+use Path::Class::Dir;
+use Path::Class::File;
+use URI;
+
+use FindBin;
+use lib "$FindBin::Bin/../lib";
+
+use_ok( 'Plerd' );
+use Plerd::Init;
+
+# Regression test for GitHub issue #57: GitHub-style fenced code blocks.
+# Text::MultiMarkdown does not understand ``` or ~~~ fences -- it collapsed a
+# backtick-fenced block into a single inline <code> span (so every line
+# wrapped together) and ignored ~~~ entirely. Plerd now pre-processes fences
+# into <pre><code> blocks before handing the body to Markdown.
+
+my $blog_dir = Path::Class::Dir->new( "$FindBin::Bin/code_blocks_blog" );
+$blog_dir->rmtree;
+Plerd::Init::initialize( $blog_dir->stringify, 0 );
+
+my $source = <<'END';
+title: Code
+
+Backtick fence with a language:
+
+```perl
+my $x = 1;
+if ( $x < 2 ) { say "ok"; }
+```
+
+Tilde fence:
+
+~~~
+plain text
+second line
+~~~
+
+Inline `code` is untouched.
+END
+
+Path::Class::File->new( $blog_dir, 'source', '2022-07-31-code.md' )->spew(
+    iomode => '>:encoding(utf8)', $source,
+);
+
+my $plerd = Plerd->new(
+    path         => $blog_dir->stringify,
+    title        => 'Test Blog',
+    author_name  => 'Nobody',
+    author_email => 'nobody@example.com',
+    base_uri     => URI->new( 'http://blog.example.com/' ),
+);
+
+$plerd->publish_all;
+my ( $post ) = @{ $plerd->posts };
+my $body = $post->body;
+
+like( $body, qr{<pre><code class="language-perl">},
+    'Backtick fence becomes a <pre><code> block with a language class.' );
+like( $body, qr{my \$x = 1;\nif},
+    'Code lines are preserved on separate lines, not wrapped together.' );
+like( $body, qr{\$x &lt; 2},
+    'Code special characters are HTML-escaped.' );
+like( $body, qr{<pre><code>\s*plain text\nsecond line},
+    'Tilde fence also becomes a <pre><code> block.' );
+like( $body, qr{<code>code</code>},
+    'Inline code spans still work.' );
+unlike( $body, qr{<p><code>perl},
+    'Fence is not collapsed into a single inline code span.' );
+
+$blog_dir->rmtree;
+
+done_testing();