Add alerts (Issue #84)

spec/fixtures/alert.txt

@@ -0,0 +1,67 @@
+## Alert
+
+Alerts are a Markdown extension based on the blockquote syntax that
+you can use to emphasize critical information. On GitHub, they are
+displayed with distinctive colors and icons to indicate the significance
+of the content.
+
+Use alerts only when they are crucial for user success and limit them
+to one or two per article to prevent overloading the reader. Additionally,
+you should avoid placing alerts consecutively. Alerts cannot be nested
+within other elements.
+
+To add an alert, use a special blockquote line specifying the alert type
+and an optional title, followed by the alert information in a standard
+blockquote.
+
+There are five types of alert: 
+
+* NOTE
+* TIP
+* IMPORTANT
+* WARNING
+* CAUTION
+
+```````````````````````````````` example alert
+> [!NOTE]
+> Useful information that users should know, even when skimming content.
+.
+<div class="alert alert-note"><p class="alert-title">NOTE</p>
+<p>Useful information that users should know, even when skimming content.</p>
+</div>
+````````````````````````````````
+
+An optional title can be added after the closing bracket.
+
+```````````````````````````````` example alert
+> [!NOTE] What is a note?
+> Useful information that users should know, even when skimming content.
+.
+<div class="alert alert-note"><p class="alert-title">What is a note?</p>
+<p>Useful information that users should know, even when skimming content.</p>
+</div>
+````````````````````````````````
+
+Empty spaces after the brackets are ignored.
+
+```````````````````````````````` example alert
+> [!NOTE]  
+> Useful information that users should know, even when skimming content.
+.
+<div class="alert alert-note"><p class="alert-title">NOTE</p>
+<p>Useful information that users should know, even when skimming content.</p>
+</div>
+````````````````````````````````
+
+Alert-like block quotes which don't use one of the five listed
+alert types are just block quotes.
+
+```````````````````````````````` example alert
+> [!FOO]  
+> Not a real alert.
+.
+<blockquote>
+<p>[!FOO]<br />
+Not a real alert.</p>
+</blockquote>
+````````````````````````````````

spec/markd_spec.cr

@@ -17,6 +17,9 @@ describe_spec("fixtures/gfm-extensions.txt", gfm: true)
 
 describe_spec("fixtures/gfm-regression.txt", gfm: true)
 
+# Alert spec examples
+describe_spec("fixtures/alert.txt", gfm: true)
+
 describe Markd do
   # Thanks Ryan Westlund <[email protected]> feedback via email.
   it "should escape unsafe html" do

src/markd/node.cr

@@ -14,6 +14,7 @@ module Markd
       List
       Item
       BlockQuote
+      Alert
       ThematicBreak
       Code
       CodeBlock
@@ -45,6 +46,7 @@ module Markd
       Type::List,
       Type::Item,
       Type::BlockQuote,
+      Type::Alert,
       Type::CustomInLine,
       Type::CustomBlock,
       Type::Table,

src/markd/parsers/block.cr

@@ -9,6 +9,7 @@ module Markd::Parser
     RULES = {
       Node::Type::Document      => Rule::Document.new,
       Node::Type::BlockQuote    => Rule::BlockQuote.new,
+      Node::Type::Alert         => Rule::BlockQuote.new, # Alerts and BlockQuotes are the same
       Node::Type::Heading       => Rule::Heading.new,
       Node::Type::CodeBlock     => Rule::CodeBlock.new,
       Node::Type::HTMLBlock     => Rule::HTMLBlock.new,

src/markd/renderer.cr

@@ -53,6 +53,7 @@ module Markd
     abstract def list(node : Node, entering : Bool) : Nil
     abstract def item(node : Node, entering : Bool) : Nil
     abstract def block_quote(node : Node, entering : Bool) : Nil
+    abstract def alert(node : Node, entering : Bool) : Nil
     abstract def thematic_break(node : Node, entering : Bool) : Nil
     abstract def code_block(node : Node, entering : Bool, formatter : T?) : Nil forall T
     abstract def code(node : Node, entering : Bool) : Nil
@@ -86,6 +87,8 @@ module Markd
             item(node, entering)
           when Node::Type::BlockQuote
             block_quote(node, entering)
+          when Node::Type::Alert
+            alert(node, entering)
           when Node::Type::ThematicBreak
             thematic_break(node, entering)
           when Node::Type::CodeBlock

src/markd/renderers/html_renderer.cr

@@ -63,6 +63,19 @@ module Markd
       newline
     end
 
+    def alert(node : Node, entering : Bool) : Nil
+      newline
+      if entering
+        tag("div", {"class" => "alert alert-#{node.data["alert"].to_s.downcase}"})
+        tag("p", {"class" => "alert-title"}) do
+          output(node.data["title"].as(String))
+        end
+      else
+        tag("div", end_tag: true)
+      end
+      newline
+    end
+
     def table(node : Node, entering : Bool) : Nil
       has_body = node.data["has_body"]
       newline

src/markd/rule.cr

@@ -105,6 +105,8 @@ module Markd
     TABLE_HEADING_SEPARATOR = /^(\|?\s*:{0,1}-:{0,1}+\s*)+(\||\s*)$/
     TABLE_CELL_SEPARATOR    = /(?<!\\)\|/
 
+    ADMONITION_START = /^> \[!((?:NOTE|TIP|IMPORTANT|CAUTION|WARNING)+)](\s*.*)?$/
+
     # Match Value
     #
     # - None: no match

src/markd/rules/block_quote.cr

@@ -6,7 +6,15 @@ module Markd::Rule
       if match?(parser)
         seek(parser)
         parser.close_unmatched_blocks
-        parser.add_child(Node::Type::BlockQuote, parser.next_nonspace)
+        if parser.gfm && (match = parser.line.match(Rule::ADMONITION_START))
+          node = parser.add_child(Node::Type::Alert, parser.next_nonspace)
+          # This is an alert
+          node.data["alert"] = match[1]
+          node.data["title"] = (match[2]? && !match[2].strip.empty?) ? match[2].strip : match[1]
+          parser.advance_offset(parser.line.size, false)
+        else
+          parser.add_child(Node::Type::BlockQuote, parser.next_nonspace)
+        end
 
         MatchValue::Container
       else