Better article ID parsing

A new ArticleIdParser class takes in an HTTP::Request object and parses
the article ID from it. It intentinoally fails on tag, user, and search
pages and attempts to only catch articles.

src/actions/articles/show.cr

@@ -2,7 +2,7 @@ require "json"
 
 class Articles::Show < BrowserAction
   fallback do
-    post_id = maybe_post_id(context.request)
+    post_id = ArticleIdParser.parse(context.request)
     case post_id
     in Monads::Just
       response = client_class.post_data(post_id.value!)
@@ -18,27 +18,6 @@ class Articles::Show < BrowserAction
     end
   end
 
-  def maybe_post_id(request : HTTP::Request)
-    from_params = post_id_from_params(request.query_params)
-    from_path = post_id_from_path(request.path)
-    from_path.or(from_params)
-  end
-
-  def post_id_from_path(request_path : String)
-    Monads::Try(Regex::MatchData)
-      .new(->{ request_path.match(/([0-9a-f]+)$/i) })
-      .to_maybe
-      .fmap(->(matches : Regex::MatchData) { matches[1] })
-  end
-
-  def post_id_from_params(params : URI::Params)
-    maybe_uri = Monads::Try(String)
-      .new(->{ params["redirectUrl"] })
-      .to_maybe
-      .fmap(->(url : String) { URI.parse(url) })
-      .bind(->(uri : URI) { post_id_from_path(uri.path) })
-  end
-
   def client_class
     if use_local?
       LocalClient

src/classes/article_id_parser.cr

@@ -0,0 +1,31 @@
+class ArticleIdParser
+  include Monads
+
+  ID_REGEX = /[\/\-]([0-9a-f]+)/i
+
+  def self.parse(request : HTTP::Request)
+    new.parse(request)
+  end
+
+  def parse(request : HTTP::Request) : Maybe
+    from_params = post_id_from_params(request.query_params)
+    from_path = post_id_from_path(request.path)
+    from_path.or(from_params)
+  end
+
+  private def post_id_from_path(request_path : String)
+    return Nothing(String).new if request_path.starts_with?("/tag/")
+    Try(Regex::MatchData)
+      .new(->{ request_path.match(ID_REGEX) })
+      .to_maybe
+      .fmap(->(matches : Regex::MatchData) { matches[1] })
+  end
+
+  private def post_id_from_params(params : URI::Params)
+    maybe_uri = Try(String)
+      .new(->{ params["redirectUrl"] })
+      .to_maybe
+      .fmap(->(url : String) { URI.parse(url) })
+      .bind(->(uri : URI) { post_id_from_path(uri.path) })
+  end
+end