Add handling of parsing the post ID from threadmark links that don't …

…include

the post ID at all (ie: the very first post in the thread).
If no ID is provided when requesting a post from a page, return the first post.
Fixes issue where the only threadmark of a thread is the very first post of
the thread.

TallyCore/Adapters/XenForoAdapter.cs

@@ -437,11 +437,14 @@ private HtmlNode GetPostFromPageById(HtmlDocument page, string id)
             if (id == null)
                 throw new ArgumentNullException(nameof(id));
 
+            var postsList = GetPostsFromPage(page);
+
+            if (id == "")
+                return postsList.FirstOrDefault();
+
             if (!id.StartsWith("post-"))
                 id = "post-" + id;
 
-            var postsList = GetPostsFromPage(page);
-
             return postsList.FirstOrDefault(a => a.Id == id);
         }
         #endregion
@@ -576,18 +579,28 @@ private string GetPostIdFromUrl(string url)
             if (url == null)
                 throw new ArgumentNullException(nameof(url));
 
+            // For links directly to post ID
             // Format: https://forums.sufficientvelocity.com/posts/4062860/
             Regex postLinkRegex = new Regex(@"posts/(?<postId>\d+)/");
             var m = postLinkRegex.Match(url);
             if (m.Success)
                 return m.Groups["postId"].Value;
 
+            // For long-form links.  May not include page number if the post is on the first page of the thread.
             // Format: https://forums.sufficientvelocity.com/threads/a-villain-in-a-world-of-heroes.18001/page-109#post-4062860
+            // Format: https://forums.sufficientvelocity.com/threads/a-villain-in-a-world-of-heroes.18001/#post-4062860
             postLinkRegex = new Regex(@"/(page-\d+)?#post-(?<postId>\d+)");
             m = postLinkRegex.Match(url);
             if (m.Success)
                 return m.Groups["postId"].Value;
 
+            // Long form link for the very first post in the thread.
+            // Format: https://forums.sufficientvelocity.com/threads/a-villain-in-a-world-of-heroes.18001/
+            postLinkRegex = new Regex(@"/threads/[^\/]+/$");
+            m = postLinkRegex.Match(url);
+            if (m.Success)
+                return "";
+
             throw new ArgumentException("Unable to extract post ID from link:\n" + url, nameof(url));
         }