Fixed appending empty channel exports

Several bugs related to appending empty channel exports have been fixed:
- Previously, the partition index hadn’t been adapted if the existing channel export had been empty. This had resulted in an uncaught exception as the target file had already existed.
- Previously, the HTML last message detection hadn’t worked if the export had been empty, resulting in an uncaught exception.
- Previously, empty TXT exports had been detected as message containing because the footer hadn’t been taken into account. This had resulted in an uncaught exception as no valid timestamp could have been retrieved.
- Previously, empty TXT & CSV exports would have been detected as message containing if they had contained enough empty lines. This has been fixed by testing whether the respective file has enough non-empty lines in a functional and lazy way.

Additionally, there have been multiple other improvements:
- The CSV last message detection has been improved to now use the last non-empty line. Previously, it wouldn’t have worked (and thrown an exception) if there had been any trailing empty lines.
- The info messages that a channel export is being appended had previously not been logged if the export had been empty. Therefore, new specific info logs have been added for this situation.
- If an empty channel had previously been exported and is still empty, the export will now be aborted (and a respective new info message will be logged).
- A comment has been added to the HTML template to prevent it from being changed without corresponding changes in the HTML last message detection.

DiscordChatExporter.Core/Exporting/ChannelExporter.cs

@@ -55,24 +55,32 @@ public class ChannelExporter(DiscordClient discord)
                     );
                     if (lastMessageSnowflake != null)
                     {
-                        request.LastPriorMessage = lastMessageSnowflake.Value;
-
-                        if (!request.Channel.MayHaveMessagesAfter(request.LastPriorMessage.Value))
+                        if (!request.Channel.MayHaveMessagesAfter(lastMessageSnowflake.Value))
                         {
                             logger.IncrementCounter(ExportResult.UpdateExportSkip);
                             logger.LogInfo(request, "Existing export already up to date");
                             return;
                         }
-
+                        request.LastPriorMessage = lastMessageSnowflake.Value;
                         logger.LogInfo(
                             request,
                             "Appending existing export starting at "
                                 + lastMessageSnowflake.Value.ToDate()
                         );
+                    }
+                    else
+                    {
+                        if (request.Channel.IsEmpty)
+                        {
+                            logger.IncrementCounter(ExportResult.UpdateExportSkip);
+                            logger.LogInfo(request, "Existing empty export already up to date");
+                            return;
+                        }
+                        logger.LogInfo(request, "Appending existing empty export.");
+                    }
                     currentPartitionIndex = MessageExporter.GetPartitionCount(
                         request.OutputFilePath
                     );
-                    }
                     break;
                 default:
                     throw new InvalidOperationException(

DiscordChatExporter.Core/Exporting/CsvMessageWriter.cs

@@ -142,10 +142,12 @@ internal partial class CsvMessageWriter(Stream stream, ExportContext context)
     {
         try
         {
-            var fileLines = File.ReadAllLines(filePath)
-                .SkipWhile(string.IsNullOrWhiteSpace)
-                .ToArray();
-            if (fileLines.Length <= HeaderSize)
+            var fileLines = File.ReadAllLines(filePath);
+            var fileContainsMessages = fileLines
+                .Where(line => !string.IsNullOrWhiteSpace(line))
+                .Skip(HeaderSize)
+                .Any();
+            if (!fileContainsMessages)
                 return null;
 
             const string columnPattern = "(?:[^\"]?(?:\"\")?)*";
@@ -155,7 +157,9 @@ internal partial class CsvMessageWriter(Stream stream, ExportContext context)
             );
             var messageDateRegex = new Regex(messageDatePattern);
 
-            var timestampMatch = messageDateRegex.Match(fileLines[^1]);
+            var timestampMatch = messageDateRegex.Match(
+                fileLines.Last(line => !string.IsNullOrWhiteSpace(line))
+            );
             var timestampString = timestampMatch.Groups[1].Value;
             var timestamp = DateTimeOffset.Parse(timestampString);
             return Snowflake.FromDate(timestamp, true);

DiscordChatExporter.Core/Exporting/HtmlMessageWriter.cs

@@ -179,6 +179,8 @@ internal partial class HtmlMessageWriter(Stream stream, ExportContext context, s
             var messageDateRegex = MessageDateRegex();
 
             var timestampMatches = messageDateRegex.Matches(fileContent);
+            if (timestampMatches.Count == 0)
+                return null;
             var timestampString = timestampMatches[^1].Groups[1].Value;
             var timestamp = DateTimeOffset.Parse(timestampString);
             return Snowflake.FromDate(timestamp, true);

DiscordChatExporter.Core/Exporting/MessageGroupTemplate.cshtml

@@ -240,6 +240,7 @@
                             }
 
                             @* Timestamp *@
+                            @* Don't change this without changing the corresponding detection in the HtmlMessageWriter *@
                             <span class="chatlog__timestamp" title="@FormatDate(message.Timestamp, "f")"><a href="#chatlog__message-container-@message.Id">@FormatDate(message.Timestamp)</a></span>
                         </div>
                     }

DiscordChatExporter.Core/Exporting/PlainTextMessageWriter.cs

@@ -16,6 +16,7 @@ internal partial class PlainTextMessageWriter(Stream stream, ExportContext conte
     : MessageWriter(stream, context)
 {
     private const int HeaderSize = 4;
+    private const int FooterSize = 3;
 
     private readonly TextWriter _writer = new StreamWriter(stream);
 
@@ -306,13 +307,17 @@ internal partial class PlainTextMessageWriter(Stream stream, ExportContext conte
     public static Snowflake? GetLastMessageDate(string filePath)
     {
         var fileLines = File.ReadAllLines(filePath);
-        if (fileLines.SkipWhile(string.IsNullOrWhiteSpace).ToArray().Length <= HeaderSize)
+        var fileContainsMessages = fileLines
+            .Where(line => !string.IsNullOrWhiteSpace(line))
+            .Skip(HeaderSize + FooterSize)
+            .Any();
+        if (!fileContainsMessages)
             return null;
 
         var messageDateRegex = MessageDateRegex();
 
         // Find the last line with a message timestamp
-        for (var i = fileLines.Length - 1; i >= HeaderSize; i--)
+        for (var i = fileLines.Length - 1 - FooterSize; i >= HeaderSize; i--)
         {
             var timestampMatch = messageDateRegex.Match(fileLines[i]);
             if (!timestampMatch.Success)