Refactor default file name generation and add date ranges there

Closes #29

DiscordChatExporter.Cli/Verbs/ExportChatVerb.cs

@@ -1,8 +1,7 @@
 using System;
-using System.IO;
 using System.Threading.Tasks;
 using DiscordChatExporter.Cli.Verbs.Options;
-using DiscordChatExporter.Core.Models;
+using DiscordChatExporter.Core.Helpers;
 using DiscordChatExporter.Core.Services;
 using Tyrrrz.Extensions;
 
@@ -32,12 +31,16 @@ namespace DiscordChatExporter.Cli.Verbs
             var chatLog = await dataService.GetChatLogAsync(Options.GetToken(), Options.ChannelId, 
                 Options.After, Options.Before);
 
-            // Generate file path if not set
+            // Generate file path if not set or is a directory
             var filePath = Options.FilePath;
             if (filePath == null || filePath.EndsWith("/") || filePath.EndsWith("\\"))
             {
-                filePath += $"{chatLog.Guild.Name} - {chatLog.Channel.Name}.{Options.ExportFormat.GetFileExtension()}"
-                    .Replace(Path.GetInvalidFileNameChars(), '_');
+                // Generate default file name
+                var defaultFileName = ExportHelper.GetDefaultExportFileName(Options.ExportFormat, chatLog.Guild,
+                    chatLog.Channel, Options.After, Options.Before);
+
+                // Append the file name to the file path
+                filePath += defaultFileName;
             }
 
             // Export

DiscordChatExporter.Core/Helpers/ExportHelper.cs

@@ -0,0 +1,52 @@
+using System;
+using System.IO;
+using System.Text;
+using DiscordChatExporter.Core.Models;
+
+namespace DiscordChatExporter.Core.Helpers
+{
+    public static class ExportHelper
+    {
+        public static string GetDefaultExportFileName(ExportFormat format, Guild guild, Channel channel,
+            DateTime? from = null, DateTime? to = null)
+        {
+            var result = new StringBuilder();
+
+            // Append guild and channel names
+            result.Append($"{guild.Name} - {channel.Name}");
+
+            // Append date range
+            if (from != null || to != null)
+            {
+                result.Append(" (");
+
+                // Both 'from' and 'to' are set
+                if (from != null && to != null)
+                {
+                    result.Append($"{from:yyyy-MM-dd} to {to:yyyy-MM-dd}");
+                }
+                // Only 'from' is set
+                else if (from != null)
+                {
+                    result.Append($"after {from:yyyy-MM-dd}");
+                }
+                // Only 'to' is set
+                else
+                {
+                    result.Append($"before {to:yyyy-MM-dd}");
+                }
+
+                result.Append(")");
+            }
+
+            // Append extension
+            result.Append($".{format.GetFileExtension()}");
+
+            // Replace invalid chars
+            foreach (var invalidChar in Path.GetInvalidFileNameChars())
+                result.Replace(invalidChar, '_');
+
+            return result.ToString();
+        }
+    }
+}

DiscordChatExporter.Gui/ViewModels/Dialogs/ExportSetupViewModel.cs

@@ -1,7 +1,7 @@
 using System;
 using System.Collections.Generic;
-using System.IO;
 using System.Linq;
+using DiscordChatExporter.Core.Helpers;
 using DiscordChatExporter.Core.Models;
 using DiscordChatExporter.Core.Services;
 using DiscordChatExporter.Gui.ViewModels.Framework;
@@ -59,10 +59,10 @@ namespace DiscordChatExporter.Gui.ViewModels.Dialogs
                 To = From;
 
             // Generate default file name
-            var ext = SelectedFormat.GetFileExtension();
-            var defaultFileName = $"{Guild.Name} - {Channel.Name}.{ext}".Replace(Path.GetInvalidFileNameChars(), '_');
+            var defaultFileName = ExportHelper.GetDefaultExportFileName(SelectedFormat, Guild, Channel, From, To);
 
             // Prompt for output file path
+            var ext = SelectedFormat.GetFileExtension();
             var filter = $"{ext.ToUpperInvariant()} files|*.{ext}";
             FilePath = _dialogManager.PromptSaveFilePath(filter, defaultFileName);