Streaming exporter

Fixes #125
Closes #177

DiscordChatExporter.Core.Services/Logic/ExportLogic.cs

@@ -0,0 +1,72 @@
+using System;
+using System.IO;
+using System.Text;
+using DiscordChatExporter.Core.Models;
+
+namespace DiscordChatExporter.Core.Services.Logic
+{
+    public static class ExportLogic
+    {
+        public static string GetDefaultExportFileName(ExportFormat format,
+            Guild guild, Channel channel,
+            DateTimeOffset? after = null, DateTimeOffset? before = null)
+        {
+            var buffer = new StringBuilder();
+
+            // Append guild and channel names
+            buffer.Append($"{guild.Name} - {channel.Name} [{channel.Id}]");
+
+            // Append date range
+            if (after != null || before != null)
+            {
+                buffer.Append(" (");
+
+                // Both 'after' and 'before' are set
+                if (after != null && before != null)
+                {
+                    buffer.Append($"{after:yyyy-MM-dd} to {before:yyyy-MM-dd}");
+                }
+                // Only 'after' is set
+                else if (after != null)
+                {
+                    buffer.Append($"after {after:yyyy-MM-dd}");
+                }
+                // Only 'before' is set
+                else
+                {
+                    buffer.Append($"before {before:yyyy-MM-dd}");
+                }
+
+                buffer.Append(")");
+            }
+
+            // Append extension
+            buffer.Append($".{format.GetFileExtension()}");
+
+            // Replace invalid chars
+            foreach (var invalidChar in Path.GetInvalidFileNameChars())
+                buffer.Replace(invalidChar, '_');
+
+            return buffer.ToString();
+        }
+
+        public static string GetExportPartitionFilePath(string baseFilePath, int partitionIndex)
+        {
+            // First partition - no changes
+            if (partitionIndex <= 0)
+                return baseFilePath;
+
+            // Inject partition index into file name
+            var fileNameWithoutExt = Path.GetFileNameWithoutExtension(baseFilePath);
+            var fileExt = Path.GetExtension(baseFilePath);
+            var fileName = $"{fileNameWithoutExt} [part {partitionIndex + 1}]{fileExt}";
+
+            // Generate new path
+            var dirPath = Path.GetDirectoryName(baseFilePath);
+            if (!string.IsNullOrWhiteSpace(dirPath))
+                return Path.Combine(dirPath, fileName);
+
+            return fileName;
+        }
+    }
+}