Rework of the file export (#894)

Co-authored-by: Thorsten Sommer <SommerEngineering@users.noreply.github.com>

app/MindWork AI Studio/Tools/CsvWriter.cs

@@ -0,0 +1,64 @@
+using System.Globalization;
+
+namespace AIStudio.Tools;
+
+/// <summary>
+/// Writes rows of character-separated values. Fields are quoted according to RFC 4180 using the
+/// separator of the respective file.
+/// </summary>
+public static class CsvWriter
+{
+    /// <summary>
+    /// The separator a spreadsheet expects from a CSV file written for the given language.
+    /// </summary>
+    /// <remarks>
+    /// Wherever a comma separates the decimals of a number, it cannot separate the columns of a
+    /// file as well: German Excel therefore expects a semicolon and puts a comma-separated file
+    /// into a single column. This is the same rule Excel itself follows when it writes a CSV, so
+    /// we ask the culture rather than keeping a list of languages of our own.
+    /// </remarks>
+    /// <param name="ietfTag">The IETF tag of the language, for example "de-DE".</param>
+    /// <returns>The separator to write with.</returns>
+    public static char SeparatorFor(string ietfTag)
+    {
+        if (string.IsNullOrWhiteSpace(ietfTag))
+            return ',';
+
+        try
+        {
+            var culture = CultureInfo.GetCultureInfo(ietfTag);
+            return culture.NumberFormat.NumberDecimalSeparator is "," ? ';' : ',';
+        }
+        catch (CultureNotFoundException)
+        {
+            return ',';
+        }
+    }
+
+    /// <summary>
+    /// Joins the given fields into one row.
+    /// </summary>
+    /// <param name="separator">The separator between two fields.</param>
+    /// <param name="fields">The fields of the row.</param>
+    /// <returns>The row, without a line ending.</returns>
+    public static string ToRow(char separator, params string[] fields) => string.Join(separator, fields.Select(field => ToField(field, separator)));
+
+    /// <summary>
+    /// Quotes one field according to RFC 4180.
+    /// </summary>
+    private static string ToField(string text, char separator)
+    {
+        if (string.IsNullOrEmpty(text))
+            return string.Empty;
+
+        // Quoting the complete field is important for long and multi-line AI
+        // answers: neither separators nor line breaks within an answer may
+        // create another column or row.
+        if (!text.Contains(separator) && !text.Contains('"') && !text.Contains('\n') && !text.Contains('\r'))
+            return text;
+
+        return $"""
+                "{text.Replace("\"", "\"\"")}"
+                """;
+    }
+}