More project base

src/GuiApp/Avalonia/FileTime.GuiApp/Converters/FormatSizeConverter.cs

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+using System.Globalization;
+using Avalonia.Data.Converters;
+
+namespace FileTime.GuiApp.Converters
+{
+    public class FormatSizeConverter : IValueConverter
+    {
+        private const long OneKiloByte = 1024;
+        private const long OneMegaByte = OneKiloByte * 1024;
+        private const long OneGigaByte = OneMegaByte * 1024;
+        private const long OneTerraByte = OneGigaByte * 1024;
+
+        public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+        {
+            return (value, int.TryParse(parameter?.ToString(), out var prec)) switch
+            {
+                (long size, true) => ToSizeString(size, prec),
+                (long size, false) => ToSizeString(size),
+                (null, _) => "...",
+                _ => value
+            };
+        }
+
+        public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+        {
+            throw new NotImplementedException();
+        }
+
+        public static string ToSizeString(long fileSize, int precision = 1)
+        {
+            var fileSizeD = (decimal)fileSize;
+            var (size, suffix) = fileSize switch
+            {
+                > OneTerraByte => (fileSizeD / OneTerraByte, "T"),
+                > OneGigaByte => (fileSizeD / OneGigaByte, "G"),
+                > OneMegaByte => (fileSizeD / OneMegaByte, "M"),
+                > OneKiloByte => (fileSizeD / OneKiloByte, "K"),
+                _ => (fileSizeD, "B")
+            };
+
+            var result = string.Format("{0:N" + precision + "}", size).Replace(',', '.');
+
+            if (result.Contains('.')) result = result.TrimEnd('0').TrimEnd('.');
+            return result + " " + suffix;
+        }
+    }
+}