Make decode even faster

MediaBrowser.Common/Hex.cs

@@ -1,5 +1,5 @@
 using System;
-using System.Globalization;
+using System.Diagnostics.CodeAnalysis;
 
 namespace MediaBrowser.Common
 {
@@ -11,6 +11,23 @@ namespace MediaBrowser.Common
         internal const string HexCharsLower = "0123456789abcdef";
         internal const string HexCharsUpper = "0123456789ABCDEF";
 
+        internal const int LastHexSymbol = 0x66; // 102: f
+
+        /// <summary>
+        /// Map from an ASCII char to its hex value shifted,
+        /// e.g. <c>b</c> -> 11. 0xFF means it's not a hex symbol.
+        /// </summary>
+        /// <value></value>
+        internal static ReadOnlySpan<byte> HexLookup => new byte[] {
+            0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
+            0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
+            0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
+            0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
+            0xFF, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
+            0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
+            0xFF, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f
+        };
+
         /// <summary>
         /// Encodes <c>bytes</c> as a hex string.
         /// </summary>
@@ -41,17 +58,37 @@ namespace MediaBrowser.Common
         /// <returns>The decoded bytes.</returns>
         public static byte[] Decode(ReadOnlySpan<char> str)
         {
-            byte[] bytes = new byte[str.Length / 2];
-            int j = 0;
-            for (int i = 0; i < str.Length; i += 2)
+            if (str.Length == 0)
             {
-                bytes[j++] = byte.Parse(
-                    str.Slice(i, 2),
-                    NumberStyles.HexNumber,
-                    CultureInfo.InvariantCulture);
+                return Array.Empty<byte>();
+            }
+
+            var unHex = HexLookup;
+
+            int byteLen = str.Length / 2;
+            byte[] bytes = new byte[byteLen];
+            int i = 0;
+            for (int j = 0; j < byteLen; j++)
+            {
+                byte a;
+                byte b;
+                if (str[i] > LastHexSymbol
+                    || (a = unHex[str[i++]]) == 0xFF
+                    || str[i] > LastHexSymbol
+                    || (b = unHex[str[i++]]) == 0xFF)
+                {
+                    ThrowArgumentException(nameof(str));
+                    break; // Unreachable
+                }
+
+                bytes[j] = (byte)((a << 4) | b);
             }
 
             return bytes;
         }
+
+        [DoesNotReturn]
+        private static void ThrowArgumentException(string paramName)
+            => throw new ArgumentException("Character is not a hex symbol.", paramName);
     }
 }

benches/Jellyfin.Common.Benches/HexDecodeBenches.cs

@@ -9,10 +9,13 @@ namespace Jellyfin.Common.Benches
     [MemoryDiagnoser]
     public class HexDecodeBenches
     {
-        private const int N = 1000000;
-        private readonly string data;
+        [Params(0, 10, 100, 1000, 10000, 1000000)]
+        public int N { get; set; }
 
-        public HexDecodeBenches()
+        private string data;
+
+        [GlobalSetup]
+        public void GlobalSetup()
         {
             var tmp = new byte[N];
             new Random(42).NextBytes(tmp);