Optimize ObjectId hex formatting

LiteDB.Tests/Document/ObjectId_Tests.cs

@@ -1,4 +1,6 @@
-using FluentAssertions;
+using System;
+using FluentAssertions;
+using LiteDB;
 using Xunit;
 
 namespace LiteDB.Tests.Document
@@ -41,5 +43,44 @@ public void ObjectId_Equals_Null_Does_Not_Throw()
             oid1.Equals(null).Should().BeFalse();
             oid1.Equals(oid0).Should().BeFalse();
         }
+
+        [Fact]
+        public void ObjectId_ToString_Minimizes_Allocations()
+        {
+            var objectId = ObjectId.NewObjectId();
+
+            objectId.ToString();
+
+            GC.Collect();
+            GC.WaitForPendingFinalizers();
+            GC.Collect();
+
+            var before = GC.GetAllocatedBytesForCurrentThread();
+            var hex = objectId.ToString();
+            var allocated = GC.GetAllocatedBytesForCurrentThread() - before;
+
+            hex.Should().HaveLength(24);
+            allocated.Should().BeLessThan(220);
+        }
+
+        [Fact]
+        public void ObjectId_FromHex_Minimizes_Allocations()
+        {
+            var original = ObjectId.NewObjectId();
+            var hex = original.ToString();
+
+            _ = new ObjectId(hex);
+
+            GC.Collect();
+            GC.WaitForPendingFinalizers();
+            GC.Collect();
+
+            var before = GC.GetAllocatedBytesForCurrentThread();
+            var parsed = new ObjectId(hex);
+            var allocated = GC.GetAllocatedBytesForCurrentThread() - before;
+
+            parsed.Should().Be(original);
+            allocated.Should().BeLessThan(220);
+        }
     }
-}
+}

LiteDB/Document/ObjectId.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Buffers;
 using System.Diagnostics;
 using System.Runtime.CompilerServices;
 using System.Security;
@@ -120,22 +121,30 @@ public ObjectId(byte[] bytes, int startIndex = 0)
                 bytes[startIndex + 11];
         }
 
+        private const int ObjectIdByteLength = 12;
+        private const int ObjectIdStringLength = ObjectIdByteLength * 2;
+
         /// <summary>
         /// Convert hex value string in byte array
         /// </summary>
         private static byte[] FromHex(string value)
         {
             if (string.IsNullOrEmpty(value)) throw new ArgumentNullException(nameof(value));
-            if (value.Length != 24) throw new ArgumentException(string.Format("ObjectId strings should be 24 hex characters, got {0} : \"{1}\"", value.Length, value));
+            if (value.Length != ObjectIdStringLength) throw new ArgumentException(string.Format("ObjectId strings should be 24 hex characters, got {0} : \"{1}\"", value.Length, value));
 
-            var bytes = new byte[12];
+            var hex = value.AsSpan();
 
-            for (var i = 0; i < 24; i += 2)
-            {
-                bytes[i / 2] = Convert.ToByte(value.Substring(i, 2), 16);
-            }
+#if NET8_0_OR_GREATER
+            return Convert.FromHexString(hex);
+#else
+            Span<byte> buffer = stackalloc byte[ObjectIdByteLength];
+            WriteBytesFromHex(hex, buffer);
 
-            return bytes;
+            var result = new byte[ObjectIdByteLength];
+            buffer.CopyTo(result);
+
+            return result;
+#endif
         }
 
         #endregion
@@ -199,34 +208,123 @@ public int CompareTo(ObjectId other)
         /// </summary>
         public void ToByteArray(byte[] bytes, int startIndex)
         {
-            bytes[startIndex + 0] = (byte)(this.Timestamp >> 24);
-            bytes[startIndex + 1] = (byte)(this.Timestamp >> 16);
-            bytes[startIndex + 2] = (byte)(this.Timestamp >> 8);
-            bytes[startIndex + 3] = (byte)(this.Timestamp);
-            bytes[startIndex + 4] = (byte)(this.Machine >> 16);
-            bytes[startIndex + 5] = (byte)(this.Machine >> 8);
-            bytes[startIndex + 6] = (byte)(this.Machine);
-            bytes[startIndex + 7] = (byte)(this.Pid >> 8);
-            bytes[startIndex + 8] = (byte)(this.Pid);
-            bytes[startIndex + 9] = (byte)(this.Increment >> 16);
-            bytes[startIndex + 10] = (byte)(this.Increment >> 8);
-            bytes[startIndex + 11] = (byte)(this.Increment);
+            this.WriteBytes(bytes.AsSpan(startIndex, ObjectIdByteLength));
         }
 
         public byte[] ToByteArray()
         {
-            var bytes = new byte[12];
+            var bytes = new byte[ObjectIdByteLength];
 
-            this.ToByteArray(bytes, 0);
+            this.WriteBytes(bytes);
 
             return bytes;
         }
 
         public override string ToString()
         {
-            return BitConverter.ToString(this.ToByteArray()).Replace("-", "").ToLower();
+#if NET8_0_OR_GREATER
+            Span<byte> buffer = stackalloc byte[ObjectIdByteLength];
+
+            this.WriteBytes(buffer);
+
+            return Convert.ToHexString(buffer).ToLowerInvariant();
+#else
+            Span<byte> buffer = stackalloc byte[ObjectIdByteLength];
+
+            this.WriteBytes(buffer);
+
+            var rented = ArrayPool<char>.Shared.Rent(ObjectIdStringLength);
+
+            try
+            {
+                var chars = rented.AsSpan(0, ObjectIdStringLength);
+                WriteHexLower(buffer, chars);
+
+                return new string(rented, 0, ObjectIdStringLength);
+            }
+            finally
+            {
+                ArrayPool<char>.Shared.Return(rented);
+            }
+#endif
+        }
+
+        private void WriteBytes(Span<byte> destination)
+        {
+            if (destination.Length < ObjectIdByteLength)
+            {
+                throw new ArgumentException("Destination span is too short.", nameof(destination));
+            }
+
+            destination[0] = (byte)(this.Timestamp >> 24);
+            destination[1] = (byte)(this.Timestamp >> 16);
+            destination[2] = (byte)(this.Timestamp >> 8);
+            destination[3] = (byte)(this.Timestamp);
+            destination[4] = (byte)(this.Machine >> 16);
+            destination[5] = (byte)(this.Machine >> 8);
+            destination[6] = (byte)(this.Machine);
+            destination[7] = (byte)(this.Pid >> 8);
+            destination[8] = (byte)(this.Pid);
+            destination[9] = (byte)(this.Increment >> 16);
+            destination[10] = (byte)(this.Increment >> 8);
+            destination[11] = (byte)(this.Increment);
         }
 
+#if !NET8_0_OR_GREATER
+        private static void WriteHexLower(ReadOnlySpan<byte> source, Span<char> destination)
+        {
+            if (destination.Length < ObjectIdStringLength)
+            {
+                throw new ArgumentException("Destination span is too short.", nameof(destination));
+            }
+
+            for (var i = 0; i < source.Length; i++)
+            {
+                var value = source[i];
+                destination[i * 2] = GetHexCharacter(value >> 4);
+                destination[i * 2 + 1] = GetHexCharacter(value & 0x0F);
+            }
+        }
+
+        private static char GetHexCharacter(int value)
+        {
+            return (char)(value < 10 ? '0' + value : 'a' + (value - 10));
+        }
+
+        private static void WriteBytesFromHex(ReadOnlySpan<char> hex, Span<byte> destination)
+        {
+            if (destination.Length < ObjectIdByteLength)
+            {
+                throw new ArgumentException("Destination span is too short.", nameof(destination));
+            }
+
+            for (var i = 0; i < destination.Length; i++)
+            {
+                var high = ParseHexDigit(hex[i * 2]);
+                var low = ParseHexDigit(hex[i * 2 + 1]);
+
+                destination[i] = (byte)((high << 4) | low);
+            }
+        }
+
+        private static int ParseHexDigit(char c)
+        {
+            if ((uint)(c - '0') <= 9)
+            {
+                return c - '0';
+            }
+
+            var lowered = (char)(c | 0x20);
+
+            if ((uint)(lowered - 'a') <= 5)
+            {
+                return lowered - 'a' + 10;
+            }
+
+            throw new FormatException(string.Format("Invalid hex character '{0}' in ObjectId.", c));
+        }
+#endif
+
         #endregion
 
         #region Operators