Optimize random string generator

Common/Utils/StringUtils.cs

@@ -1,37 +1,67 @@
-using System.Security.Cryptography;
+using System.Buffers;
+using System.Runtime.InteropServices;
+using System.Security.Cryptography;
 
 namespace ZapMe.Utils;
 
 public static class StringUtils
 {
-    /// <summary>
-    /// Generates a cryptographically secure random string of <paramref name="length"/> length, consisting of these characters: abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_
-    /// </summary>
-    /// <param name="length">Length of the generated string</param>
-    /// <returns></returns>
-    public static string GenerateUrlSafeRandomString(int length)
+    private static void FillUrlSafeRandomString(Span<char> buffer, int length)
     {
         const string chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_";
 
-        Span<byte> data = stackalloc byte[(length * 3 / 4) + 4];
-
-        using (RandomNumberGenerator rng = RandomNumberGenerator.Create())
+        int[] array = ArrayPool<int>.Shared.Rent((length / 5) + 1);
+        try
         {
-            rng.GetBytes(data);
-        }
+            Span<int> data = new(array);
 
-        Span<char> buffer = stackalloc char[length + 3];
+            using (RandomNumberGenerator rng = RandomNumberGenerator.Create())
+            {
+                rng.GetBytes(MemoryMarshal.AsBytes(data));
+            }
 
-        for (int i = 0, j = 0; j < length; i += 3, j += 4)
-        {
-            int bitBuffer = (data[i + 2] << 16) | (data[i + 1] << 8) | data[i];
+            int i = 0;
+            int j = 0;
+            do
+            {
+                int bitBuffer = data[i++];
+
+                buffer[j + 0] = chars[bitBuffer & 0x3F];
+                buffer[j + 1] = chars[(bitBuffer >> 6) & 0x3F];
+                buffer[j + 2] = chars[(bitBuffer >> 12) & 0x3F];
+                buffer[j + 3] = chars[(bitBuffer >> 18) & 0x3F];
+                buffer[j + 4] = chars[(bitBuffer >> 24) & 0x3F];
+
+                j += 5;
+            }
+            while ((j + 5) < length);
 
-            buffer[j + 0] = chars[bitBuffer & 0x3F];
-            buffer[j + 1] = chars[(bitBuffer >> 6) & 0x3F];
-            buffer[j + 2] = chars[(bitBuffer >> 12) & 0x3F];
-            buffer[j + 3] = chars[bitBuffer >> 18];
+            int remainder = length - j;
+            if (remainder == 0)
+            {
+                return;
+            }
+
+            int lastBitBuffer = data[i];
+            do
+            {
+                buffer[j++] = chars[(lastBitBuffer >> (--remainder * 6)) & 0x3F];
+            }
+            while (remainder > 0);
+        }
+        finally
+        {
+            ArrayPool<int>.Shared.Return(array);
         }
+    }
 
-        return new string(buffer[..length]);
+    /// <summary>
+    /// Generates a cryptographically secure random string of <paramref name="length"/> length, consisting of these characters: abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_
+    /// </summary>
+    /// <param name="length">Length of the generated string</param>
+    /// <returns></returns>
+    public static string GenerateUrlSafeRandomString(int length)
+    {
+        return String.Create(length, length, FillUrlSafeRandomString);
     }
 }