Alternative implemention of the Mod97-10 calculation

test/IbanNet.Benchmark/AltMod.cs

@@ -0,0 +1,39 @@
+using System.Diagnostics.Contracts;
+using System.Runtime.CompilerServices;
+
+namespace IbanNet.Benchmark;
+
+internal static class AltMod
+{ 
+    /// <summary>Checks the Mod97 constraint.</summary>
+    [Pure]
+    public static int Mod97(char[] iban)
+    {
+        ulong num = 0;
+
+        // Calculate the first 4 characters (country and checksum) last
+        for (var i = 4; i < iban.Length; i++)
+        {
+            num = Next(num, iban[i]);
+
+            // If we wait longer, we could overflow.
+            if (num >> 57 is not 0) num %= 97;
+        }
+
+        // If we wait longer, we could overflow.
+        if (num >> 44 is not 0) num %= 97;
+
+        for (var i = 0; i < 4; i++)
+        {
+            num = Next(num, iban[i]);
+        }
+
+        return (int)(num % 97);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        static ulong Next(ulong num, char ch)
+            => ch <= '9'
+            ? (num * 10) + ch - '0'
+            : (num * 100) + ch - 'A' + 10;
+    }
+}

test/IbanNet.Benchmark/Mod9710Benchmark.cs

@@ -21,11 +21,18 @@ public static IEnumerable<object> TestCases()
         ];
     }
 
+    [Benchmark(Baseline = true)]
+    [ArgumentsSource(nameof(TestCases))]
+    public int Test(TestCase buffer)
+    {
+        return Mod9710.Compute(buffer.Buffer);
+    }
+
     [Benchmark]
     [ArgumentsSource(nameof(TestCases))]
-    public void Test(TestCase buffer)
+    public int Alt(TestCase buffer)
     {
-        Mod9710.Compute(buffer.Buffer);
+        return AltMod.Mod97(buffer.Buffer);
     }
 
     public sealed class TestCase