From 4cd1709a386bfc10f5d088d7ecf447965f32907c Mon Sep 17 00:00:00 2001
From: Brant Burnett <bburnett@centeredgesoftware.com>
Date: Thu, 16 May 2024 19:36:38 -0400
Subject: [PATCH] Redesign LeftShiftOverflows test

The current test was based on the C++ test, but the overhead of the
LeftShiftOverflowsMasks lookup is much higher on C#, especially on
legacy .NET Framework. In particular, this new approach avoids range
checks and theoretical exceptions.
---
 Snappier.Benchmarks/LeftShiftOverflows.cs | 17 ++++++++++++
 Snappier.Tests/HelpersTests.cs            | 34 ++++++++++++++++++++++-
 Snappier/Internal/Helpers.cs              | 15 ++++------
 3 files changed, 55 insertions(+), 11 deletions(-)
 create mode 100644 Snappier.Benchmarks/LeftShiftOverflows.cs

diff --git a/Snappier.Benchmarks/LeftShiftOverflows.cs b/Snappier.Benchmarks/LeftShiftOverflows.cs
new file mode 100644
index 0000000..b0151fd
--- /dev/null
+++ b/Snappier.Benchmarks/LeftShiftOverflows.cs
@@ -0,0 +1,17 @@
+using BenchmarkDotNet.Attributes;
+using Snappier.Internal;
+
+namespace Snappier.Benchmarks
+{
+    public class LeftShiftOverflows
+    {
+        private byte value = 24;
+        private int shift = 7;
+
+        [Benchmark(Baseline = true)]
+        public bool Current()
+        {
+            return Helpers.LeftShiftOverflows(value, shift);
+        }
+    }
+}
diff --git a/Snappier.Tests/HelpersTests.cs b/Snappier.Tests/HelpersTests.cs
index 66af6dd..d11ed5b 100644
--- a/Snappier.Tests/HelpersTests.cs
+++ b/Snappier.Tests/HelpersTests.cs
@@ -1,7 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
-using System.Text;
 using Snappier.Internal;
 using Xunit;
 
@@ -9,6 +8,39 @@ namespace Snappier.Tests
 {
     public class HelpersTests
     {
+        #region LeftShiftOverflows
+
+        [Theory]
+        [InlineData(2, 31)]
+        [InlineData(0xff, 25)]
+        public void LeftShiftOverflows_True(byte value, int shift)
+        {
+            // Act
+
+            var result = Helpers.LeftShiftOverflows(value, shift);
+
+            // Assert
+
+            Assert.True(result);
+        }
+
+        [Theory]
+        [InlineData(1, 31)]
+        [InlineData(0xff, 24)]
+        [InlineData(0, 31)]
+        public void LeftShiftOverflows_False(byte value, int shift)
+        {
+            // Act
+
+            var result = Helpers.LeftShiftOverflows(value, shift);
+
+            // Assert
+
+            Assert.False(result);
+        }
+
+        #endregion
+
         #region Log2FloorNonZero
 
         public static IEnumerable<object[]> Log2FloorNonZeroValues() =>
diff --git a/Snappier/Internal/Helpers.cs b/Snappier/Internal/Helpers.cs
index 2118a1a..c3a58d8 100644
--- a/Snappier/Internal/Helpers.cs
+++ b/Snappier/Internal/Helpers.cs
@@ -45,17 +45,12 @@ public static int MaxCompressedLength(int sourceBytes)
             return 32 + sourceBytes + sourceBytes / 6 + 1;
         }
 
-        private static ReadOnlySpan<byte> LeftShiftOverflowsMasks =>
-        [
-            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-            0x00, 0x80, 0xc0, 0xe0, 0xf0, 0xf8, 0xfc, 0xfe
-        ];
-
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static bool LeftShiftOverflows(byte value, int shift) =>
-            (value & LeftShiftOverflowsMasks[shift]) != 0;
+        public static bool LeftShiftOverflows(byte value, int shift)
+        {
+            Debug.Assert(shift < 32);
+            return (value & ~(0xffff_ffffu >>> shift)) != 0;
+        }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static uint ExtractLowBytes(uint value, int numBytes)