From c9b56f85acc7a5e4ea7a115c2927f63508daeeab Mon Sep 17 00:00:00 2001
From: Jason Nelson <jason@carbonmade.com>
Date: Wed, 18 Apr 2018 15:40:50 -0700
Subject: [PATCH] Pack GifLogicalScreen value directly

---
 .../Sections/GifLogicalScreenDescriptor.cs    | 28 +++++++++++++++----
 .../GifLogicalScreenDescriptorTests.cs        | 23 +++++++++++++++
 2 files changed, 45 insertions(+), 6 deletions(-)
 create mode 100644 tests/ImageSharp.Tests/Formats/Gif/Sections/GifLogicalScreenDescriptorTests.cs

diff --git a/src/ImageSharp/Formats/Gif/Sections/GifLogicalScreenDescriptor.cs b/src/ImageSharp/Formats/Gif/Sections/GifLogicalScreenDescriptor.cs
index 35056c6e7..1cfec4763 100644
--- a/src/ImageSharp/Formats/Gif/Sections/GifLogicalScreenDescriptor.cs
+++ b/src/ImageSharp/Formats/Gif/Sections/GifLogicalScreenDescriptor.cs
@@ -104,14 +104,30 @@ namespace SixLabors.ImageSharp.Formats.Gif
 
         public static byte GetPackedValue(bool globalColorTableFlag, int colorResolution, bool sortFlag, int globalColorTableSize)
         {
-            PackedField field = default;
+            /*
+            Global Color Table Flag    | 1 Bit
+            Color Resolution           | 3 Bits
+            Sort Flag                  | 1 Bit
+            Size of Global Color Table | 3 Bits
+            */
 
-            field.SetBit(0, globalColorTableFlag);     // 0   : Global Color Table Flag     | 1 bit
-            field.SetBits(1, 3, globalColorTableSize); // 1-3 : Color Resolution            | 3 bits
-            field.SetBit(4, sortFlag);                 // 4   : Sort Flag                   | 1 bits
-            field.SetBits(5, 3, globalColorTableSize); // 5-7 : Size of Global Color Table  | 3 bits
+            byte value = 0;
 
-            return field.Byte;
+            if (globalColorTableFlag)
+            {
+                value |= 1 << 7;
+            }
+
+            value |= (byte)(colorResolution << 4);
+
+            if (sortFlag)
+            {
+                value |= 1 << 3;
+            }
+
+            value |= (byte)globalColorTableSize;
+
+            return value;
         }
     }
 }
\ No newline at end of file
diff --git a/tests/ImageSharp.Tests/Formats/Gif/Sections/GifLogicalScreenDescriptorTests.cs b/tests/ImageSharp.Tests/Formats/Gif/Sections/GifLogicalScreenDescriptorTests.cs
new file mode 100644
index 000000000..c6458d22f
--- /dev/null
+++ b/tests/ImageSharp.Tests/Formats/Gif/Sections/GifLogicalScreenDescriptorTests.cs
@@ -0,0 +1,23 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using SixLabors.ImageSharp.Formats.Gif;
+
+using Xunit;
+
+namespace SixLabors.ImageSharp.Tests.Formats.Gif
+{
+    public class GifLogicalScreenDescriptorTests
+    {
+        [Fact]
+        public void TestPackedValue()
+        {
+            Assert.Equal(0, GifLogicalScreenDescriptor.GetPackedValue(false, 0, false, 0));
+            Assert.Equal(128, GifLogicalScreenDescriptor.GetPackedValue(true, 0, false, 0)); // globalColorTableFlag
+            Assert.Equal(8, GifLogicalScreenDescriptor.GetPackedValue(false, 0, true, 0)); // sortFlag
+            Assert.Equal(48, GifLogicalScreenDescriptor.GetPackedValue(false, 3, false, 0));
+            Assert.Equal(155, GifLogicalScreenDescriptor.GetPackedValue(true, 1, true, 3));
+            Assert.Equal(55, GifLogicalScreenDescriptor.GetPackedValue(false, 3, false, 7));
+        }
+    }
+}
\ No newline at end of file