Correct return type of channel methods in Moments and PerceptualHash (#1621)

Author: Gounlaf

src/Magick.NET.Core/Statistics/IMoments.cs

@@ -12,12 +12,12 @@ public interface IMoments
     /// Gets the moments for the all the channels.
     /// </summary>
     /// <returns>The moments for the all the channels.</returns>
-    IChannelMoments Composite();
+    IChannelMoments? Composite();
 
     /// <summary>
     /// Gets the moments for the specified channel.
     /// </summary>
     /// <param name="channel">The channel to get the moments for.</param>
     /// <returns>The moments for the specified channel.</returns>
-    IChannelMoments GetChannel(PixelChannel channel);
+    IChannelMoments? GetChannel(PixelChannel channel);
 }

src/Magick.NET.Core/Statistics/IPerceptualHash.cs

@@ -13,7 +13,7 @@ public interface IPerceptualHash
     /// </summary>
     /// <param name="channel">The channel to get the has for.</param>
     /// <returns>The perceptual hash for the specified channel.</returns>
-    IChannelPerceptualHash GetChannel(PixelChannel channel);
+    IChannelPerceptualHash? GetChannel(PixelChannel channel);
 
     /// <summary>
     /// Returns the sum squared difference between this hash and the other hash.

src/Magick.NET/Statistics/Moments.cs

@@ -29,14 +29,14 @@ internal Moments(MagickImage image, IntPtr list)
     /// </summary>
     /// <returns>The moments for the all the channels.</returns>
     public IChannelMoments Composite()
-        => GetChannel(PixelChannel.Composite);
+        => _channels[PixelChannel.Composite];
 
     /// <summary>
     /// Gets the moments for the specified channel.
     /// </summary>
     /// <param name="channel">The channel to get the moments for.</param>
     /// <returns>The moments for the specified channel.</returns>
-    public IChannelMoments GetChannel(PixelChannel channel)
+    public IChannelMoments? GetChannel(PixelChannel channel)
     {
         _channels.TryGetValue(channel, out var moments);
         return moments;

src/Magick.NET/Statistics/PerceptualHash.cs

@@ -63,7 +63,7 @@ internal bool Isvalid
     /// </summary>
     /// <param name="channel">The channel to get the has for.</param>
     /// <returns>The perceptual hash for the specified channel.</returns>
-    public IChannelPerceptualHash GetChannel(PixelChannel channel)
+    public IChannelPerceptualHash? GetChannel(PixelChannel channel)
     {
         _channels.TryGetValue(channel, out var perceptualHash);
         return perceptualHash;
@@ -78,10 +78,19 @@ public double SumSquaredDistance(IPerceptualHash other)
     {
         Throw.IfNull(nameof(other), other);
 
+        var red = other.GetChannel(PixelChannel.Red);
+        var green = other.GetChannel(PixelChannel.Green);
+        var blue = other.GetChannel(PixelChannel.Blue);
+
+        if (red is null || green is null || blue is null)
+        {
+            throw new NotSupportedException("other IPerceptualHash must have Red, Green and Blue channel");
+        }
+
         return
-          _channels[PixelChannel.Red].SumSquaredDistance(other.GetChannel(PixelChannel.Red)) +
-          _channels[PixelChannel.Green].SumSquaredDistance(other.GetChannel(PixelChannel.Green)) +
-          _channels[PixelChannel.Blue].SumSquaredDistance(other.GetChannel(PixelChannel.Blue));
+          _channels[PixelChannel.Red].SumSquaredDistance(red) +
+          _channels[PixelChannel.Green].SumSquaredDistance(green) +
+          _channels[PixelChannel.Blue].SumSquaredDistance(blue);
     }
 
     /// <summary>

tests/Magick.NET.Tests/Statistics/MomentsTests/TheGetChannelMethod.cs

@@ -0,0 +1,26 @@
+// Copyright Dirk Lemstra https://github.com/dlemstra/Magick.NET.
+// Licensed under the Apache License, Version 2.0.
+
+using ImageMagick;
+using Xunit;
+
+namespace Magick.NET.Tests;
+
+public partial class MomentsTests
+{
+    public class TheGetChannelMethod
+    {
+        [Fact]
+        public void ShouldReturnNullWhenChannelDoesNotExist()
+        {
+            using var image = new MagickImage(Files.ImageMagickJPG);
+            var moments = image.Moments();
+
+            Assert.NotNull(moments.GetChannel(PixelChannel.Red));
+            Assert.NotNull(moments.GetChannel(PixelChannel.Green));
+            Assert.NotNull(moments.GetChannel(PixelChannel.Blue));
+
+            Assert.Null(moments.GetChannel(PixelChannel.Black));
+        }
+    }
+}

tests/Magick.NET.Tests/Statistics/PerceptualHashTests/TheGetChannelMethod.cs

@@ -0,0 +1,25 @@
+// Copyright Dirk Lemstra https://github.com/dlemstra/Magick.NET.
+// Licensed under the Apache License, Version 2.0.
+
+using ImageMagick;
+using Xunit;
+
+namespace Magick.NET.Tests;
+
+public partial class PerceptualHashTests
+{
+    public class TheGetChannelMethod
+    {
+        [Fact]
+        public void ShouldReturnNullWhenChannelDoesNotExist()
+        {
+            var hash = new PerceptualHash("81b4488652898d48a7a9622346206e620f8a646682939835e986ec98c78f887ae8c67f81b1e884c58a0d18af2d622718fd35623ffdeac9a78cbaedaa81d888434e824c683ad781c37895978c8688c426628ed61b216279b81b48887318a1628af43622a2619d162372");
+
+            Assert.NotNull(hash.GetChannel(PixelChannel.Red));
+            Assert.NotNull(hash.GetChannel(PixelChannel.Green));
+            Assert.NotNull(hash.GetChannel(PixelChannel.Blue));
+
+            Assert.Null(hash.GetChannel(PixelChannel.Black));
+        }
+    }
+}

tests/Magick.NET.Tests/Statistics/PerceptualHashTests/TheSumSquaredDistanceMethod.cs

@@ -1,15 +1,38 @@
 // Copyright Dirk Lemstra https://github.com/dlemstra/Magick.NET.
 // Licensed under the Apache License, Version 2.0.
 
+using System;
 using ImageMagick;
 using Xunit;
 
 namespace Magick.NET.Tests;
 
 public partial class PerceptualHashTests
 {
+    public class TestPerceptualHash : IPerceptualHash
+    {
+        public IChannelPerceptualHash GetChannel(PixelChannel channel)
+            => null;
+
+        public double SumSquaredDistance(IPerceptualHash other)
+        {
+            throw new System.NotImplementedException();
+        }
+    }
+
     public class TheSumSquaredDistanceMethod
     {
+        [Fact]
+        public void ShouldThrowNotSupportedExceptionIfCustomImplementationDoesNotHaveExpectedChannels()
+        {
+            using var image = new MagickImage(Files.ImageMagickJPG);
+            var phash = image.PerceptualHash();
+            Assert.NotNull(phash);
+
+            var exception = Assert.Throws<NotSupportedException>(() => phash.SumSquaredDistance(new TestPerceptualHash()));
+            Assert.Equal("other IPerceptualHash must have Red, Green and Blue channel", exception.Message);
+        }
+
         [Fact]
         public void ShouldReturnTheDifference()
         {