Add :ignore filter approach

Author: RiRiSharp

src/AngleSharp.Diffing.Tests/DiffBuilderTest.cs

@@ -141,7 +141,7 @@ public void Test008(string controlHtml, string testHtml, DiffResult expectedDiff
             .WithTest(testHtml)
             .WithOptions(a => a // Most important thing to note here is we do not have a ignore attribute comparer
                 .AddSearchingNodeMatcher()
-                .AddAttributeNameMatcher()
+                .AddMatcher(AttributeNameMatcher.Match, StrategyType.Generalized)
                 .AddElementComparer(enforceTagClosing: false)
                 .AddMatcher(PostfixedAttributeMatcher.Match, StrategyType.Specialized)
                 .AddComparer(AttributeComparer.Compare, StrategyType.Generalized)

src/AngleSharp.Diffing/Core/HtmlDifferenceEngine.cs

@@ -161,13 +161,7 @@ void MarkSelectedSourcesAsMatched(in AttributeComparison comparison)
 
         void UpdateUnmatchedTracking()
         {
-            // Filter out unmatched :ignore attributes, they were meant to be ignored after all
-            // https://github.com/AngleSharp/AngleSharp.Diffing/issues/48
-            var controlsUnmatched = controls
-                .GetUnmatched()
-                .Where(c => !IgnoreAttributeComparer.IsIgnoreAttribute(c.Attribute));
-
-            Context.MissingAttributeSources.AddRange(controlsUnmatched);
+            Context.MissingAttributeSources.AddRange(controls.GetUnmatched());
             Context.UnexpectedAttributeSources.AddRange(tests.GetUnmatched());
         }
     }

src/AngleSharp.Diffing/Strategies/AttributeStrategies/DiffingStrategyPipelineBuilderExtensions.cs

@@ -23,6 +23,7 @@ public static IDiffingStrategyCollection IgnoreDiffAttributes(this IDiffingStrat
     public static IDiffingStrategyCollection AddAttributeNameMatcher(this IDiffingStrategyCollection builder)
     {
         builder.AddMatcher(AttributeNameMatcher.Match, StrategyType.Generalized);
+        builder.AddMatcher(IgnoreAttributeMatcher.Match, StrategyType.Generalized);
         return builder;
     }
 

src/AngleSharp.Diffing/Strategies/AttributeStrategies/IgnoreAttributeMatcher.cs

@@ -0,0 +1,29 @@
+namespace AngleSharp.Diffing.Strategies.AttributeStrategies;
+
+/// <summary>
+/// Ignore Attribute matcher strategy.
+/// </summary>
+public static class IgnoreAttributeMatcher
+{
+    private const string DIFF_IGNORE_POSTFIX = ":ignore";
+
+    /// <summary>
+    /// Attribute name matcher strategy.
+    /// </summary>
+    public static IEnumerable<AttributeComparison> Match(IDiffContext context, SourceMap controlSources, SourceMap testSources)
+    {
+        if (controlSources is null)
+            throw new ArgumentNullException(nameof(controlSources));
+        if (testSources is null)
+            throw new ArgumentNullException(nameof(testSources));
+
+        foreach (var control in controlSources.GetUnmatched())
+        {
+            // An unmatched :ignore attribute can just be matched with itself if it isn't
+            // matched with a "test" attribute of the same name already.
+            // this means an ignored attribute is ignored even if it does not appear in the test html.
+            if (control.Attribute.Name.EndsWith(DIFF_IGNORE_POSTFIX, StringComparison.OrdinalIgnoreCase))
+                yield return new AttributeComparison(control, control);
+        }
+    }
+}