Ignore unmatched :ignore attributes

src/AngleSharp.Diffing.Tests/DiffBuilderTest.cs

@@ -1,3 +1,8 @@
+using AngleSharp.Diffing.Strategies;
+using AngleSharp.Diffing.Strategies.AttributeStrategies;
+using AngleSharp.Diffing.Strategies.TextNodeStrategies;
+using AngleSharp.Diffing.TestData;
+
 namespace AngleSharp.Diffing;
 
 
@@ -116,4 +121,38 @@ public void Test006(string control, string test)
 
         diffs.ShouldBeEmpty();
     }
-}
+
+    [Theory(DisplayName =
+        "When a control element has ':ignore', elements with and without that attribute should return empty diffs")]
+    [MemberData(nameof(IgnoreAttributeTestData.ControlAndHtmlData), MemberType = typeof(IgnoreAttributeTestData))]
+    public void Test007(string controlHtml, string testHtml)
+    {
+        var diffs = DiffBuilder.Compare(controlHtml).WithTest(testHtml).Build();
+        Assert.Empty(diffs);
+    }
+
+    [Theory(DisplayName =
+        "When a control element has ':ignore', but IgnoreAttributeComparer is not active, diffs should be found")]
+    [MemberData(nameof(IgnoreAttributeTestData.ControlHtmlAndDiffData), MemberType = typeof(IgnoreAttributeTestData))]
+    public void Test008(string controlHtml, string testHtml, DiffResult expectedDiffResult)
+    {
+        var diffs = DiffBuilder
+            .Compare(controlHtml)
+            .WithTest(testHtml)
+            .WithOptions(a => a // Most important thing to note here is we do not have a ignore attribute comparer
+                .AddSearchingNodeMatcher()
+                .AddMatcher(AttributeNameMatcher.Match, StrategyType.Generalized)
+                .AddElementComparer(enforceTagClosing: false)
+                .AddMatcher(PostfixedAttributeMatcher.Match, StrategyType.Specialized)
+                .AddComparer(AttributeComparer.Compare, StrategyType.Generalized)
+                .AddClassAttributeComparer()
+                .AddBooleanAttributeComparer(BooleanAttributeComparision.Strict)
+                .AddStyleAttributeComparer())
+            .Build()
+            .ToList();
+
+        Assert.Single(diffs);
+        Assert.Equal(DiffTarget.Attribute, diffs[0].Target);
+        Assert.Equal(expectedDiffResult, diffs[0].Result);
+    }
+}

src/AngleSharp.Diffing.Tests/TestData/IgnoreAttributeTestData.cs

@@ -0,0 +1,45 @@
+namespace AngleSharp.Diffing.TestData;
+
+internal static class IgnoreAttributeTestData
+{
+    public static TheoryData<string, string> ControlAndHtmlData()
+    {
+        var theoryData = new TheoryData<string, string>();
+        foreach (var (controlHtml, expectedHtml, _) in TestCases)
+        {
+            theoryData.Add(controlHtml, expectedHtml);
+        }
+
+        return theoryData;
+    }
+
+    public static TheoryData<string, string, DiffResult> ControlHtmlAndDiffData()
+    {
+        var theoryData = new TheoryData<string, string, DiffResult>();
+        foreach (var (controlHtml, expectedHtml, expectedDiffResult) in TestCases)
+        {
+            theoryData.Add(controlHtml, expectedHtml, expectedDiffResult);
+        }
+
+        return theoryData;
+    }
+
+    private static readonly IEnumerable<(string controlHtml, string expectedHtml, DiffResult expectedDiffResult)>
+        TestCases =
+        [
+            ("<div class:ignore></div>", "<div class=\"ian-fleming\"></div>", DiffResult.Different),
+            ("<div class:ignore></div>", "<div class=\"\"></div>", DiffResult.Different),
+            ("<div class:ignore></div>", "<div class></div>", DiffResult.Different),
+            ("<div class:ignore></div>", "<div></div>", DiffResult.Missing),
+            ("<input required:ignore/>", "<input required=\"required\"/>", DiffResult.Different),
+            ("<input required:ignore/>", "<input required=\"\"/>", DiffResult.Different),
+            ("<input required:ignore/>", "<input required/>", DiffResult.Different),
+            ("<input required:ignore/>", "<input/>", DiffResult.Missing),
+            ("<button onclick:ignore/></button>", "<button onclick=\"alert(1)\"></button>", DiffResult.Different),
+            ("<button onclick:ignore/></button>", "<button/></button>", DiffResult.Missing),
+            ("<a aria-disabled:ignore/></a>", "<a aria-disabled=\"true\"/></a>", DiffResult.Different),
+            ("<a aria-disabled:ignore/></a>", "<a/></a>", DiffResult.Missing),
+            ("<span style:ignore/></span>", "<span style=\"color:red;\"/></span>", DiffResult.Different),
+            ("<span style:ignore/></span>", "<span/></span>", DiffResult.Missing),
+        ];
+}

src/AngleSharp.Diffing/Core/HtmlDifferenceEngine.cs

@@ -1,4 +1,5 @@
 using AngleSharp.Diffing.Core.Diffs;
+using AngleSharp.Diffing.Strategies.AttributeStrategies;
 
 namespace AngleSharp.Diffing.Core;
 

src/AngleSharp.Diffing/Core/SourceMap.cs

@@ -70,7 +70,7 @@ public IEnumerable<AttributeComparisonSource> GetUnmatched()
     {
         foreach (var source in _sources.Values)
         {
-            if (!_matched.Contains(source.Attribute.Name))
+            if (IsUnmatched(source.Attribute.Name))
                 yield return source;
         }
     }

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/IgnoreAttributeComparer.cs

@@ -15,8 +15,13 @@ public static CompareResult Compare(in AttributeComparison comparison, CompareRe
         if (currentDecision.IsSameOrSkip)
             return currentDecision;
 
-        return comparison.Control.Attribute.Name.EndsWith(DIFF_IGNORE_POSTFIX, StringComparison.OrdinalIgnoreCase)
+        return IsIgnoreAttribute(comparison.Control.Attribute)
             ? CompareResult.Same
             : currentDecision;
     }
+
+    public static bool IsIgnoreAttribute(IAttr source)
+    {
+        return source.Name.EndsWith(DIFF_IGNORE_POSTFIX,  StringComparison.OrdinalIgnoreCase);
+    }
 }

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);
+        }
+    }
+}