LibWeb/CSS: Move An+B matching code into ANPlusBPattern::matches()

Not everything we want to match is an "nth element".

Also moved its serialization function's implementation out of the header
while I was at it.

Libraries/LibWeb/CSS/Selector.cpp

@@ -823,4 +823,77 @@ SelectorList adapt_nested_relative_selector_list(SelectorList const& selectors)
     return new_list;
 }
 
+// https://drafts.csswg.org/css-syntax-3/#anb-microsyntax
+bool Selector::SimpleSelector::ANPlusBPattern::matches(int index) const
+{
+    // "If both a and b are equal to zero, the pseudo-class represents no element in the document tree."
+    if (step_size == 0 && offset == 0)
+        return false;
+
+    // When "step_size == -1", selector represents first "offset" elements in document tree.
+    if (step_size == -1)
+        return !(offset <= 0 || index > offset);
+
+    // When "step_size == 1", selector represents last "offset" elements in document tree.
+    if (step_size == 1)
+        return !(offset < 0 || index < offset);
+
+    // When "step_size == 0", selector picks only the "offset" element.
+    if (step_size == 0)
+        return index == offset;
+
+    // If both are negative, nothing can match.
+    if (step_size < 0 && offset < 0)
+        return false;
+
+    // Like "a % b", but handles negative integers correctly.
+    auto const canonical_modulo = [](int a, int b) -> int {
+        int c = a % b;
+        if ((c < 0 && b > 0) || (c > 0 && b < 0)) {
+            c += b;
+        }
+        return c;
+    };
+
+    // When "step_size < 0", we start at "offset" and count backwards.
+    if (step_size < 0)
+        return index <= offset && canonical_modulo(index - offset, -step_size) == 0;
+
+    // Otherwise, we start at "offset" and count forwards.
+    return index >= offset && canonical_modulo(index - offset, step_size) == 0;
+}
+
+// https://drafts.csswg.org/css-syntax-3/#serializing-anb
+String Selector::SimpleSelector::ANPlusBPattern::serialize() const
+{
+    // 1. If A is zero, return the serialization of B.
+    if (step_size == 0)
+        return String::number(offset);
+
+    // 2. Otherwise, let result initially be an empty string.
+    StringBuilder result;
+
+    // 3.
+    // - A is 1: Append "n" to result.
+    if (step_size == 1)
+        result.append('n');
+    // - A is -1: Append "-n" to result.
+    else if (step_size == -1)
+        result.append("-n"sv);
+    // - A is non-zero: Serialize A and append it to result, then append "n" to result.
+    else if (step_size != 0)
+        result.appendff("{}n", step_size);
+
+    // 4.
+    // - B is greater than zero: Append "+" to result, then append the serialization of B to result.
+    if (offset > 0)
+        result.appendff("+{}", offset);
+    // - B is less than zero: Append the serialization of B to result.
+    else if (offset < 0)
+        result.appendff("{}", offset);
+
+    // 5. Return result.
+    return MUST(result.to_string());
+}
+
 }

Libraries/LibWeb/CSS/Selector.h

@@ -86,39 +86,8 @@ public:
             int step_size { 0 }; // "A"
             int offset = { 0 };  // "B"
 
-            // https://www.w3.org/TR/css-syntax-3/#serializing-anb
+            bool matches(int index) const;
-            String serialize() const
+            String serialize() const;
-            {
-                // 1. If A is zero, return the serialization of B.
-                if (step_size == 0) {
-                    return String::number(offset);
-                }
-
-                // 2. Otherwise, let result initially be an empty string.
-                StringBuilder result;
-
-                // 3.
-                // - A is 1: Append "n" to result.
-                if (step_size == 1)
-                    result.append('n');
-                // - A is -1: Append "-n" to result.
-                else if (step_size == -1)
-                    result.append("-n"sv);
-                // - A is non-zero: Serialize A and append it to result, then append "n" to result.
-                else if (step_size != 0)
-                    result.appendff("{}n", step_size);
-
-                // 4.
-                // - B is greater than zero: Append "+" to result, then append the serialization of B to result.
-                if (offset > 0)
-                    result.appendff("+{}", offset);
-                // - B is less than zero: Append the serialization of B to result.
-                if (offset < 0)
-                    result.appendff("{}", offset);
-
-                // 5. Return result.
-                return MUST(result.to_string());
-            }
         };
 
         struct PseudoClassSelector {

Libraries/LibWeb/CSS/SelectorEngine.cpp

@@ -1,6 +1,6 @@
 /*
  * Copyright (c) 2018-2024, Andreas Kling <andreas@ladybird.org>
- * Copyright (c) 2021-2024, Sam Atkins <sam@ladybird.org>
+ * Copyright (c) 2021-2025, Sam Atkins <sam@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -671,10 +671,6 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
     case CSS::PseudoClass::NthOfType:
     case CSS::PseudoClass::NthLastOfType: {
         auto& an_plus_b = pseudo_class.an_plus_b_patterns.first();
-        auto const step_size = an_plus_b.step_size;
-        auto const offset = an_plus_b.offset;
-        if (step_size == 0 && offset == 0)
-            return false; // "If both a and b are equal to zero, the pseudo-class represents no element in the document tree."
 
         auto const* parent = element.parent();
         if (!parent)
@@ -730,38 +726,7 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
         default:
             VERIFY_NOT_REACHED();
         }
-
+        return an_plus_b.matches(index);
-        // When "step_size == -1", selector represents first "offset" elements in document tree.
-        if (step_size == -1)
-            return !(offset <= 0 || index > offset);
-
-        // When "step_size == 1", selector represents last "offset" elements in document tree.
-        if (step_size == 1)
-            return !(offset < 0 || index < offset);
-
-        // When "step_size == 0", selector picks only the "offset" element.
-        if (step_size == 0)
-            return index == offset;
-
-        // If both are negative, nothing can match.
-        if (step_size < 0 && offset < 0)
-            return false;
-
-        // Like "a % b", but handles negative integers correctly.
-        auto const canonical_modulo = [](int a, int b) -> int {
-            int c = a % b;
-            if ((c < 0 && b > 0) || (c > 0 && b < 0)) {
-                c += b;
-            }
-            return c;
-        };
-
-        // When "step_size < 0", we start at "offset" and count backwards.
-        if (step_size < 0)
-            return index <= offset && canonical_modulo(index - offset, -step_size) == 0;
-
-        // Otherwise, we start at "offset" and count forwards.
-        return index >= offset && canonical_modulo(index - offset, step_size) == 0;
     }
     case CSS::PseudoClass::Playing: {
         if (!is<HTML::HTMLMediaElement>(element))