Answer a constant nth-child index without building the sibling list

docs/api.md

@@ -11,21 +11,21 @@ Query contexts default to the factory document when omitted. `closest()`, `first
 
 | Method | Result |
 | --- | --- |
-| [`byClass(cls, context)`](../src/nwsapi.mts#L3747) | Returns elements with the class name. |
-| [`byId(id, context)`](../src/nwsapi.mts#L3745) | Returns elements with the ID. Duplicate IDs are allowed by default. |
-| [`byTag(tag, context)`](../src/nwsapi.mts#L3746) | Returns elements with the tag name. Use `*` for all elements. |
-| [`closest(selectors, element, callback)`](../src/nwsapi.mts#L3753) | Returns the nearest match, starting with the element, or `null`. |
-| [`compile(selector, mode, callback, relative)`](../src/nwsapi.mts#L3755) | Compiles a selector into a resolver function. This is an advanced API. |
-| [`configure(option, clear)`](../src/nwsapi.mts#L3756) | Reads or changes options. Pass `true` as the second argument to clear compiled selectors. |
-| [`emit(message, proto)`](../src/nwsapi.mts#L3758) | Reports an error using the configured error policy. |
-| [`first(selectors, context, callback)`](../src/nwsapi.mts#L3749) | Returns the first matching descendant, or `null`. |
-| [`install(all)`](../src/nwsapi.mts#L3764) | Replaces native selector methods. Pass `true` to also replace collection methods. |
-| [`match(selectors, element, callback)`](../src/nwsapi.mts#L3750) | Returns whether the element matches. |
-| [`registerCombinator(combinator, resolver)`](../src/nwsapi.mts#L3771) | Adds a relationship between elements using trusted resolver code. |
-| [`registerOperator(operator, resolver)`](../src/nwsapi.mts#L3796) | Adds an attribute operator using a resolver with `p1`, `p2`, and `p3` fields. |
-| [`registerSelector(name, rexp, func)`](../src/nwsapi.mts#L3818) | Adds a selector pattern and a compiler callback that returns `source` and `status`. |
-| [`select(selectors, context, callback)`](../src/nwsapi.mts#L3751) | Returns an array of matching descendants, or an empty array. |
-| [`uninstall()`](../src/nwsapi.mts#L3765) | Restores the native methods saved by `install()`. |
+| [`byClass(cls, context)`](../src/nwsapi.mts#L3774) | Returns elements with the class name. |
+| [`byId(id, context)`](../src/nwsapi.mts#L3772) | Returns elements with the ID. Duplicate IDs are allowed by default. |
+| [`byTag(tag, context)`](../src/nwsapi.mts#L3773) | Returns elements with the tag name. Use `*` for all elements. |
+| [`closest(selectors, element, callback)`](../src/nwsapi.mts#L3780) | Returns the nearest match, starting with the element, or `null`. |
+| [`compile(selector, mode, callback, relative)`](../src/nwsapi.mts#L3782) | Compiles a selector into a resolver function. This is an advanced API. |
+| [`configure(option, clear)`](../src/nwsapi.mts#L3783) | Reads or changes options. Pass `true` as the second argument to clear compiled selectors. |
+| [`emit(message, proto)`](../src/nwsapi.mts#L3785) | Reports an error using the configured error policy. |
+| [`first(selectors, context, callback)`](../src/nwsapi.mts#L3776) | Returns the first matching descendant, or `null`. |
+| [`install(all)`](../src/nwsapi.mts#L3791) | Replaces native selector methods. Pass `true` to also replace collection methods. |
+| [`match(selectors, element, callback)`](../src/nwsapi.mts#L3777) | Returns whether the element matches. |
+| [`registerCombinator(combinator, resolver)`](../src/nwsapi.mts#L3798) | Adds a relationship between elements using trusted resolver code. |
+| [`registerOperator(operator, resolver)`](../src/nwsapi.mts#L3823) | Adds an attribute operator using a resolver with `p1`, `p2`, and `p3` fields. |
+| [`registerSelector(name, rexp, func)`](../src/nwsapi.mts#L3845) | Adds a selector pattern and a compiler callback that returns `source` and `status`. |
+| [`select(selectors, context, callback)`](../src/nwsapi.mts#L3778) | Returns an array of matching descendants, or an empty array. |
+| [`uninstall()`](../src/nwsapi.mts#L3792) | Restores the native methods saved by `install()`. |
 
 <details>
 <summary>Configuration</summary>
@@ -60,22 +60,22 @@ These exports support extensions and debugging. Prefer query methods and `config
 
 | Member | Purpose |
 | --- | --- |
-| [`CFG`](../src/nwsapi.mts#L3733) | Contains the compiler syntax settings. |
-| [`Config`](../src/nwsapi.mts#L3759) | Contains the active options. Use `configure()` to change them. |
-| [`M_BODY`](../src/nwsapi.mts#L3736) | Contains the matching resolver body template. |
-| [`M_TEST`](../src/nwsapi.mts#L3740) | Contains the matching resolver test template. |
-| [`matchLambdas`](../src/nwsapi.mts#L3725) | Caches compiled matching functions, not DOM results. |
-| [`matchResolvers`](../src/nwsapi.mts#L3728) | Caches matching plans, not DOM results. |
-| [`N_BODY`](../src/nwsapi.mts#L3737) | Exposes the matching resolver body template. |
-| [`N_TEST`](../src/nwsapi.mts#L3741) | Contains the alternate resolver test template. |
-| [`Operators`](../src/nwsapi.mts#L3767) | Contains registered attribute operators. |
-| [`S_BODY`](../src/nwsapi.mts#L3735) | Contains the selection resolver body template. |
-| [`S_TEST`](../src/nwsapi.mts#L3739) | Contains the selection resolver test template. |
-| [`selectLambdas`](../src/nwsapi.mts#L3726) | Caches compiled selection functions, not DOM results. |
-| [`Selectors`](../src/nwsapi.mts#L3768) | Contains registered selector extensions. |
-| [`selectResolvers`](../src/nwsapi.mts#L3729) | Caches selection plans, not DOM results. |
-| [`Snapshot`](../src/nwsapi.mts#L3760) | Contains the document state and helpers used by compiled selectors. |
-| [`Version`](../src/nwsapi.mts#L3762) | Contains the engine version string. |
+| [`CFG`](../src/nwsapi.mts#L3760) | Contains the compiler syntax settings. |
+| [`Config`](../src/nwsapi.mts#L3786) | Contains the active options. Use `configure()` to change them. |
+| [`M_BODY`](../src/nwsapi.mts#L3763) | Contains the matching resolver body template. |
+| [`M_TEST`](../src/nwsapi.mts#L3767) | Contains the matching resolver test template. |
+| [`matchLambdas`](../src/nwsapi.mts#L3752) | Caches compiled matching functions, not DOM results. |
+| [`matchResolvers`](../src/nwsapi.mts#L3755) | Caches matching plans, not DOM results. |
+| [`N_BODY`](../src/nwsapi.mts#L3764) | Exposes the matching resolver body template. |
+| [`N_TEST`](../src/nwsapi.mts#L3768) | Contains the alternate resolver test template. |
+| [`Operators`](../src/nwsapi.mts#L3794) | Contains registered attribute operators. |
+| [`S_BODY`](../src/nwsapi.mts#L3762) | Contains the selection resolver body template. |
+| [`S_TEST`](../src/nwsapi.mts#L3766) | Contains the selection resolver test template. |
+| [`selectLambdas`](../src/nwsapi.mts#L3753) | Caches compiled selection functions, not DOM results. |
+| [`Selectors`](../src/nwsapi.mts#L3795) | Contains registered selector extensions. |
+| [`selectResolvers`](../src/nwsapi.mts#L3756) | Caches selection plans, not DOM results. |
+| [`Snapshot`](../src/nwsapi.mts#L3787) | Contains the document state and helpers used by compiled selectors. |
+| [`Version`](../src/nwsapi.mts#L3789) | Contains the engine version string. |
 
 </details>
 

src/nwsapi.mts

@@ -2179,7 +2179,7 @@
 
       loop += mode || mode === null ? '{' + source + '}' : source
 
-      if (mode || (mode === null && selector.includes(':nth'))) {
+      if (mode || selector.includes(':nth')) {
         loop += reNthElem.test(selector) ? 's.nthElement(null, 2);' : ''
         loop += reNthType.test(selector) ? 's.nthOfType(null, 2);' : ''
       }
@@ -2587,6 +2587,33 @@
                                 : 'n>' + (b - 1)
                               : 'false'
                     }
+                    // A constant index needs no index. nth(Element|OfType)
+                    // builds the sibling list of the parent to number the
+                    // element within it, which is the right trade for an an+b
+                    // form that has to know where the element sits, and pure
+                    // overhead for ':nth-child(3)', which only has to know
+                    // whether three steps back runs out of siblings.
+                    //
+                    // Only for the -child forms: of-type has to compare the
+                    // name of every sibling it steps over, and reading
+                    // localName through the host on each one costs more than
+                    // the list it avoids.
+                    if (test == 'n==' + a && a >= 1 && !expr) {
+                      test = type ? 'next' : 'previous'
+                      source =
+                        'n=1,o=e;' +
+                        'while(n<=' +
+                        a +
+                        '&&(o=o.' +
+                        test +
+                        'ElementSibling))++n;' +
+                        'if(n==' +
+                        a +
+                        '){' +
+                        source +
+                        '}'
+                      break
+                    }
                     expr = expr ? 'OfType' : 'Element'
                     type = type ? 'true' : 'false'
                     source =

test/repo/e2e/upstream/fixtures/nth-constant.html

@@ -0,0 +1,35 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>Constant sibling indexes</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<body>
+<script>
+for (const pseudo of ['nth-child', 'nth-last-child', 'nth-of-type', 'nth-last-of-type']) {
+  test(t => {
+    const engine = NW.Dom;
+    const parent = document.createElement('main');
+    parent.innerHTML = '<i></i>text<!-- gap --><b></b><i></i><i></i>';
+    document.body.appendChild(parent);
+    t.add_cleanup(() => parent.remove());
+    for (let round = 0; round < 3; round++) {
+      for (const index of ['0', '-1', '1', '2', '3', '4', '5', '0n+3', '2n', 'n+3']) {
+        const selector = `:${pseudo}(${index})`;
+        // Zero is never a valid sibling index. Chromium currently returns
+        // every child for :nth-child(0), even without NWSAPI installed.
+        const expected = index === '0' ? [] : Array.from(parent.querySelectorAll(selector));
+        assert_array_equals(engine.select(selector, parent), expected, selector);
+        assert_equals(engine.first(selector, parent), expected[0] || null, selector);
+        for (const element of parent.children) {
+          assert_equals(engine.match(selector, element), expected.includes(element), selector);
+        }
+      }
+      parent.prepend(document.createElement('i'));
+      parent.lastElementChild.remove();
+    }
+    const orphan = document.createElement('i');
+    assert_true(engine.match(`:${pseudo}(1)`, orphan));
+    assert_false(engine.match(`:${pseudo}(2)`, orphan));
+  }, `${pseudo} handles constant indexes, formulas, detached nodes, and mutations`);
+}
+</script>

test/repo/e2e/upstream/manifest.mts

@@ -36,6 +36,11 @@ export const manifest: Array<{
   install?: boolean
   legacyMap?: boolean
 }> = [
+  {
+    path: '/_repo/test/repo/e2e/upstream/fixtures/nth-constant.html',
+    note: 'local WPT regression: constant sibling indexes agree with native queries after mutation',
+    install: false,
+  },
   {
     path: '/_repo/test/repo/e2e/upstream/fixtures/legacy-dom.html',
     note: 'local WPT regression: legacy host reads agree with native selectors and refresh after mutations',

test/repo/unit/nth-constant.test.mts

@@ -0,0 +1,71 @@
+import { JSDOM } from 'jsdom'
+import { expect, test } from 'vitest'
+import factory from '../../../src/nwsapi.js'
+
+for (const pseudo of [
+  'nth-child',
+  'nth-last-child',
+  'nth-of-type',
+  'nth-last-of-type',
+]) {
+  test(`${pseudo} preserves constant and formula results after mutations`, t => {
+    const { window } = new JSDOM(
+      '<main><i></i>text<!-- gap --><b></b><i></i><i></i></main>',
+      {
+        url: 'https://example.test/',
+      },
+    )
+    t.onTestFinished(() => window.close())
+    const { document } = window
+    const engine = factory(window)
+    const parent = document.querySelector('main')!
+    for (let round = 0; round < 3; round++) {
+      for (const index of [
+        '0',
+        '-1',
+        '1',
+        '2',
+        '3',
+        '4',
+        '5',
+        '0n+3',
+        '2n',
+        'n+3',
+      ]) {
+        const selector = `:${pseudo}(${index})`
+        const elements = [...parent.children]
+        const expected = elements.filter(element => {
+          const siblings = pseudo.includes('of-type')
+            ? elements.filter(
+                sibling => sibling.localName === element.localName,
+              )
+            : elements
+          const position = pseudo.includes('last')
+            ? siblings.length - siblings.indexOf(element)
+            : siblings.indexOf(element) + 1
+          return index === '2n'
+            ? position % 2 === 0
+            : index === 'n+3'
+              ? position >= 3
+              : position === (index === '0n+3' ? 3 : Number(index))
+        })
+        expect(
+          engine.select(selector, parent),
+          `${selector}, round ${round}`,
+        ).toEqual(expected)
+        expect(engine.first(selector, parent)).toBe(expected[0] ?? null)
+        for (const element of parent.children) {
+          expect(engine.match(selector, element)).toBe(
+            expected.includes(element),
+          )
+        }
+      }
+      parent.prepend(document.createElement('i'))
+      parent.lastElementChild!.remove()
+      parent.remove()
+    }
+    const orphan = document.createElement('i')
+    expect(engine.match(`:${pseudo}(1)`, orphan)).toBe(true)
+    expect(engine.match(`:${pseudo}(2)`, orphan)).toBe(false)
+  })
+}