Optimize DOM selector lookups by pre-warming by selectors' parents

src/run.js

@@ -485,9 +485,13 @@ const minimalcss = async options => {
     );
   }
 
+  // This is the protection against ever looking up the same CSS selector
+  // more than once. We can confidently pre-populate it with a couple that
+  // we're confident about.
+  const decisionsCache = { '*': true, body: true, html: true, '': true };
+
   // Now, let's loop over ALL links and process their ASTs compared to
   // the DOMs.
-  const decisionsCache = {};
   const isSelectorMatchToAnyElement = selectorString => {
     // Here's the crucial part. Decide whether to keep the selector
     // Find at least 1 DOM that contains an object that matches
@@ -519,7 +523,6 @@ const minimalcss = async options => {
       return;
     }
     const ast = stylesheetAsts[href];
-
     csstree.walk(ast, {
       visit: 'Rule',
       enter: function(node, item, list) {
@@ -548,6 +551,40 @@ const minimalcss = async options => {
             const selectorString = utils.reduceCSSSelector(
               csstree.generate(node)
             );
+
+            // Before we begin, do a little warmup of the decision cache.
+            // From a given selector, e.g. `div.foo p.bar`, we can first look
+            // up if there's an point by first doing a lookup for `div.foo`
+            // because if that doesn't exist we *know*  we can ignore more
+            // "deeper" selectors like `div.foo p.bar` and `div.foo span a`.
+            const parentSelectors = utils.getParentSelectors(selectorString);
+
+            // If "selectorString" was `.foo .bar span`, then
+            // this `parentSelectors` array will be
+            // `['.foo', '.foo .bar']`.
+            // If `selectorString` was just `.foo`, then
+            // this `parentSelectors` array will be `[]`.
+            let bother = true;
+            parentSelectors.forEach(selectorParentString => {
+              if (bother) {
+                // Is it NOT in the decision cache?
+                if (selectorParentString in decisionsCache === false) {
+                  decisionsCache[
+                    selectorParentString
+                  ] = isSelectorMatchToAnyElement(selectorParentString);
+                }
+                // What was the outcome of that? And if the outcome was
+                // that it was NOT there, set the 'bother' to false which
+                // will popoulate the decision cache immediately.
+                if (!decisionsCache[selectorParentString]) {
+                  bother = false;
+                  decisionsCache[selectorString] = false;
+                }
+              } else {
+                decisionsCache[selectorParentString] = false;
+              }
+            });
+
             if (selectorString in decisionsCache === false) {
               decisionsCache[selectorString] = isSelectorMatchToAnyElement(
                 selectorString
@@ -612,9 +649,9 @@ const minimalcss = async options => {
   // it too.
   csso.compress(allCombinedAst, cssoOptions);
   postProcessOptimize(allCombinedAst);
-
+  const finalCss = csstree.generate(allCombinedAst);
   const returned = {
-    finalCss: csstree.generate(allCombinedAst),
+    finalCss,
     stylesheetContents
   };
   return Promise.resolve(returned);

src/utils.js

@@ -1,3 +1,19 @@
+const csstree = require('css-tree');
+
+/**
+ * Simple single argument memoize function. Only works if the first and
+ * only argument is a hashable. E.g. a string.
+ */
+function memoize(fn) {
+  const cache = {};
+  return argument => {
+    if (argument in cache === false) {
+      cache[argument] = fn(argument);
+    }
+    return cache[argument];
+  };
+}
+
 /**
  * Reduce a CSS selector to be without any pseudo class parts.
  * For example, from `a:hover` return `a`. And from `input::-moz-focus-inner`
@@ -10,25 +26,25 @@
  * @param {string} selector
  * @return {string}
  */
-const reduceCSSSelector = selector => {
+const reduceCSSSelector = memoize(selector => {
   return selector.split(
     /:(?=([^"'\\]*(\\.|["']([^"'\\]*\\.)*[^"'\\]*['"]))*[^"']*$)/g
   )[0];
-};
+});
 
 /**
  * Remove the ' and/or " at the beginning and end of a string if it has it.
  * @param {string} string
  * @return {string}
  */
-const unquoteString = string => {
+const unquoteString = memoize(string => {
   const first = string.charAt(0);
   const last = string.charAt(string.length - 1);
   if (first === last && (first === '"' || first === "'")) {
     return string.substring(1, string.length - 1);
   }
   return string;
-};
+});
 
 /**
  * Removes all sequences of two-or-more semicolons separated by zero-or-more
@@ -43,4 +59,39 @@ const removeSequentialSemis = css => {
   return css;
 };
 
-module.exports = { reduceCSSSelector, removeSequentialSemis, unquoteString };
+/**
+ * Given a string CSS selector (e.g. '.foo .bar .baz') return it with the
+ * last piece (split by whitespace) omitted (e.g. '.foo .bar').
+ * If there is no parent, return an empty string.
+ *
+ * @param {string} selector
+ * @return {string[]}
+ */
+function getParentSelectors(selector) {
+  if (!selector) return [];
+  const parentSelectors = [];
+  const selectorAst = csstree.parse(selector, { context: 'selector' });
+
+  let generatedCSS;
+  while (selectorAst.children.tail) {
+    selectorAst.children.prevUntil(
+      selectorAst.children.tail,
+      (node, item, list) => {
+        list.remove(item);
+        return node.type === 'Combinator' || node.type === 'WhiteSpace';
+      }
+    );
+    generatedCSS = csstree.generate(selectorAst);
+    if (generatedCSS) {
+      parentSelectors.push(generatedCSS);
+    }
+  }
+  return parentSelectors.reverse();
+}
+
+module.exports = {
+  reduceCSSSelector,
+  removeSequentialSemis,
+  unquoteString,
+  getParentSelectors
+};

tests/examples/nested-selectors.css

@@ -0,0 +1,14 @@
+#foo
+.link, #foo  p  {
+  color: red;
+}
+
+.one {
+  color: red;
+}
+.one .two {
+  color: orange;
+}
+.one .two .three {
+  color: green;
+}

tests/examples/nested-selectors.html

@@ -0,0 +1,13 @@
+
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8">
+    <link href="nested-selectors.css" rel="stylesheet">
+  </head>
+  <body>
+    <div id="foo">
+      <p>Text</p>
+    </div>
+  </body>
+</html>

tests/main.test.js

@@ -163,6 +163,11 @@ test('form elements', async () => {
   expect(finalCss).toMatch('option:selected');
 });
 
+test('nested selectors and domLookupsTotal', async () => {
+  const { finalCss } = await runMinimalcss('nested-selectors');
+  expect(finalCss).toMatch('#foo p{color:red}');
+});
+
 test('invalid css', async () => {
   expect.assertions(1);
 

tests/utils.test.js

@@ -31,3 +31,31 @@ test('Test removeSequentialSemis', () => {
   // multiple semicolon sequences
   expect(f('a;b;;c;;;d;;;;')).toEqual('a;b;c;d;');
 });
+
+test('Test parentSelectors', async () => {
+  const f = utils.getParentSelectors;
+  // Simplest possible
+  expect(f('.foo .bar')).toEqual(['.foo']);
+  // Slightly less simple
+  expect(f('.foo .bar .baz')).toEqual(['.foo', '.foo .bar']);
+  // Empty array
+  expect(f('.foo')).toEqual([]);
+  // Less trivial
+  expect(f('.ui.dropdown>.dropdown.icon:before')).toEqual(['.ui.dropdown']);
+  expect(f('.ui.vertical.menu .dropdown.item>.dropdown.icon:before')).toEqual([
+    '.ui.vertical.menu',
+    '.ui.vertical.menu .dropdown.item'
+  ]);
+  expect(
+    f(
+      '.ui.search.selection>.icon.input:not([class*="left icon"])>.icon~.remove.icon'
+    )
+  ).toEqual([
+    '.ui.search.selection',
+    '.ui.search.selection>.icon.input:not([class*="left icon"])',
+    '.ui.search.selection>.icon.input:not([class*="left icon"])>.icon'
+  ]);
+  expect(f('.ui[class*="right aligned"].search>.results')).toEqual([
+    '.ui[class*="right aligned"].search'
+  ]);
+});