Cache repeated directory lookups for 12% speedup

packages/eslint-plugin/lib/utilities/index.js

@@ -68,7 +68,18 @@ function getImportDetailsForName(name, context) {
 
 const INDEX_FILE = /^index\./;
 const IS_FILE = /(^|\/).*\..*$/;
+
+// Calling `normalizeSource` repeatedly with the same arguments is expensive as
+// it traverses the file system upwards in search for a specific directory. By
+// caching the normalized value we trade a little bit of additional memory with
+// a 12% speedup in total linting times for bigger projects.
+const normalizationCache = new Map();
+
 function normalizeSource(source, context) {
+  if (normalizationCache.has(source)) {
+    return normalizationCache.get(source);
+  }
+
   const sourceRoot = pkgDir.sync(source);
   const packageRoot = pkgDir.sync(context.getFilename());
   const relativeSource =
@@ -84,7 +95,9 @@ function normalizeSource(source, context) {
     ? sourceDir
     : join(sourceDir, sourceBasename.split('.').slice(0, -1).join('.'));
 
-  return sourceWithoutExtension.replace(/^node_modules\//, '');
+  const normalized = sourceWithoutExtension.replace(/^node_modules\//, '');
+  normalizationCache.set(source, normalized);
+  return normalized;
 }
 
 function findDefinition(name, context) {