perf: use a Set for visited dirs, only when resolving real paths

__tests__/symlinks.test.ts

@@ -116,11 +116,25 @@ const fsWithSymlinkInRootDir = {
   }),
 };
 
+const fsWithManySymlinksToSameDir = {
+  "/shared/target": {
+    "file-1": "file contents",
+    nested: { "file-2": "file contents" },
+  },
+  "/many/links": Object.fromEntries(
+    Array.from({ length: 50 }, (_, i) => [
+      `link-${i}`,
+      mock.symlink({ path: resolveSymlinkRoot("/shared/target") }),
+    ])
+  ),
+};
+
 const mockFs = {
   ...fsWithRelativeSymlinks,
   ...fsWithRecursiveSymlinks,
   ...fsWithRecursiveRelativeSymlinks,
   ...fsWithSymlinkInRootDir,
+  ...fsWithManySymlinksToSameDir,
 
   "/sym/linked": {
     "file-1": "file contents",
@@ -301,6 +315,24 @@ for (const type of apiTypes) {
       );
     });
 
+    test(`crawl a directory reached via many symlinks only once (real paths: true)`, async (t) => {
+      const api = new fdir().withErrors().withSymlinks().crawl("/many/links");
+      const files = await api[type]();
+      t.expect(files.sort()).toStrictEqual(
+        normalize(["/shared/target/file-1", "/shared/target/nested/file-2"])
+      );
+    });
+
+    test(`count a directory reached via many symlinks only once (real paths: true)`, async (t) => {
+      const api = new fdir()
+        .withErrors()
+        .withSymlinks()
+        .onlyCounts()
+        .crawl("/many/links");
+      const counts = await api[type]();
+      t.expect(counts.files).toBe(2);
+    });
+
     test(`resolve symlinks (real paths: false)`, async (t) => {
       const api = new fdir()
         .withErrors()

src/api/functions/resolve-symlink.ts

@@ -89,5 +89,6 @@ function isRecursive(path: string, resolved: string, state: WalkerState) {
 }
 
 function isRecursiveUsingRealPaths(resolved: string, state: WalkerState) {
-  return state.visited.includes(resolved + state.options.pathSeparator);
+  const { visited } = state;
+  return !!visited && visited.has(resolved + state.options.pathSeparator);
 }

src/api/functions/walk-directory.ts

@@ -24,7 +24,7 @@ const walkAsync: WalkDirectoryFunction = (
 
   const { fs } = state;
 
-  state.visited.push(crawlPath);
+  if (state.visited) state.visited.add(crawlPath);
   state.counts.directories++;
 
   // Perf: Node >= 10 introduced withFileTypes that helps us
@@ -45,7 +45,7 @@ const walkSync: WalkDirectoryFunction = (
 ) => {
   const { fs } = state;
   if (currentDepth < 0) return;
-  state.visited.push(crawlPath);
+  if (state.visited) state.visited.add(crawlPath);
   state.counts.directories++;
 
   let entries: Dirent[] = [];

src/api/walker.ts

@@ -49,7 +49,9 @@ export class Walker<TOutput extends Output> {
         this.callbackInvoker(state, error, callback)
       ),
       symlinks: new Map(),
-      visited: [""].slice(0, 0),
+      // Perf: only needed for recursion detection when resolving symlinks to
+      // real paths. A Set keeps that check O(1) instead of O(directories).
+      visited: options.useRealPaths ? new Set<string>() : undefined,
       controller: new Aborter(),
       fs: options.fs || nativeFs,
     };

src/types.ts

@@ -66,7 +66,7 @@ export type WalkerState = {
   fs: FSLike;
 
   symlinks: Map<string, string>;
-  visited: string[];
+  visited?: Set<string>;
 };
 
 export type ResultCallback<TOutput extends Output> = (