Add getParentDirectory filter

src/Filters/GetParentDirectory.js

@@ -0,0 +1,49 @@
+/**
+ * Returns the parent directory of a given path.
+ *
+ * For directory paths (ending with /): returns the parent directory
+ * For file paths: returns the directory containing the file
+ *
+ * Examples:
+ *   /mydir/           -> /
+ *   /mydir/slug.html  -> /mydir/
+ *   /mydir/index.md   -> /mydir/
+ *   /a/b/c/           -> /a/b/
+ *   /file.html        -> /
+ *
+ * @param {string} path - The path to get the parent directory of
+ * @returns {string} The parent directory path
+ */
+export default function getParentDirectory(path) {
+	if (!path || typeof path !== "string") {
+		return "";
+	}
+
+	// If path ends with /, it's a directory - get its parent
+	if (path.endsWith("/") && path.length > 1) {
+		// Remove trailing slash, then find parent
+		let withoutTrailing = path.slice(0, -1);
+		let lastSlash = withoutTrailing.lastIndexOf("/");
+		if (lastSlash === -1) {
+			return "/";
+		}
+		return withoutTrailing.slice(0, lastSlash + 1) || "/";
+	}
+
+	// Handle root directory
+	if (path === "/") {
+		return "/";
+	}
+
+	// Otherwise it's a file path - get the directory containing it
+	let lastSlash = path.lastIndexOf("/");
+	if (lastSlash === -1) {
+		// No slash found, no parent directory
+		return "";
+	}
+	if (lastSlash === 0) {
+		// File is at root level
+		return "/";
+	}
+	return path.slice(0, lastSlash + 1);
+}

src/defaultConfigExtended.js

@@ -8,6 +8,7 @@ import MemoizeUtil from "./Util/MemoizeFunction.js";
 import urlFilter from "./Filters/Url.js";
 import getLocaleCollectionItem from "./Filters/GetLocaleCollectionItem.js";
 import getCollectionItemIndex from "./Filters/GetCollectionItemIndex.js";
+import getParentDirectory from "./Filters/GetParentDirectory.js";
 import { FilterPlugin as InputPathToUrlFilterPlugin } from "./Plugins/InputPathToUrl.js";
 
 /**
@@ -98,4 +99,9 @@ export default function (config) {
 
 		return urlFilter.call(this, url, pathPrefix);
 	});
+
+	// Returns the parent directory of a path
+	// For directories (/mydir/): returns parent (/mydir/ -> /)
+	// For files (/mydir/file.html): returns containing directory (/mydir/file.html -> /mydir/)
+	config.addFilter("getParentDirectory", getParentDirectory);
 }

test/GetParentDirectoryTest.js

@@ -0,0 +1,55 @@
+import test from "ava";
+import getParentDirectory from "../src/Filters/GetParentDirectory.js";
+
+test("Directory paths - returns parent directory", (t) => {
+	// Examples from issue #3794
+	t.is(getParentDirectory("/mydir/"), "/");
+
+	// Nested directories
+	t.is(getParentDirectory("/a/b/"), "/a/");
+	t.is(getParentDirectory("/a/b/c/"), "/a/b/");
+	t.is(getParentDirectory("/a/b/c/d/"), "/a/b/c/");
+});
+
+test("File paths - returns containing directory", (t) => {
+	// Examples from issue #3794
+	t.is(getParentDirectory("/mydir/slug.html"), "/mydir/");
+	t.is(getParentDirectory("/mydir/index.md"), "/mydir/");
+
+	// Nested file paths
+	t.is(getParentDirectory("/a/b/file.html"), "/a/b/");
+	t.is(getParentDirectory("/a/b/c/page.md"), "/a/b/c/");
+});
+
+test("Root level paths", (t) => {
+	// Root directory
+	t.is(getParentDirectory("/"), "/");
+
+	// File at root
+	t.is(getParentDirectory("/file.html"), "/");
+	t.is(getParentDirectory("/index.md"), "/");
+});
+
+test("Edge cases", (t) => {
+	// Empty and invalid inputs
+	t.is(getParentDirectory(""), "");
+	t.is(getParentDirectory(null), "");
+	t.is(getParentDirectory(undefined), "");
+
+	// Non-string inputs
+	t.is(getParentDirectory(123), "");
+	t.is(getParentDirectory({}), "");
+
+	// Relative paths (no leading slash)
+	t.is(getParentDirectory("file.html"), "");
+	t.is(getParentDirectory("dir/file.html"), "dir/");
+	t.is(getParentDirectory("a/b/c/"), "a/b/");
+});
+
+test("Single segment paths", (t) => {
+	// Directory with single segment
+	t.is(getParentDirectory("/foo/"), "/");
+
+	// File with single segment at root
+	t.is(getParentDirectory("/foo"), "/");
+});