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website/src/lib/engine/workspace.ts

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+/** `\` and `/` alike. */
+function parts(path: string): string[] {
+    return path.split(/[\\/]+/).filter((part) => part !== '');
+}
+
+/** The name of the folder, the last part of its path. */
+export function folderName(folder: string): string {
+    return parts(folder).at(-1) ?? folder;
+}
+
+/**
+ * Where a file is in the open folder: the folders between them, as a path (`''` if it is in the
+ * folder itself). `undefined` for a file that is not in the folder (or not on the disk).
+ */
+export function subfolder(
+    folder: string | undefined,
+    path: string | undefined
+): string | undefined {
+    if (folder === undefined || path === undefined) {
+        return undefined;
+    }
+    const root = parts(folder);
+    const file = parts(path);
+    if (file.length <= root.length || root.some((part, i) => part !== file[i])) {
+        return undefined;
+    }
+    return file.slice(root.length, -1).join('/');
+}