In-Memory File System with Non-Recursive mkdir and Missing-Path Handling

Implement an in-memory file system as a class. It keeps directories and files in memory and answers operations on absolute, Unix-style paths such as `/a/b/c`. The report names `mkdir` and `ls` and likens the task to a well-known problem whose interface also appends content to a file and reads a file back, so the interface below follows that standard form. | Method | Behavior | | --- | --- | | `ls(path) -> list[str]` | If `path` names a directory, return the names of the entries directly inside it, in lexicographic order. `path` may also name a file (see below). | | `mkdir(path) -> None` | Create a new, empty directory at `path`. | | `add_content_to_file(path, content) -> None` | If no file exists at `path`, create one containing `content`; otherwise append `content` to it. | | `read_content_from_file(path) -> str` | Return the full content of the file at `path`. | The version in this round is stricter than the standard form. The report lists the cases below as the ones that failed when the statement was read too quickly: - **Paths that do not exist.** Any operation can receive a path in which a directory along the way, or the target file or directory itself, does not exist. Detect it and respond as the specification requires; never crash. - **`mkdir` is not `mkdir -p`.** Only the last component of the path may be missing. Every parent directory must already exist, and `mkdir` never creates missing parents. - **`ls` on a file.** The path passed to `ls` may name a file rather than a directory. Settle the expected result of each case before writing the methods. An illustrative session, with failures shown as `error` because their exact form is one of the clarifying questions: ```text mkdir("/a") -> ok mkdir("/a/b/c") -> error: /a/b does not exist mkdir("/a/b") -> ok add_content_to_file("/a/b/f", "hi") -> ok, creates the file add_content_to_file("/a/b/f", "!") -> ok, appends read_content_from_file("/a/b/f") -> "hi!" ls("/a") -> ["b"] ls("/a/x") -> error: /a/x does not exist ``` ```hint One way to walk a path Every method starts by walking from the root along the path. Decide what that shared walk returns, and how it reports the two different ways it can fail: a component that is missing, and a component that exists but is a file. ``` ```hint List the failures per method For each method, write down every state the path can be in (missing parent, missing target, target is a file, target is a directory, the root itself) and the agreed result for each, before writing any method body. ``` ### Constraints and Clarifications - Paths are absolute and start with `/`. The path `/` alone is the root directory, which always exists. - Names are non-empty and do not contain `/`. A file and a directory can never have the same path. - Lexicographic order means ordinary string comparison of the names. - How a failed operation is reported was not given; settle it before coding. ### Clarifying Questions - How should a failed operation be reported: an exception, a return value such as `False` or an error string, or a specific message? Must each kind of failure be distinguishable? - What should `ls` return for a path that names a file: a list containing only the file's name, or an error? - What should `mkdir` do when the path already exists, as a directory or as a file? - Does `add_content_to_file` require the parent directory to exist, given that `mkdir` does not create parents? What if the path names an existing directory? - Can paths contain a trailing slash, repeated slashes, `.` or `..`? ### What a Strong Answer Covers - Settles the error contract and the result of every reported edge case before coding - A tree of directory nodes with a clear distinction between files and directories - One path-resolution routine shared by all methods that tells a missing component apart from a file used as a directory, and handles the root - Correct non-recursive `mkdir` and file-creation rules, with no partial changes left behind by a failed call - Sorted listing, the complexity of each operation, and tests for each reported edge case ### Follow-up Questions - Add a `-p` mode to `mkdir` that creates missing parents. If a component halfway along the path is a file, what state can a naive implementation leave behind, and how do you prevent it? - `ls` is called far more often than `mkdir` on directories with many entries. How would you avoid sorting on every call? - Add `rm` and `mv`. What checks does moving a directory need so that it cannot be moved into its own subtree? - How would you make the file system safe for concurrent callers?