Read and Write Files with Shared Content

In-Memory Deduplicated File System

Design an in-memory file system that supports writing and reading files. In many data environments, multiple files share identical content (e.g., duplicate configurations, mirrored datasets). To optimize memory, your system must store identical content payloads only once in memory.

Implement the DeduplicatedFileSystem class:

• write_file(path: str, content: str) -> None: Saves content at path. If path already exists, overwrite it. Identical contents across different paths (or subsequent overwrites) must share the same underlying storage. If an overwrite causes a previous content payload to have zero remaining references across all files, that payload should no longer be retained in memory.
• read_file(path: str) -> str | None: Returns the exact, case-sensitive content of the file at path. If the file does not exist, return None.
• delete_file(path: str) -> bool: Deletes the file at path. If deleted successfully, return True. If path does not exist, return False. Any content payload that is no longer referenced by any file after deletion must be removed from memory.
• count_unique_contents() -> int: Returns the number of distinct content payloads currently retained in memory.

Examples

fs = DeduplicatedFileSystem()

fs.write_file("/a/b/c.txt", "market_data_v1")
fs.write_file("/a/b/e.txt", "market_data_v1")
fs.count_unique_contents()  # Returns 1 (both paths share "market_data_v1")

fs.read_file("/a/b/c.txt")   # Returns "market_data_v1"

fs.write_file("/a/b/c.txt", "market_data_v2")
fs.count_unique_contents()  # Returns 2 ("market_data_v1" used by /a/b/e.txt, "market_data_v2" by /a/b/c.txt)

fs.delete_file("/a/b/e.txt")
fs.count_unique_contents()  # Returns 1 ("market_data_v1" is freed because no file references it)

fs.read_file("/nonexistent") # Returns None

Constraints

• path is a non-empty string consisting of valid path characters (/, alphanumeric, ., _).
• content can be any string, including the empty string "".
• Path and content comparisons are case-sensitive.
• At most 20,000 total calls to write_file, read_file, delete_file, and count_unique_contents.