How should I practice File Storage Service?

Start by reading the full statement, solve the visible examples, then compare the pattern against related Cresta phone screen problems in FastPrep.

Is File Storage Service associated with Cresta?

FastPrep tags this as a Cresta phone screen coding practice problem when the problem catalog has that company and stage context.

File Storage Service

Cresta● MediumNEW GRADPHONE SCREEN

Problem statement

Source note: This comes from a Cresta forward-deployed engineer phone screen on CoderPad, where the candidate chose Python and got only a task description, with no starter code and no tests. From memory, and warning that details may differ, the author describes a FileStorage class with four methods: store a file and return a unique ID, renaming the new file with a suffix when a file of that name is already stored; return a file's content and fail when it does not exist; delete a file and say whether it existed; and list each file's ID, name, size and upload time.

The report did not say how IDs look, what the suffix looks like or where it goes, which names count as existing, how a failure is returned, or the list order. Practice choices here: one operation list stands in for the class; IDs are file-1, file-2 and so on and are never reused; the suffix is a space and (k) placed before the extension that starts at the last dot, and a name whose last dot is its first character (such as .env) has no extension, so its copy is .env (1); the smallest free k is used, checking only files still stored; a missing file returns ERROR; content is short ASCII text whose size is its length; the upload time is passed in; the list follows upload order in a pipe and semicolon text form; and the size limits are ours.

The judged task matches the report at about 75%.

Build a small in-memory file storage service. Process operations in order and return one string for each operation.

["UPLOAD", fileName, content, timestamp] stores a file and returns its new ID. IDs are file-1, file-2, and so on, in upload order. An ID is never reused, even after its file is deleted. timestamp is the upload time; store it as given.
["GET", fileId] returns the file's content, or ERROR if no stored file has that ID. A file with empty content returns an empty string.
["DELETE", fileId] deletes the file and returns true, or returns false if no stored file has that ID.
["LIST"] returns the metadata of every stored file in upload order, as fileId|fileName|fileSize|uploadTimestamp entries joined by ;. fileSize is the number of characters in the content, which equals its size in bytes. With no stored files, return an empty string.

Duplicate names. If a file that is currently stored already has the name fileName, store the new file under a new name instead:

Split fileName at its last dot into a stem and an extension that starts with that dot. If the name has no dot, or its last dot is its first character, the whole name is the stem and the extension is empty.
Use the smallest k >= 1 for which stem + " (" + k + ")" + extension is not the name of a stored file.

For example, a second a.txt becomes a (1).txt, a copy of archive.tar.gz becomes archive.tar (1).gz, and a copy of .env becomes .env (1). Names of deleted files are free again. GET and LIST report the stored name.

Function

runFileStorage(operations: String[][]) → String[]

Examples

Example 1

operations = [["UPLOAD","a.txt","hello","10"],["UPLOAD","a.txt","world","12"],["UPLOAD","b","","12"],["LIST"],["GET","file-2"],["DELETE","file-1"],["GET","file-1"],["DELETE","file-1"],["UPLOAD","a.txt","again","20"],["LIST"]]return = ["file-1","file-2","file-3","file-1|a.txt|5|10;file-2|a (1).txt|5|12;file-3|b|0|12","world","true","ERROR","false","file-4","file-2|a (1).txt|5|12;file-3|b|0|12;file-4|a.txt|5|20"]

The second a.txt is renamed a (1).txt. b has empty content, so its size is 0. After file-1 is deleted, getting it returns ERROR and deleting it again returns false. The name a.txt is free again, so the last upload keeps it and gets the new ID file-4.

Example 2

operations = [["UPLOAD","report.txt","v1","1"],["UPLOAD","report.txt","v2","2"],["UPLOAD","report.txt","v3","3"],["DELETE","file-2"],["UPLOAD","report.txt","v4","4"],["UPLOAD","report (1).txt","v5","5"],["GET","file-3"],["LIST"]]return = ["file-1","file-2","file-3","true","file-4","file-5","v3","file-1|report.txt|2|1;file-3|report (2).txt|2|3;file-4|report (1).txt|2|4;file-5|report (1) (1).txt|2|5"]

Uploads 2 and 3 become report (1).txt and report (2).txt. Deleting file-2 frees report (1).txt, so the fourth report.txt takes that name again. Uploading report (1).txt while that name is taken adds a suffix to its own stem: report (1) (1).txt.

Example 3

operations = [["UPLOAD","archive.tar.gz","x1","7"],["UPLOAD","archive.tar.gz","x2","7"],["UPLOAD",".env","k","8"],["UPLOAD",".env","k2","9"],["UPLOAD","notes","","9"],["UPLOAD","notes","n","9"],["GET","file-5"],["GET","file-9"],["LIST"]]return = ["file-1","file-2","file-3","file-4","file-5","file-6","","ERROR","file-1|archive.tar.gz|2|7;file-2|archive.tar (1).gz|2|7;file-3|.env|1|8;file-4|.env (1)|2|9;file-5|notes|0|9;file-6|notes (1)|1|9"]

The extension starts at the last dot, so the copy of archive.tar.gz is archive.tar (1).gz. In .env the only dot is the first character, so there is no extension and the copy is .env (1). notes has no dot. Getting file-5 returns its empty content, while file-9 never existed.

Constraints

1 <= operations.length <= 300, and at most 2 operations are LIST.
Every operation has one of the four forms above.
fileName has 1 to 20 characters, each a letter, digit, ., _, -, space, ( or ), and it does not start or end with a space.
content has 0 to 20 characters, each a lowercase letter or digit.
timestamp is a decimal integer from 0 to 10^6 without leading zeros, and timestamps never decrease from one upload to the next.
fileId in GET and DELETE has 1 to 12 characters, each a lowercase letter, digit or -.