ZPOPMAX Command Migration

[prabhavdogra]

Summary by CodeRabbit

• New Features
  • Introduced the new ZPOPMAX command, which lets you remove and return the highest-scoring items from a sorted set, with an option to specify the number of items.
  • Enhanced the consistency and clarity of command output formatting for a more predictable user experience.

[coderabbitai[bot]]

Walkthrough

The pull request introduces a new command implementation for ZPOPMAX that removes and returns the highest-scoring members from a sorted set. The new command includes functions for argument parsing (parseZPOPMAXArgs), evaluation (evalZPOPMAX), and execution (executeZPOPMAX), along with its registration in the command registry. Additionally, utility enhancements are made in the commands file with the addition of a variable (cmdResEmptySlice) and a function (cmdResSlice) to construct command results from string slices.

Changes

File Path	Summary of Changes
internal/cmd/cmd_zpopmax.go	Introduces ZPOPMAX command implementation. Adds new command metadata, registration in the init function, and functions: evalZPOPMAX, parseZPOPMAXArgs, and executeZPOPMAX for processing, validating arguments, and executing the command.
internal/cmd/cmds.go	Adds utility improvements: a new variable cmdResEmptySlice for initializing an empty result slice and a new function cmdResSlice to convert a slice of strings into a command result.

Sequence Diagram(s)

sequenceDiagram
    participant U as User
    participant CR as Command Registry
    participant EX as executeZPOPMAX
    participant EV as evalZPOPMAX
    participant PS as parseZPOPMAXArgs
    participant SS as SortedSet

    U->>CR: Submit "ZPOPMAX" command with args
    CR->>EX: Invoke executeZPOPMAX
    EX->>EV: Delegate processing
    EV->>PS: Parse and validate arguments
    PS-->>EV: Return key and count (or error)
    EV->>SS: Call PopMax(key, count)
    SS-->>EV: Return highest-scoring members
    EV-->>EX: Return command result
    EX-->>CR: Command execution complete
    CR-->>U: Output result

Poem

I’m a rabbit in a realm of code,
Where lines and logic form my abode.
With ZPOPMAX I hop and pop so high,
Sorting treasures like carrots in the sky.
I nibble on bytes with a joyful gleam,
Happy to sprout a brand-new command dream!

✨ Finishing Touches

• [ ] 📝 Generate Docstrings

---

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[CLAassistant]

All committers have signed the CLA.

[prabhavdogra]

@lucifercr07 @arpitbbhayani @JyotinderSingh @ayushsatyam146 I’ve observed an issue and here’s what I observed:

• When executing in an empty set:

localhost:7379> zpopmax set

the key "set" isn’t present in the store, so our code correctly returns an empty slice using:

if obj == nil {
    return cmdResEmptySlice, nil
}

• This behavior aligns with Redis, where an empty result is expected (see attached screenshot).

• However, after that, we marshal the response with:

if b, err = proto.Marshal(r); err != nil {
    return err
}

if _, err := h.conn.Write(b); err != nil {
    return err
}

Since cmdResEmptySlice (or the equivalent message) is empty, proto.Marshal(r) produces an empty byte array (length 0). Consequently, the call to h.conn.Write(b) effectively sends no data over the connection.

Possible Solutions:

1. Force a Non-Nil Empty Payload: Instead of returning a nil or zero-length slice, we could define cmdResEmptySlice as a non-nil but empty slice (e.g., []byte{}) so that the write call is invoked and the client receives an explicit empty message.

2. Send a Sentinel Value: If acceptable for the protocol, we could send a specific marker (like a specific header or flag) to indicate an empty result, ensuring that the client gets a response even if no data payload is present.

3. Explicit Write Check: Modify the write logic to check if the marshalled data has length zero and, if so, perform an explicit write of an empty payload (or sentinel) to trigger the client-side handling.

How to replicate:

• Fetch this branch of dicedb locally
• Fetch dice-db cli change from: https://github.com/DiceDB/dicedb-cli/pull/35
• Launch the dicedb and dicedb-cli
• Run: localhost:7379> zpopmax set

Let me know if any other clarifications/discussions are required on this

[arpitbbhayani]

We have already merged ZPOPMAX with Tests. Thanks for the patch. Closing this issue as this is not redundant.