Implementing a Vector Clock in JavaScript

Vector clocks are a crucial tool for maintaining causality in distributed systems. They allow us to track the order of events across multiple processes without relying on a central time source, preventing inconsistencies and ensuring reliable message delivery. This challenge asks you to implement a basic vector clock data structure and its core operations in JavaScript.

Problem Description

You are tasked with creating a VectorClock class in JavaScript. This class should represent a vector clock and provide methods for initialization, updating, comparing, and merging vector clocks. The vector clock will be represented as an object where keys are process IDs (strings) and values are integer timestamps.

Key Requirements:

• Initialization: The constructor should accept an array of process IDs (strings) and initialize the vector clock with all processes having a timestamp of 0.
• Increment: A method to increment the timestamp of the local process.
• Receive: A method to update the vector clock after receiving a message from another process. This involves updating the timestamp of the sending process based on the received vector clock.
• Compare: A method to compare two vector clocks and determine if one event happened before another (happens-before relationship).
• Merge: A method to merge two vector clocks, taking the maximum timestamp for each process.

Expected Behavior:

• The VectorClock class should be robust and handle various scenarios, including processes with different IDs.
• The receive method should correctly update the vector clock based on the received clock.
• The compare method should accurately determine the happens-before relationship.
• The merge method should produce a valid vector clock representing the combined state.

Edge Cases to Consider:

• Process IDs that are not present in the initial list.
• Empty process ID lists during initialization.
• Comparing clocks with different sets of process IDs.
• Handling cases where timestamps are equal.

Examples

Example 1:

Input:
processIds = ["A", "B", "C"]
clock1 = new VectorClock(processIds);
clock2 = new VectorClock(processIds);

clock1.increment();
clock2.receive(clock1);

Output:
clock1: { A: 1, B: 0, C: 0 }
clock2: { A: 0, B: 1, C: 0 }
Explanation: clock1 increments its own timestamp (A). clock2 receives clock1 and updates its timestamp for process A.

Example 2:

Input:
processIds = ["X", "Y"]
clock1 = new VectorClock(processIds);
clock2 = new VectorClock(processIds);

clock1.increment();
clock2.increment();
clock1.receive(clock2);

Output:
clock1: { X: 1, Y: 1 }
clock2: { X: 1, Y: 1 }
Explanation: Both clocks increment. clock1 receives clock2, and both clocks now have timestamps of 1 for both processes.

Example 3: (Comparison)

Input:
processIds = ["P1", "P2"]
clock1 = new VectorClock(processIds);
clock2 = new VectorClock(processIds);

clock1.increment();
clock2.increment();

result = clock1.compare(clock2);

Output:
result: false
Explanation: clock1's timestamp for P1 is 1, and clock2's is 1.  Since they are equal, the happens-before relationship is not true.

Constraints

• Process IDs will be strings.
• Timestamps will be non-negative integers.
• The number of process IDs will be between 1 and 100.
• The implementation should be reasonably efficient for a small number of processes. Performance is not the primary focus, but avoid excessively complex or inefficient algorithms.

Notes

• Consider using a JavaScript object to represent the vector clock.
• The compare method should return true if the first clock represents an event that happened before the event represented by the second clock. It should return false otherwise.
• Think about how to handle process IDs that are not present in both clocks during comparison and merging. You can assume that a missing process ID has a timestamp of 0.
• Focus on correctness and clarity of the code. Good variable names and comments are encouraged.
• The increment method should only increment the timestamp of the local process (the process that owns the vector clock).