Welcome back to another exciting session where we learn about enhancing existing functionality without causing regressions. Today, our scenario involves designing a voting system. We'll start with the basic implementation of the voting system and gradually introduce additional elements of complexity using PHP.

In our initial task, we created a simple voting system in PHP with a set of basic functionalities:

• function registerCandidate(string $candidateId): bool: This function is used to add new candidates to our system.
• function vote(int $timestamp, string $voterId, string $candidateId): bool: This function facilitates users casting their votes. Each vote is given a timestamp.
• function getVotes(string $candidateId): ?int: This function retrieves the total number of votes for a given candidate.
• function topNCandidates(int $n): array: We also want to add a leaderboard functionality to our system. This function returns the top n candidates sorted by the number of votes.

Let's jump into the PHP code and begin the implementation of our starter task. Here, we use PHP's arrays as the core of our design. Associative arrays allow us to have dynamic structures keyed based on candidate IDs and voter IDs, which will greatly simplify our design.

Now that we have a basic voting system, our goal is to enhance this system with additional functionalities:

• function getVotingHistory(string $voterId): ?array: Provides a detailed voting history for a specified voter, returning an associative array with candidates and the number of votes they received from the voter. Returns null if the voter ID does not exist.
• function blockVoterRegistration(int $timestamp): bool: Implements a mechanism to halt any new voter registrations past a specified timestamp, effectively freezing the voter list as of that moment.
• function changeVote(int $timestamp, string $voterId, string $oldCandidateId, string $newCandidateId): bool: Enables voters to change their vote from one candidate to another, given the change is made within a 24-hour window from their last vote, ensuring both the old and new candidates are registered and that the voter initially voted for the old candidate.

We proceed to enhance our existing VotingSystem class to accommodate the new functionalities.

First, let's incorporate the methods to get the voting history and to block further voter registrations:

With the introduction of the blockVoterRegistration functionality, we must revisit our vote function to ensure it respects the new rules set by this feature. Specifically, we need to ensure that no votes are cast after the voter registration has been blocked. This is critical in maintaining the integrity of the voting system, especially in scenarios where registration deadlines are enforced. Here's how we modify the vote function to incorporate this change:

This update ensures that our voting system behaves as expected, even with the new functionality to block further voter registrations beyond a certain timestamp. It's a perfect demonstration of how new features can necessitate revisits and revisions to existing code to enhance functionality while ensuring backward compatibility.

The changeVote method allows voters to change their vote, adhering to specific rules. Here's a step-by-step explanation of implementing this functionality:

• Verify Candidate and Voter Validity: Check if both the old and new candidate IDs exist in the system, and verify that the voter has previously voted for the old candidate.

• Timestamp Constraints: Ensure that the voter is trying to change their vote within an allowable time frame after their initial vote.

• Update Votes: If all conditions are met, subtract one vote from the old candidate, add one vote to the new candidate, and update the voter's voting record.

Note: In the implementation of the changeVote method, we don't use blockTime because blockTime is intended to halt new voter registrations and votes after the specified timestamp. However, the changeVote method is focused on changing an existing vote. Since it's assumed that the voter has already registered and cast their vote before the blockTime, the focus here is on validating the vote change within the 24-hour window, irrespective of blockTime. Therefore, blockTime does not need to be considered in this context where changes to existing votes are allowed.

Congratulations! You've successfully enhanced the voting system by adding functionalities to view voting history, block new candidate registrations, and, most importantly, enable vote changes under specific conditions. Each of these features was developed with careful consideration to maintain the integrity and backward compatibility of the system in a PHP context. Continue exploring with practice sessions and further experimentation to refine your skills in developing complex, functionality-rich applications in PHP. Happy coding!