Welcome back to a fascinating session where we will learn about enhancing existing functionality without causing regressions. Our scenario today involves designing a voting system. We'll start with the basic implementation of the voting system and gradually introduce additional elements of complexity.

In our initial task, we create a simple voting system in TypeScript with a set of basic functionalities. Let's define the methods with TypeScript type annotations:

• registerCandidate(candidateId: string): boolean: This method is used for adding new candidates to our system.
• vote(timestamp: number, voterId: string, candidateId: string): boolean: This method is designed to facilitate users in casting their votes. Each vote is given a timestamp.
• getVotes(candidateId: string): number | null: This method retrieves the total number of votes for a given candidate.
• topNCandidates(n: number): string[]: We also want to add a leaderboard functionality to our system. This method returns the top n candidates sorted by the number of votes.

Let's jump into the TypeScript code and begin the implementation of our starter task. Here, we use TypeScript's type system to define class fields and method parameters, providing more robust type-checking and better code maintenance.

Now that we have a basic voting system, our goal is to enhance this system with additional functionalities. The methods will be correctly typed using TypeScript:

• getVotingHistory(voterId: string): Record<string, number> | null: Provides a detailed voting history for a specified voter, returning an object mapping each candidate to the number of times voted for by the voter. Returns null if the voter ID does not exist.
• blockVoterRegistration(timestamp: number): boolean: Implements a mechanism to halt any new voter registrations past a specified timestamp, effectively freezing the voter list as of that moment.
• changeVote(timestamp: number, voterId: string, oldCandidateId: string, newCandidateId: string): boolean: 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. Let's incorporate the methods to get the voting history and to block further voter registrations.

The getVotingHistory method retrieves a voter's voting history as an object, mapping each candidate ID to the number of times the voter has voted for them. If the voter exists, the method accesses their votes array, which contains the IDs of the candidates they voted for. It then iterates through this array, using an object called votingHistory to count the occurrences of each candidate ID. For every candidate in the array, the method increments their count in votingHistory. Finally, it returns this object, providing a summary of the voter’s activity.

With the introduction of the blockVoterRegistration functionality, we must revisit our vote method to ensure it respects the new rules set by this feature. Here’s how we modify the vote method in TypeScript 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 while maintaining 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 in TypeScript:

• 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 timeframe 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.

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. Continue exploring with practice sessions and further experimentation to refine your skills in developing complex, functionality-rich applications using TypeScript. Happy Coding!