Introduction

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.

Starter Task Review

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

  • 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): Array<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.
Initial Solution Development

Let's jump into the JavaScript code and begin the implementation of our starter task. Here, we use JavaScript objects and Maps as the core of our design. These structures will allow us to dynamically manage lists keyed based on candidate IDs and voter IDs, greatly simplifying our design.

class VotingSystem {
  constructor() {
    this.candidates = new Map(); // Stores candidateId as key and votes as value
    this.voters = new Map(); // Tracks each voter's voting history
  }

  registerCandidate(candidateId) {
    if (this.candidates.has(candidateId)) {
      return false; // Candidate is already registered
    }
    this.candidates.set(candidateId, 0); // Initialize candidates with 0 votes
    return true;
  }

  vote(timestamp, voterId, candidateId) {
    if (!this.candidates.has(candidateId)) {
      return false; // Returns false if candidate is not registered
    }

    if (!this.voters.has(voterId)) {
      this.voters.set(voterId, { votes: [], timestamps: [] });
    }

    const voterData = this.voters.get(voterId);
    voterData.votes.push(candidateId); // Record the vote
    voterData.timestamps.push(timestamp); // Record the time of the vote
    this.candidates.set(candidateId, this.candidates.get(candidateId) + 1); // Increment vote count for the candidate
    return true;
  }

  getVotes(candidateId) {
    return this.candidates.get(candidateId) || null; // Retrieve vote count for a candidate, or null if not found
  }

  topNCandidates(n) {
    return Array.from(this.candidates.keys())
      .sort((a, b) => this.candidates.get(b) - this.candidates.get(a))
      .slice(0, n); // Return top `n` candidates based on votes
  }
}
Introduce New Methods

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

  • getVotingHistory(voterId: string): Object | 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.
Implementing New Methods

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:

class VotingSystem {
  // existing methods here…

  getVotingHistory(voterId) {
    if (!this.voters.has(voterId)) {
      return null;
    }
    const voterData = this.voters.get(voterId);
    const votingHistory = {};
    voterData.votes.forEach((candidateId) => {
      if (!votingHistory[candidateId]) {
        votingHistory[candidateId] = 0;
      }
      votingHistory[candidateId]++;
    });
    return votingHistory; // Returns an object with each candidate voted for, and the number of times they were voted for
  }

  blockVoterRegistration(timestamp) {
    this.blockTime = timestamp; // Records the timestamp after which no registration is allowed
    return true;
  }
}
Updating the "vote" Method

With the introduction of the blockVoterRegistration functionality, we must revisit our vote method 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 method to incorporate this change:

class VotingSystem {
  // existing methods here…

  vote(timestamp, voterId, candidateId) {
    // Check if blockTime is set and if the vote attempt is after the block timestamp
    if (this.blockTime && timestamp >= this.blockTime) {
      return false; // Vote attempt is blocked due to the registration freeze
    }

    if (!this.candidates.has(candidateId)) {
      return false; // Returns false if the candidate is not registered
    }

    if (!this.voters.has(voterId)) {
      this.voters.set(voterId, { votes: [], timestamps: [] });
    }

    const voterData = this.voters.get(voterId);
    voterData.votes.push(candidateId); // Record the vote
    voterData.timestamps.push(timestamp); // Record the time of the vote
    this.candidates.set(candidateId, this.candidates.get(candidateId) + 1); // Increment vote count for the candidate
    return true;
  }
}

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.

Implementing "changeVote" Method

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 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.
class VotingSystem {
  // existing methods here…

  changeVote(timestamp, voterId, oldCandidateId, newCandidateId) {
    // Verify existence in the voting system
    if (!this.candidates.has(oldCandidateId) || !this.candidates.has(newCandidateId)) {
      return false;
    }

    // Check if the voterId is valid and has previously voted
    if (!this.voters.has(voterId) || !this.voters.get(voterId).votes.length) {
      return false; // Voter is either not registered or has not voted yet
    }

    const voterData = this.voters.get(voterId); 

    // Confirm the voter has voted for the old candidate
    const lastVoteIndex = voterData.votes.lastIndexOf(oldCandidateId);
    if (lastVoteIndex === -1) {
      return false;
    }

    // Ensure the operation is within the permitted timeframe
    if (timestamp - voterData.timestamps[lastVoteIndex] > 86400) { // 24 hours in seconds
      return false;
    }

    // Perform the vote change
    voterData.votes.splice(lastVoteIndex, 1); // Remove the old vote
    voterData.timestamps.splice(lastVoteIndex, 1); // Remove the corresponding timestamp
    voterData.votes.push(newCandidateId); // Append the new vote
    voterData.timestamps.push(timestamp); // Append the new timestamp

    // Update candidates' vote counts
    this.candidates.set(oldCandidateId, this.candidates.get(oldCandidateId) - 1);
    this.candidates.set(newCandidateId, this.candidates.get(newCandidateId) + 1);

    return true;
  }
}
Conclusion

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. Happy Coding!

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