Problem 7 FizzBuzz Multithreading Problem

Four threads, one ordered output — coordinated by a shared counter

Four threads each own one job: print the number, print "fizz" (n%3==0, not %5), "buzz" (n%5==0, not %3), or "fizzbuzz" (n%15==0). They must cooperate so the combined output is exactly 1 2 fizz 4 buzz fizz 7 … in order. The mechanism is a single shared counter current guarded by a lock + condition: each thread waits until current is a value that belongs to it, prints, advances the counter, and wakes the others. The earlier version was broken twice over — the method was named Solution() and printed the literal string "Solution" on multiples of 15, and the driver never joined its threads, so the program could exit before finishing.

Correct Java

import java.util.concurrent.locks.*;
import java.util.function.IntConsumer;

class FizzBuzz {
    private final int n;
    private int current = 1;
    private final Lock lock = new ReentrantLock();
    private final Condition cv = lock.newCondition();
    FizzBuzz(int n) { this.n = n; }

    private void run(java.util.function.IntPredicate mine, Runnable print) throws InterruptedException {
        while (true) {
            lock.lock();
            try {
                while (current <= n && !mine.test(current)) cv.await(); // not my turn
                if (current > n) return;                                // done
                print.run();
                current++;
                cv.signalAll();
            } finally { lock.unlock(); }
        }
    }
    public void fizz(Runnable p)     throws InterruptedException { run(i -> i%3==0 && i%5!=0, p); }
    public void buzz(Runnable p)     throws InterruptedException { run(i -> i%5==0 && i%3!=0, p); }
    public void fizzbuzz(Runnable p) throws InterruptedException { run(i -> i%15==0, p); }
    public void number(IntConsumer p) throws InterruptedException {
        while (true) {
            lock.lock();
            try {
                while (current <= n && (current%3==0 || current%5==0)) cv.await();
                if (current > n) return;
                p.accept(current);
                current++;
                cv.signalAll();
            } finally { lock.unlock(); }
        }
    }
}
// driver: start all four threads, then join EACH so main waits for completion.

Correct Go

Go expresses the same coordination with sync.Mutex + sync.Cond, and a sync.WaitGroup replaces the explicit joins.

type FizzBuzz struct{ n, cur int; mu sync.Mutex; cv *sync.Cond }
func (f *FizzBuzz) step(mine func(int) bool, print func(int)) {
    for {
        f.mu.Lock()
        for f.cur <= f.n && !mine(f.cur) { f.cv.Wait() }
        if f.cur > f.n { f.mu.Unlock(); return }
        print(f.cur); f.cur++; f.cv.Broadcast()
        f.mu.Unlock()
    }
}
// number: mine = func(i int) bool { return i%3!=0 && i%5!=0 }
// wg.Add(4); go each step; wg.Wait()

Pitfalls

• while (!mine) await() must be a loop, not an if — a thread can wake (signalAll) on a value that still isn't its turn (a spurious-ish wakeup), and must re-check.
• Every worker also needs the current > n exit check inside the loop, or threads hang at the end.
• The driver must join all four threads (Java) / wg.Wait() (Go), or output is truncated.

Takeaways

• Order is enforced by a guarded shared counter: wait-until-mine, print, advance, signalAll.
• Guard the wait in a while loop; always include the termination check.
• Join/Wait for every worker so the program doesn't exit early.

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Re-authored for correctness for this guide (the prior version printed "Solution" and never joined its threads). Pattern: LeetCode 1195 "Fizz Buzz Multithreaded". See also: Condition Variables, Mutex Lock.

I'm working on the problem **Problem 7 FizzBuzz Multithreading Problem** (Concurrency). Give me a HINT LADDER: start with the tiniest nudge, then wait. Only reveal the next, stronger hint when I ask. Do NOT show the full solution unless I type 'show solution'. Keep me doing the thinking. If you're unsure or a claim isn't standard, say so and reason from first principles instead of guessing.

Explain the optimal approach to **Problem 7 FizzBuzz Multithreading Problem** with a VISUAL walkthrough: trace it on a small concrete example using ASCII art / a step-by-step diagram, narrate what changes each step, then give time & space complexity with a one-line derivation. If you're unsure or a claim isn't standard, say so and reason from first principles instead of guessing.

I'll paste my solution to **Problem 7 FizzBuzz Multithreading Problem**. Review it for correctness, missed edge cases, and time/space complexity, then coach me toward the optimal — don't just rewrite it. Ask me to paste my code now. If you're unsure or a claim isn't standard, say so and reason from first principles instead of guessing.

Give me 2 problems that use the SAME underlying pattern as **Problem 7 FizzBuzz Multithreading Problem**. For each, let me attempt first, then review my answer and name the trigger signal that reveals the pattern. If you're unsure or a claim isn't standard, say so and reason from first principles instead of guessing.