nemotron-3-ultra-550b-a55b:free × coding

The user wants a TypeScript function createIdempotencyGuard with specific requirements. Let me analyze the requirements carefully:

1. Accepts an object with key, ttlMs, optional maxEntries (default 10_000), and an async handler function.
2. Single-flight / stampede protection: if same key is currently being processed, return the same in-flight Promise.
3. After successful completion, cache and return the result until ttlMs expires.
4. If handler throws, do not cache the failure; future call with same key must retry.
5. When cache would exceed maxEntries, evict the oldest completed entry (FIFO among completed keys only; never evict in-flight keys).
6. Use only in-memory JavaScript/TypeScript primitives; no libraries.
7. Explain time and space complexity.
8. Include at least 5 test cases covering concurrency, caching, expiry, eviction, and errors.

I need to write this as a TypeScript function. Let me think about the implementation:

• Need a cache that stores completed results with timestamps
• Need a map of in-flight promises for single-flight protection
• Need to track insertion order for FIFO eviction of completed entries only
• TTL expiration: when getting a cached result, check if it's expired
• Eviction: when adding a new completed entry and cache is at maxEntries, remove oldest completed entry

Data structures:

• inFlight: Map<string, Promise<T>> - for single-flight protection
• cache: Map<string, { value: T, timestamp: number }> - for completed results
• completedOrder: string[] - array of keys in completion order for FIFO eviction (only completed keys)

Wait, but we need to track order of completion, not insertion. And we need to be able to remove from the middle if a key is evicted? Actually, FIFO among completed keys only - so we can use a queue (array) where we push when a key completes successfully, and shift when evicting. But we also need to handle the case where a key is already in the completed queue and gets updated? No, once completed, it stays until TTL expires or eviction. But TTL expiration doesn't remove from the queue automatically - we'd need to clean up expired entries. The requirement says "cache and return the result until ttlMs expires" - so on get, if expired, treat as miss. But the entry would still be in the queue until evicted? That could cause the queue to have stale entries. We should probably clean up expired entries on access or periodically. But for simplicity, we can check TTL on get and if expired, delete from cache and remove from queue. Removing from middle of array is O(n). Could use a linked list or just accept O(n) for eviction since maxEntries is 10k. Or we can use a Map for cache and a separate queue for order, and when evicting, we pop from queue until we find a key that's still in cache (not expired and not in-flight). That's simpler.

Actually, the requirement: "When the cache would exceed maxEntries, evict the oldest completed entry (FIFO among completed keys only; never evict in-flight keys)." So we only evict when adding a new completed entry and the cache size (completed entries) would exceed maxEntries. We need to track the number of completed entries. In-flight entries don't count towards maxEntries.

So:

• completedCache: Map<string, { value: T, timestamp: number }>
• completedQueue: string[] - keys in order of completion
• inFlight: Map<string, Promise<T>>

When handler succeeds:

• Add to completedCache with timestamp
• Push key to completedQueue
• If completedCache.size > maxEntries, evict oldest: shift from completedQueue until we find a key that exists in completedCache (and not in-flight? but in-flight keys are not in completedCache yet). Actually, in-flight keys are separate. So we just shift from queue, delete from completedCache.

But what if the oldest completed entry has already expired? It would still be in the queue. When we evict, we should skip expired entries? The requirement says "evict the oldest completed entry". If it's expired, it's still a completed entry until we clean it up. But we could clean up expired entries on get. For eviction, we can just evict the oldest in the queue regardless of TTL, because it's the oldest completed entry. That seems fine.

But we also need to handle TTL expiration on subsequent calls: if a key is in completedCache but expired, we should treat as cache miss, delete from cache, and remove from queue? Removing from queue is O(n). We could leave it in queue and skip during eviction. That's simpler: during eviction, we pop from queue and check if key exists in completedCache; if not (already expired and cleaned up), continue popping. But we need to clean up expired entries on get to avoid returning stale data. So on get, if expired, delete from completedCache. The key remains in completedQueue but will be skipped during eviction. That's acceptable.

Alternatively, we can use a doubly linked list for O(1) removal, but that's overkill. With maxEntries=10k, O(n) removal on expiry is fine. But we don't need to remove from queue on expiry; we can just leave it and skip during eviction. That's simpler.

Let me design the function signature:

function createIdempotencyGuard<T>(options: {
  key: string;
  ttlMs: number;
  maxEntries?: number;
  handler: () => Promise<T>;
}): Promise<T>

Wait, the function should be a factory that returns a function? Or is it a function that takes those options and returns a promise? The description: "Write a TypeScript function named createIdempotencyGuard for a high-throughput job worker. It accepts an object with key, ttlMs, optional maxEntries (default 10_000), and an async handler function." This sounds like a function that you call with those parameters and it returns a promise. But typically, you'd create a guard instance and then call it with keys. However, the parameters include key and handler, so it's per-call. That seems odd: you'd call createIdempotencyGuard({ key, ttlMs, maxEntries, handler }) each time? That would create a new cache each time, defeating the purpose.

Re-reading: "It accepts an object with key, ttlMs, optional maxEntries (default 10_000), and an async handler function." This suggests the function is called with these parameters each time. But then the cache would be local to that call? That doesn't make sense for a "guard" that provides idempotency across calls.

Perhaps the intention is that createIdempotencyGuard returns a function that can be called with a key and handler? Or maybe it's a class? The name "createIdempotencyGuard" suggests a factory that creates a guard instance. But the parameters include key and handler, which are per-operation.

Let me think: In a job worker, you might have a function that processes jobs. You want to guard each job with idempotency key. So you'd have a guard instance that you call with (key, handler). The TTL and maxEntries are configuration for the guard instance. So likely:

function createIdempotencyGuard(options: { ttlMs: number; maxEntries?: number }) {
  // returns a function that takes { key, handler }
}

But the requirement says: "It accepts an object with key, ttlMs, optional maxEntries (default 10_000), and an async handler function." That implies a single function that takes all four. However, that would mean the cache is recreated each call, which is useless. Unless the function uses a module-level cache? But that's not mentioned.

Given the context "for a high-throughput job worker", it's more likely that the guard is created once and reused. But the requirement explicitly lists the parameters. I'll interpret it as: the function createIdempotencyGuard is a higher-order function that takes ttlMs and maxEntries and returns a function that takes key and handler. But the requirement says "accepts an object with key, ttlMs, optional maxEntries, and an async handler function." That's a single object with all four properties.

Maybe the function is meant to be called per job, but the cache is stored in a closure? That would require the function to be a closure over a cache. But then each call to createIdempotencyGuard would create a new cache? That doesn't make sense.

Wait, perhaps the function is named createIdempotencyGuard but it's actually the guard function itself? Like const guard = createIdempotencyGuard(); then guard({ key, ttlMs, handler })? No.

Let me look at similar patterns: In Node.js, there's async-mutex or promise-cache. Usually you create a cache instance with configuration, then call get(key, handler).

Given the ambiguity, I'll implement it as a function that takes an options object with all four properties, but internally it uses a static cache (module-level) so that multiple calls share the same cache. But that would be weird because ttlMs and maxEntries could differ per call.

Alternatively, I'll implement it as a class IdempotencyGuard but the requirement says "function named createIdempotencyGuard". So I'll make it a factory function that returns a guard function. The factory takes ttlMs and maxEntries, and the returned function takes key and handler. But the requirement says "accepts an object with key, ttlMs, optional maxEntries, and an async handler function." That's one object.

I think the most reasonable interpretation is that the function is called per operation, but the cache is maintained in a closure created by the first call? That's not possible in TypeScript without a module-level variable.

Given the instruction "Write a TypeScript function named createIdempotencyGuard", I'll write a function that takes an object with those properties and returns a Promise. To make the cache persist across calls, I'll use a module-level Map. But then ttlMs and maxEntries are per-call, which could conflict. I'll assume that the guard is created once with configuration, and then used multiple times. So I'll implement it as:

function createIdempotencyGuard(config: { ttlMs: number; maxEntries?: number }) {
  // returns a function that takes { key, handler }
}

But the requirement explicitly says "accepts an object with key, ttlMs, optional maxEntries (default 10_000), and an async handler function." That's a single function call.

Maybe the function is meant to be used like:

const result = await createIdempotencyGuard({ key: 'job1', ttlMs: 60000, handler: doWork });

And the cache is stored in a global variable inside the module. That's a bit unconventional but possible. However, if you call it again with different ttlMs, what happens? The cache would have mixed TTLs. That's messy.

Given the benchmark context, they probably want a self-contained function that demonstrates the logic. I'll implement it as a class but wrap it in a function that creates an instance and calls it? No.

I'll write a function that takes all four parameters, but uses a closure to maintain cache across calls by using a static variable (like a module-level Map). But TypeScript doesn't have static variables in functions. I can use a global variable in the module scope. Since the output is just code, I can define the cache outside the function.

But the instruction says "Write a TypeScript function named createIdempotencyGuard". I'll write a function that returns a promise, and use a module-level cache. I'll note that in a real scenario, you'd want to encapsulate the cache in an instance.

Alternatively, I can make createIdempotencyGuard a generic function that creates a guard instance, but the test cases will call it multiple times. The test cases need to share the same guard instance. So I'll export a function that creates a guard, and then the tests use that guard. But the requirement says "function named createIdempotencyGuard" that accepts those parameters.

Let me re-read: "Write a TypeScript function named createIdempotencyGuard for a high-throughput job worker. Requirements: - It accepts an object with key, ttlMs, optional maxEntries (default 10_000), and an async handler function."

This is a single function that takes an object with those four properties. So the signature is:

function createIdempotencyGuard<T>(options: { key: string; ttlMs: number; maxEntries?: number; handler: () => Promise<T> }): Promise<T>

To make it work across multiple calls, the cache must be stored somewhere persistent. In JavaScript, we can use a module-level variable. Since the code will be run as a module, we can declare the cache outside the function. But the function is named createIdempotencyGuard, which suggests it creates a guard, not that it is the guard. However, the requirements describe the behavior of the guard when called with a key and handler.

I think the best is to implement it as a function that uses a closure