danneu · March 24, 2023 21:04 · Mar 24, 2023
diff --git a/queue.ts b/queue.ts
@@ -0,0 +1,69 @@
+// USAGE
+
+const queue = new Queue(50)
+const middleware = () => async (ctx, next: () => Promise<void>) => {
+    const lock = await queue.take()
+    await next().finally(() => {
+        queue.release(lock)
+    })
+}
+
+// IMPL
+
+class MyLock {}
+class Ticket {
+    resolve: (_: MyLock) => void
+    constructor(resolve: (_: MyLock) => void) {
+        this.resolve = resolve
+    }
+}
+
+export default class Queue {
+    inflightCapacity: number
+    locks: Set<MyLock>
+    pending: Array<Ticket>
+    pendingCapacity: number
+
+    constructor(inflightCapacity: number) {
+        this.inflightCapacity = inflightCapacity
+        this.locks = new Set()
+        this.pending = []
+        this.pendingCapacity = 1000
+    }
+
+    // When you take(), you get a promise that resolves into a lock once a lock
+    // is available.
+    async take(): Promise<MyLock> {
+        if (this.locks.size < this.inflightCapacity) {
+            // There are locks available
+            const lock = new MyLock()
+            this.locks.add(lock)
+            return lock
+        } else if (this.pending.length >= this.pendingCapacity) {
+            // Drop request
+            throw new Error('OVER_CAPACITY')
+        } else {
+            // Pend until a lock is available
+            const promise = new Promise((resolve: (_: MyLock) => void) => {
+                const ticket = new Ticket(resolve)
+                this.pending.push(ticket)
+            })
+            return promise
+        }
+    }
+
+    release(lock: MyLock) {
+        this.locks.delete(lock)
+        for (let i = 0; i < this.inflightCapacity - this.locks.size; i++) {
+            // const ticket = this.pending.pop()
+            const ticket = this.pending.shift()
+            if (!ticket) {
+                return
+            }
+            const lock = new MyLock()
+            this.locks.add(lock)
+            ticket.resolve(lock)
+        }
+    }
+}
+