Initial service implementation

src/lib/semaphore.ts

@@ -0,0 +1,44 @@
+export class Semaphore {
+  private current = 0;
+  private readonly waiting: Array<() => void> = [];
+
+  constructor(private readonly limit: number) {}
+
+  get active(): number {
+    return this.current;
+  }
+
+  get queued(): number {
+    return this.waiting.length;
+  }
+
+  async use<T>(task: () => Promise<T>): Promise<T> {
+    await this.acquire();
+
+    try {
+      return await task();
+    } finally {
+      this.release();
+    }
+  }
+
+  private async acquire(): Promise<void> {
+    if (this.current < this.limit) {
+      this.current += 1;
+      return;
+    }
+
+    await new Promise<void>((resolve) => {
+      this.waiting.push(() => {
+        this.current += 1;
+        resolve();
+      });
+    });
+  }
+
+  private release(): void {
+    this.current -= 1;
+    const next = this.waiting.shift();
+    next?.();
+  }
+}