feat: add counting semaphore

Author: tharropoulos

src/concurrency.ts

@@ -0,0 +1,32 @@
+export class CountingSemaphore {
+  #count: number;
+  readonly #waiters: (() => void)[] = [];
+
+  constructor(count: number) {
+    if (count < 1) {
+      throw new Error("Semaphore count must be at least 1");
+    }
+    this.#count = count;
+  }
+
+  async acquire(): Promise<void> {
+    if (this.#count > 0) {
+      this.#count -= 1;
+      return;
+    }
+
+    await new Promise<void>((resolve) => {
+      this.#waiters.push(resolve);
+    });
+  }
+
+  release(): void {
+    const nextResolve = this.#waiters.shift();
+    if (nextResolve !== undefined) {
+      nextResolve();
+      return;
+    }
+
+    this.#count += 1;
+  }
+}