/**
 * FIFO Semaphore for per-hoster concurrency control.
 * acquire() blocks until a slot is available, release() frees it.
 */
class Semaphore {
  constructor(limit) {
    this.limit = Math.max(1, limit || 1);
    this.active = 0;
    this.queue = [];
  }

  acquire() {
    return new Promise((resolve) => {
      if (this.active < this.limit) {
        this.active++;
        resolve();
      } else {
        this.queue.push(resolve);
      }
    });
  }

  release() {
    if (this.queue.length > 0) {
      // Don't decrement active — hand slot directly to next waiter
      const next = this.queue.shift();
      next();
    } else {
      this.active = Math.max(0, this.active - 1);
    }
  }

  updateLimit(newLimit) {
    this.limit = Math.max(1, newLimit || 1);
    // If new limit is higher, wake up waiting tasks
    while (this.active < this.limit && this.queue.length > 0) {
      this.active++;
      const next = this.queue.shift();
      next();
    }
  }

  get pending() {
    return this.queue.length;
  }
}

module.exports = Semaphore;