import { rawAsap } from './raw';
import { Task } from '../types';

// rawAsap provides everything we need except exception management.
// RawTasks are recycled to reduce GC churn.
let freeTasks: Task[] = [];
// We queue errors to ensure they are thrown in right order (FIFO).
// Array-as-queue is good enough here, since we are just dealing with exceptions.
let pendingErrors: any[] = [];
let requestErrorThrow = rawAsap.makeRequestCallFromTimer(throwFirstError);

function throwFirstError() {
  if (pendingErrors.length) {
    throw pendingErrors.shift();
  }
}

/**
 * Calls a task as soon as possible after returning, in its own event, with priority
 * over other events like animation, reflow, and repaint. An error thrown from an
 * event will not interrupt, nor even substantially slow down the processing of
 * other events, but will be rather postponed to a lower priority event.
 * @param {{call}} task A callable object, typically a function that takes no
 * arguments.
 */
export function asap(task: Task) {
  var rawTask;
  if (freeTasks.length) {
    rawTask = freeTasks.pop();
  } else {
    rawTask = new RawTask();
  }
  rawTask.task = task;
  rawAsap(rawTask);
}

// We wrap tasks with recyclable task objects.  A task object implements
// `call`, just like a function.
class RawTask {
  private task: Task;

  public call() {
    try {
      this.task.call();
    } catch (error) {
      if ((asap as any).onerror) {
        // This hook exists purely for testing purposes.
        // Its name will be periodically randomized to break any code that
        // depends on its existence.
        (asap as any).onerror(error);
      } else {
        // In a web browser, exceptions are not fatal. However, to avoid
        // slowing down the queue of pending tasks, we rethrow the error in a
        // lower priority turn.
        pendingErrors.push(error);
        requestErrorThrow();
      }
    } finally {
      this.task = null;
      freeTasks[freeTasks.length] = this;
    }
  }
}