import { Task, Domain } from '../types';

let domain: Domain; // The domain module is executed on demand
const hasSetImmediate = typeof setImmediate === 'function';

// Use the fastest means possible to execute a task in its own turn, with
// priority over other events including network IO events in Node.js.
//
// An exception thrown by a task will permanently interrupt the processing of
// subsequent tasks. The higher level `asap` function ensures that if an
// exception is thrown by a task, that the task queue will continue flushing as
// soon as possible, but if you use `rawAsap` directly, you are responsible to
// either ensure that no exceptions are thrown from your task, or to manually
// call `rawAsap.requestFlush` if an exception is thrown.
export function rawAsap(task: Task): void {
  if (!queue.length) {
    requestFlush();
    flushing = true;
  }
  // Avoids a function call
  queue[queue.length] = task;
}

const queue: Task[] = [];

// Once a flush has been requested, no further calls to `requestFlush` are
// necessary until the next `flush` completes.
let flushing = false;

// The position of the next task to execute in the task queue. This is
// preserved between calls to `flush` so that it can be resumed if
// a task throws an exception.
let index = 0;

// If a task schedules additional tasks recursively, the task queue can grow
// unbounded. To prevent memory excaustion, the task queue will periodically
// truncate already-completed tasks.
let capacity = 1024;

// The flush function processes all tasks that have been scheduled with
// `rawAsap` unless and until one of those tasks throws an exception.
// If a task throws an exception, `flush` ensures that its state will remain
// consistent and will resume where it left off when called again.
// However, `flush` does not make any arrangements to be called again if an
// exception is thrown.
function flush() {
  while (index < queue.length) {
    var currentIndex = index;
    // Advance the index before calling the task. This ensures that we will
    // begin flushing on the next task the task throws an error.
    index = index + 1;
    queue[currentIndex].call();
    // Prevent leaking memory for long chains of recursive calls to `asap`.
    // If we call `asap` within tasks scheduled by `asap`, the queue will
    // grow, but to avoid an O(n) walk for every task we execute, we don't
    // shift tasks off the queue after they have been executed.
    // Instead, we periodically shift 1024 tasks off the queue.
    if (index > capacity) {
      // Manually shift all values starting at the index back to the
      // beginning of the queue.
      for (
        var scan = 0, newLength = queue.length - index;
        scan < newLength;
        scan++
      ) {
        queue[scan] = queue[scan + index];
      }
      queue.length -= index;
      index = 0;
    }
  }
  queue.length = 0;
  index = 0;
  flushing = false;
}

rawAsap.requestFlush = requestFlush;
function requestFlush() {
  // Ensure flushing is not bound to any domain.
  // It is not sufficient to exit the domain, because domains exist on a stack.
  // To execute code outside of any domain, the following dance is necessary.
  var parentDomain = process.domain;
  if (parentDomain) {
    if (!domain) {
      // Lazy execute the domain module.
      // Only employed if the user elects to use domains.
      domain = require('domain');
    }
    domain.active = process.domain = null as any;
  }

  // `setImmediate` is slower that `process.nextTick`, but `process.nextTick`
  // cannot handle recursion.
  // `requestFlush` will only be called recursively from `asap.js`, to resume
  // flushing after an error is thrown into a domain.
  // Conveniently, `setImmediate` was introduced in the same version
  // `process.nextTick` started throwing recursion errors.
  if (flushing && hasSetImmediate) {
    setImmediate(flush);
  } else {
    process.nextTick(flush);
  }

  if (parentDomain) {
    domain.active = process.domain = parentDomain;
  }
}