/* eslint-disable no-undefined,no-param-reassign,no-shadow */

/**
 * Throttle execution of a function. Especially useful for rate limiting
 * execution of handlers on events like resize and scroll.
 *
 * @param {number} delay -                  A zero-or-greater delay in milliseconds. For event callbacks, values around 100 or 250 (or even higher)
 *                                            are most useful.
 * @param {Function} callback -               A function to be executed after delay milliseconds. The `this` context and all arguments are passed through,
 *                                            as-is, to `callback` when the throttled-function is executed.
 * @param {object} [options] -              An object to configure options.
 * @param {boolean} [options.noTrailing] -   Optional, defaults to false. If noTrailing is true, callback will only execute every `delay` milliseconds
 *                                            while the throttled-function is being called. If noTrailing is false or unspecified, callback will be executed
 *                                            one final time after the last throttled-function call. (After the throttled-function has not been called for
 *                                            `delay` milliseconds, the internal counter is reset).
 * @param {boolean} [options.noLeading] -   Optional, defaults to false. If noLeading is false, the first throttled-function call will execute callback
 *                                            immediately. If noLeading is true, the first the callback execution will be skipped. It should be noted that
 *                                            callback will never executed if both noLeading = true and noTrailing = true.
 * @param {boolean} [options.debounceMode] - If `debounceMode` is true (at begin), schedule `clear` to execute after `delay` ms. If `debounceMode` is
 *                                            false (at end), schedule `callback` to execute after `delay` ms.
 *
 * @returns {Function} A new, throttled, function.
 */
export function throttle(delay, callback, options) {
  const {
    noTrailing = false,
    noLeading = false,
    debounceMode = undefined,
  } = options || {};
  /*
   * After wrapper has stopped being called, this timeout ensures that
   * `callback` is executed at the proper times in `throttle` and `end`
   * debounce modes.
   */
  let timeoutID;
  let cancelled = false;

  // Keep track of the last time `callback` was executed.
  let lastExec = 0;

  // Function to clear existing timeout
  function clearExistingTimeout() {
    if (timeoutID) {
      clearTimeout(timeoutID);
    }
  }

  // Function to cancel next exec
  function cancel(options) {
    const { upcomingOnly = false } = options || {};
    clearExistingTimeout();
    cancelled = !upcomingOnly;
  }

  /*
   * The `wrapper` function encapsulates all of the throttling / debouncing
   * functionality and when executed will limit the rate at which `callback`
   * is executed.
   */
  function wrapper(...arguments_) {
    let self = this;
    let elapsed = Date.now() - lastExec;

    if (cancelled) {
      return;
    }

    // Execute `callback` and update the `lastExec` timestamp.
    function exec() {
      lastExec = Date.now();
      callback.apply(self, arguments_);
    }

    /*
     * If `debounceMode` is true (at begin) this is used to clear the flag
     * to allow future `callback` executions.
     */
    function clear() {
      timeoutID = undefined;
    }

    if (!noLeading && debounceMode && !timeoutID) {
      /*
       * Since `wrapper` is being called for the first time and
       * `debounceMode` is true (at begin), execute `callback`
       * and noLeading != true.
       */
      exec();
    }

    clearExistingTimeout();

    if (debounceMode === undefined && elapsed > delay) {
      if (noLeading) {
        /*
         * In throttle mode with noLeading, if `delay` time has
         * been exceeded, update `lastExec` and schedule `callback`
         * to execute after `delay` ms.
         */
        lastExec = Date.now();
        if (!noTrailing) {
          timeoutID = setTimeout(debounceMode ? clear : exec, delay);
        }
      } else {
        /*
         * In throttle mode without noLeading, if `delay` time has been exceeded, execute
         * `callback`.
         */
        exec();
      }
    } else if (noTrailing !== true) {
      /*
       * In trailing throttle mode, since `delay` time has not been
       * exceeded, schedule `callback` to execute `delay` ms after most
       * recent execution.
       *
       * If `debounceMode` is true (at begin), schedule `clear` to execute
       * after `delay` ms.
       *
       * If `debounceMode` is false (at end), schedule `callback` to
       * execute after `delay` ms.
       */
      timeoutID = setTimeout(
        debounceMode ? clear : exec,
        debounceMode === undefined ? delay - elapsed : delay,
      );
    }
  }

  wrapper.cancel = cancel;

  // Return the wrapper function.
  return wrapper;
}

export function debounce(delay, callback, options) {
  const { atBegin = false } = options || {};
  return throttle(delay, callback, { debounceMode: atBegin !== false });
}