/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

"use strict";

/**
 * A simple undo stack manager.
 *
 * Actions are added along with the necessary code to
 * reverse the action.
 */
class UndoStack {
  /**
   * @param {integer} maxUndo Maximum number of undo steps.
   *   defaults to 50.
   */
  constructor(maxUndo) {
    this.maxUndo = maxUndo || 50;
    this._stack = [];
  }

  // Current index into the undo stack.  Is positioned after the last
  // currently-applied change.
  _index = 0;

  // The current batch depth (see startBatch() for details)
  _batchDepth = 0;

  destroy() {
    this.uninstallController();
    delete this._stack;
  }

  /**
   * Start a collection of related changes.  Changes will be batched
   * together into one undo/redo item until endBatch() is called.
   *
   * Batches can be nested, in which case the outer batch will contain
   * all items from the inner batches.  This allows larger user
   * actions made up of a collection of smaller actions to be
   * undone as a single action.
   */
  startBatch() {
    if (this._batchDepth++ === 0) {
      this._batch = [];
    }
  }

  /**
   * End a batch of related changes, performing its action and adding
   * it to the undo stack.
   */
  endBatch() {
    if (--this._batchDepth > 0) {
      return;
    }

    // Cut off the end of the undo stack at the current index,
    // and the beginning to prevent a stack larger than maxUndo.
    const start = Math.max(this._index + 1 - this.maxUndo, 0);
    this._stack = this._stack.slice(start, this._index);

    const batch = this._batch;
    delete this._batch;
    const entry = {
      do() {
        for (const item of batch) {
          item.do();
        }
      },
      undo() {
        for (let i = batch.length - 1; i >= 0; i--) {
          batch[i].undo();
        }
      },
    };
    this._stack.push(entry);
    this._index = this._stack.length;
    entry.do();
  }

  /**
   * Perform an action, adding it to the undo stack.
   *
   * @param function toDo Called to perform the action.
   * @param function undo Called to reverse the action.
   */
  do(toDo, undo) {
    this.startBatch();
    this._batch.push({ do: toDo, undo });
    this.endBatch();
  }

  /*
   * Returns true if undo() will do anything.
   */
  canUndo() {
    return this._index > 0;
  }

  /**
   * Undo the top of the undo stack.
   *
   * @return true if an action was undone.
   */
  undo() {
    if (!this.canUndo()) {
      return false;
    }
    this._stack[--this._index].undo();
    return true;
  }

  /**
   * Returns true if redo() will do anything.
   */
  canRedo() {
    return this._stack.length > this._index;
  }

  /**
   * Redo the most recently undone action.
   *
   * @return true if an action was redone.
   */
  redo() {
    if (!this.canRedo()) {
      return false;
    }
    this._stack[this._index++].do();
    return true;
  }

  /**
   * ViewController implementation for undo/redo.
   */

  /**
   * Install this object as a command controller.
   */
  installController(controllerWindow) {
    const controllers = controllerWindow.controllers;
    // Only available when running in a Firefox panel.
    if (!controllers || !controllers.appendController) {
      return;
    }

    this._controllerWindow = controllerWindow;
    controllers.appendController(this);
  }

  /**
   * Uninstall this object from the command controller.
   */
  uninstallController() {
    if (!this._controllerWindow) {
      return;
    }
    this._controllerWindow.controllers.removeController(this);
  }

  supportsCommand(command) {
    return command == "cmd_undo" || command == "cmd_redo";
  }

  isCommandEnabled(command) {
    switch (command) {
      case "cmd_undo":
        return this.canUndo();
      case "cmd_redo":
        return this.canRedo();
    }
    return false;
  }

  doCommand(command) {
    switch (command) {
      case "cmd_undo":
        return this.undo();
      case "cmd_redo":
        return this.redo();
      default:
        return null;
    }
  }

  onEvent() {}
}

exports.UndoStack = UndoStack;
