/**
 * The command supertype, which is inherited by subtype commands.
 * @constructor
 * @param {type} normalAction The normal action.
 * @param {type} undoAction The opposite of the normal action.
 * @param {type} actionValue The value, which will get passed to the action.
 * @returns {Command}
 */
function Command(normalAction, undoAction, actionValue) {
    this.execute = normalAction;
    this.undo = undoAction;
    this.value = actionValue;
}

/**
 * Class for creating brave mountain climbers.
 * @constructor
 * @param {number} start The starting height.
 * @returns {Mountaineer}
 */
function Mountaineer(start) {
    // The achieved height.
    this._currentProgress = start || 0;
    // Contains instances of inherited Commands.
    this._commandsList = [];
    // We do not push and pop commands,
    // instead we keep them all and remember the current.
    // It start with -1, because this._commands is zero based.
    this._currentCommand = -1;
}

// Add the methods for mountaineers
Mountaineer.prototype = {
    constructor: Mountaineer,
    /**
     * Execute a new command.
     * @param {type} command Instance of a command.
     * @returns {undefined}
     */
    execute: function (command) {
        this._currentProgress = command.execute(this._currentProgress);
        this._currentCommand++;
        this._commandsList[this._currentCommand] = command;
        if (this._commandsList[this._currentCommand + 1]) {
            this._commandsList.splice(this._currentCommand + 1);
        }
    },
    /**
     * Undo the current command.
     * @returns {undefined}
     */
    undo: function () {
        var cmd = this._commandsList[this._currentCommand];
        if (!cmd) {
            console.error('Nothing to undo');
            return;
        }
        this._currentProgress = cmd.undo(this._currentProgress);
        this._currentCommand--;
    },
    /**
     * Redo the undone command.
     * @returns {undefined}
     */
    redo: function () {
        var cmd = this._commandsList[this._currentCommand + 1];
        if (!cmd) {
            console.error('Nothing to redo');
            return;
        }
        this._currentProgress = cmd.execute(this._currentProgress);
        this._currentCommand++;
    },
    /**
     * Simple getter of the current progress i.e. the achieved height.
     * @returns {Number}
     */
    getCurrentProgress: function () {
        return this._currentProgress;
    }
};

/**
 * Climb up action. One of the actions which will be used in the commands.
 * @param {type} actionValue Amount of climbed progress.
 * @returns {climb.value}
 */
function climbUp(actionValue) {
    // `this` is the instance of a command.
    return actionValue + this.value;
}

/**
 * Fall down action. One of the actions which will be used in the commands.
 * @param {type} actionValue Amount of falling regression.
 * @returns {fall.value}
 */
function fallDown(actionValue) {
    // `this` is the instance of a command.
    return actionValue - this.value;
}

/**
 * Command subtype for climbing up.
 * @constructor
 * @extends Command
 * @param {type} value The value, which will be passed to the action.
 * @returns {undefined}
 */
function CommandClimbUp(value) {
    // Constructor stealing for inheritance.
    // The order of climb and fall defines which is the normal vs undo action.
    Command.call(this, climbUp, fallDown, value);
}
// Prototype chaining for inheritance.
CommandClimbUp.prototype = Object.create(Command.prototype);

/**
 * Command subtype for falling down.
 * @constructor
 * @extends Command
 * @param {type} value The value, which will be passed to the action.
 * @returns {undefined}
 */
function CommandFallDown(value) {
    // Constructor stealing for inheritance.
    // The order of climb and fall defines which is the normal vs undo action.
    Command.call(this, fallDown, climbUp, value);
}
// Prototype chaining of inheritance.
CommandFallDown.prototype = Object.create(Command.prototype);

// Lots of other commands e.g. jumping, eating, dancing, planking can be added here

var reinhold = new Mountaineer(); // start at 0m
reinhold.execute(new ClimbUpCommand(1)); // climb 1m. (progress = 1m)
reinhold.undo(); // fall down 1m. (progress = 0m)
reinhold.execute(new ClimbUpCommand(2)); // climb 2m. (progress = 2m)
reinhold.undo(); // fall down 2m. (progress = 0m)
reinhold.execute(new FallDownCommand(3)); // fall 3m. (progress = -3m)
reinhold.undo(); // climb up 3m. (progress = 0m)
reinhold.execute(new ClimbUpCommand(4)); // climb 4m. (progress = 4m)
reinhold.execute(new FallDownCommand(5)); // fall 5m. (progress = -1m)
reinhold.execute(new ClimbUpCommand(6)); // climb 6m. (progress = 0m)
reinhold.undo(); // fall 6m. (progress = -1m)
reinhold.undo(); // climb 5m. (progress = 4m)
reinhold.undo(); // fall 4m. (progress = 0m)
reinhold.undo(); // Do nothing, because there are no actions left to undo
reinhold.redo(); // climb 4m. (progress = 4m)
reinhold.redo(); // fall 5m. (progress = -1m)
reinhold.redo(); // climb 6m. (progress = 5m)
reinhold.redo(); // Do nothing, because there are no actions left to redo
reinhold.undo(); // fall 6m. (progress = -1m)
reinhold.redo(); // climb 6m. (progress = 5m)
reinhold.execute(new ClimbUpCommand(7)); // climb 7m. (progress = 12m)
reinhold.getCurrentProgress(); // 12m