/*
 * Copyright 2018 Palantir Technologies, Inc. All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/* eslint-disable no-underscore-dangle */

/**
 * The reserved type of the "Create Item" option in item lists. This is intended
 * not to conflict with any custom item type `T` that might be used in  item
 * list.
 */
export interface CreateNewItem {
    __blueprintCreateNewItemBrand: "blueprint-create-new-item";
}

/** Returns an instance of a "Create Item" object. */
export function getCreateNewItem(): CreateNewItem {
    return { __blueprintCreateNewItemBrand: "blueprint-create-new-item" };
}

/**
 * Type guard returning `true` if the provided item (e.g. the current
 * `activeItem`) is a "Create Item" option.
 */
export function isCreateNewItem<T>(item: T | CreateNewItem | null | undefined): item is CreateNewItem {
    if (item == null) {
        return false;
    }

    // see if the provided item exactly matches the `CreateNewItem` object,
    // with no superfluous keys.
    const keys = Object.keys(item);
    if (keys.length !== 1 || keys[0] !== "__blueprintCreateNewItemBrand") {
        return false;
    }
    return (item as CreateNewItem).__blueprintCreateNewItemBrand === "blueprint-create-new-item";
}

/**
 * Returns the type of the the current active item. This will be a no-op unless
 * the `activeItem` is `undefined` or a "Create Item" option, in which case
 * `null` will be returned instead.
 */
export function getActiveItem<T>(activeItem: T | CreateNewItem | null | undefined): T | null {
    return activeItem == null || isCreateNewItem(activeItem) ? null : activeItem;
}