const imageLoader = {

    /**
     * Load a list of img URLs then resolve a Promise
     *
     * @param {String[]} urls images to load
     * @param {Number} [retries=0] number of times to retry on img.onerror
     * @return {Promise<HTMLImageElement[]|String>} return the loaded image elements in preserved order, or the first URL to fail on rejection
     */
    loadImages(urls, retries = 0) {
        return new Promise((resolve, reject) => {
            const images = [];
            Promise.all(urls.map(url => this.loadImage(url, retries)))
                .then(loaded => {
                    loaded.forEach(img => {
                        const position = urls.indexOf(img.url);
                        images[position] = img;
                    });

                    resolve(images);

                }).catch(reject);
        });
    },

    /**
     * Load an image URL then resolve a Promise
     *
     * @param {String[]} url image to load
     * @param {Number} [retries=0] number of times to retry on img.onerror
     * @return {Promise<HTMLImageElement|String>} return the loaded image element, or the URL that failed on rejection
     */
    loadImage(url, retries = 0) {
        return new Promise((resolve, reject) => {
            const img = new Image();
            img.onload = () => {
                img.url = url;
                resolve(img);
            };
            img.onerror = () => {
                retries--;
                if(retries >= 0) {
                    return this.loadImage(url, retries);
                } else {
                    reject(url);
                }
            };
            img.src = url;
        });
    }
};

module.exports = imageLoader;