"use strict";

const path = require("path");
const utils = require("loader-utils");

const matchModuleImport = /^~([^\/]+|@[^\/]+[\/][^\/]+)$/;

/**
 * When libsass tries to resolve an import, it uses a special algorithm.
 * Since the sass-loader uses webpack to resolve the modules, we need to simulate that algorithm. This function
 * returns an array of import paths to try. The last entry in the array is always the original url
 * to enable straight-forward webpack.config aliases.
 *
 * @param {string} url
 * @returns {Array<string>}
 */
function importsToResolve(url) {
    const request = utils.urlToRequest(url);
    // Keep in mind: ext can also be something like '.datepicker' when the true extension is omitted and the filename contains a dot.
    // @see https://github.com/webpack-contrib/sass-loader/issues/167
    const ext = path.extname(request);

    if (matchModuleImport.test(url)) {
        return [request, url];
    }

    // libsass' import algorithm works like this:

    // In case there is a file extension...
    //   - If the file is a CSS-file, do not include it all, but just link it via @import url().
    //   - The exact file name must match (no auto-resolving of '_'-modules).
    if (ext === ".css") {
        return [];
    }
    if (ext === ".scss" || ext === ".sass") {
        return [request, url];
    }

    // In case there is no file extension...
    //   - Prefer modules starting with '_'.
    //   - File extension precedence: .scss, .sass, .css.
    const basename = path.basename(request);

    if (basename.charAt(0) === "_") {
        return [
            `${ request }.scss`, `${ request }.sass`, `${ request }.css`,
            url
        ];
    }

    const dirname = path.dirname(request);

    return [
        `${ dirname }/_${ basename }.scss`, `${ dirname }/_${ basename }.sass`, `${ dirname }/_${ basename }.css`,
        `${ request }.scss`, `${ request }.sass`, `${ request }.css`,
        url
    ];
}

module.exports = importsToResolve;