/**
 * Converts the given string to an HTML element.
 *
 * @param string the string to convert to an HTML element
 * @param query the type of element to return
 * @returns {HTMLElement} the HTML element described by the given string
 */
const stringToHtml = function(string, query) {
    return (new DOMParser()).parseFromString(string, "text/html").body.querySelector(query);
}

/**
 * Alias for `document.querySelector`.
 *
 * @param q {string} the query string
 * @returns {HTMLElement} the element identified by the query string
 */
const $ = q => document.querySelector(q);

/**
 * Runs the given function once the page is loaded.
 *
 * This function can be used multiple times. It does not overwrite existing callbacks for the page load event. If the
 * page has already loaded when this function is invoked, `fun` is invoked immediately inside this function.
 *
 * @param fun {function(...*): *} the function to run
 */
const doAfterLoad = function(fun) {
    if (document.readyState === "complete") {
        fun();
        return;
    }

    const oldOnLoad = onload || (() => {
    });

    onload = (() => {
        oldOnLoad();
        fun();
    });
};


/**
 * Creates a navigation element for navigating through the website.
 *
 * Fetches entries asynchronously from the website's API.
 *
 * @param [highlightPath] {String} the path to highlight together with its parents
 * @param [cb] {Function} the callback to execute on the fetched entries, to prevent the need to re-fetch elsewhere
 * @returns {HTMLElement} a base navigation element that will eventually be filled with contents
 */
const nav = function(highlightPath = "", cb = undefined) {
    const base = stringToHtml(
        `<ul><li><a href="https://fwdekker.com/">` +
        `<div class="logo"><img class="logo" src="https://fwdekker.com/favicon.png" alt="FWDekker" /></div>` +
        `<b>FWDekker</b>` +
        `</a></li></ul>`,
        "ul"
    );

    fetch("https://fwdekker.com/api/nav/")
        .then(it => it.json())
        .then(json => {
            if (cb !== undefined) cb(json);

            json.entries.forEach(entry => base.appendChild(stringToHtml(unpackEntry(entry, "/", highlightPath), "li")));
        })
        .catch(e => {
            console.error("Failed to fetch navigation elements", e);
            return [];
        });

    const nav = stringToHtml(
        `<nav>` +
        `<input id="nav-hamburger-checkbox" type="checkbox" hidden />` +
        `<label id="nav-hamburger-label" for="nav-hamburger-checkbox">&#9776;</label>` +
        `</nav>`,
        "nav"
    );
    nav.appendChild(base);
    return nav;
};

/**
 * Unpacks a navigation entry returned from the navigation API into an HTML element.
 *
 * @param entry {Object} the entry to unpack
 * @param [path] {number} the current path traversed, found by joining the names of the entries with `/`s; always starts
 * and ends with a `/`
 * @param [highlightPath] {String} the path to highlight together with its parents
 * @returns {string} the navigation list entry as HTML, described by its children
 */
const unpackEntry = function(entry, path = "/", highlightPath = "") {
    const shouldHighlight = highlightPath.startsWith(`${path + entry.name}/`);
    const isExternalLink = !(/^https:\/\/.*fwdekker.com/.test(entry.link)) && entry.link !== "#";
    const formattedName = (isExternalLink ? "&#9099; " : "") + entry.name;

    if (entry.entries.length === 0)
        return "" +
            `<li ${shouldHighlight ? "class=\"currentPage\"" : ""}><a href="${entry.link}">${formattedName}</a></li>`;

    const depth = path.split("/").length - 2;  // -1 because count parts, then another -1 because of leading `/`
    const arrow = depth === 0 ? "&#9662;" : "&#9656;";

    return "" +
        `<li class="${shouldHighlight ? "currentPage" : ""}">` +
        `<a href="${entry.link}">${formattedName} ${arrow}</a>` +
        `<ul>${entry.entries.map(it => unpackEntry(it, `${path + entry.name}/`, highlightPath)).join("")}</ul>` +
        `</li>`;
};


/**
 * Creates a header element with the given title and description.
 *
 * @param [title] {string} the title to display, possibly including HTML
 * @param [description] {string} the description to display, possibly including HTML
 * @returns {HTMLElement} a header element
 */
const header = function({title, description}) {
    if (title === undefined && description === undefined)
        return stringToHtml(`<header></header>`, "header");

    return stringToHtml(
        `<header><section class="container">` +
        (title !== undefined ? `<h1>${title}</h1>` : "") +
        (description !== undefined ? `<p><em>${description}</em></p>` : "") +
        `</section></header>`,
        "header"
    );
};


/**
 * Creates a footer element with the given data.
 *
 * Setting an argument to `undefined` or not giving that argument will cause the default value to be used. Setting an
 * argument to `null` will result in a footer without the corresponding element.
 *
 * @param [author] {string|null|undefined} the author
 * @param [authorURL] {string|null|undefined} the URL to link the author's name to
 * @param [license] {string|null|undefined} the type of license
 * @param [licenseURL] {string|null|undefined} the URL to the license file
 * @param [vcs] {string|null|undefined} the type of version control
 * @param [vcsURL] {string|null|undefined} the URL to the repository
 * @param [version] {string|null|undefined} the page version
 * @param [privacyPolicyURL] {string|null|undefined} the URL to the privacy policy
 * @returns {HTMLElement} a footer element
 */
const footer = function(
    {
        author = undefined,
        authorURL = undefined,
        license = undefined,
        licenseURL = undefined,
        vcs = undefined,
        vcsURL = undefined,
        version = undefined,
        privacyPolicyURL = undefined
    }) {
    if (author === undefined) author = "F.W.&nbsp;Dekker";
    if (authorURL === undefined) authorURL = "https://fwdekker.com/";
    if (license === undefined) license = "MIT&nbsp;License";
    if (licenseURL === undefined && vcsURL !== undefined) licenseURL = `${vcsURL}src/branch/master/LICENSE`;
    if (vcs === undefined && vcsURL !== undefined) vcs = "git";
    if (privacyPolicyURL === undefined) privacyPolicyURL = "https://fwdekker.com/privacy/";

    return stringToHtml(
        `<footer><section class="container">` +
        footerLink("Made by ", author, authorURL, ". ") +
        footerLink("Licensed under the ", license, licenseURL, ". ") +
        footerLink("Source code available on ", vcs, vcsURL, ". ") +
        footerLink("Consider reading the ", privacyPolicyURL && "privacy policy", privacyPolicyURL, ". ") +
        `<div id="footerVersion">${version || ""}</div>` +
        `</section></footer>`,
        "footer");
};

/**
 * Constructs a link that is used in footers.
 *
 * @param prefix {string} the text to display before the text if the text is not undefined
 * @param text {string|null} the text to display, or `null` if the returned element should be empty
 * @param url {string|null} the URL to link the text to, or `null` if the text should not be a link
 * @param suffix {string} the text to display after the text if the text is not undefined
 * @returns {string} a footer link element
 */
const footerLink = function(prefix, text, url, suffix) {
    if (text === null) return "";

    return `${prefix}${url !== null ? `<a href="${url}">${text}</a>` : text}${suffix}`;
};


/**
 * Runs the functions `nav`, `header`, and `footer` after the page has loaded using properties defined in the meta tags.
 *
 * The remaining functions are invoked only if the corresponding `target` meta property is set to select an existing
 * element. The HTML element returned by the function is then added as a child to the element specified by the query
 * selector in the `target` property. The parameters to the function invocation can be set using other meta properties.
 * The format of meta properties is `fwd:<function>:<property>`, where `property` is the same as the name of the
 * parameter of that function, except that instead of camelcase words are separated by dashes. For example, `vcsURL`
 * becomes `vcs-url`.
 *
 * Meta properties can be set by including `<meta name="fwd:<function>:<property>" content="<value>" />` in the HTML
 * page on which this module is included. The `<value>` is then passed without modification as a parameter to the
 * function. Leaving out the `<value>` by writing `<meta name="fwd:<function>:property" />` instead results in passing
 * `null` as the value. Not including the meta tag at all corresponds to passing `undefined` to the function. See the
 * documentation of the respective functions for more details on the parameters that they accept.
 *
 * Note that the function is invoked only if `fwd:<function>:target` is a query selector for an existing element. This
 * means that it is possible to mix how the functions are invoked; for example, one can use meta properties to pass
 * parameters to `nav`, but also invoke `header` in a separate function manually.
 */
doAfterLoad(() => {
    const getMetaProperty = (name) => {
        const element = $(`meta[name="${name}"]`);
        return element === null ? undefined : element.getAttribute("content");
    };

    const navTarget = $(getMetaProperty("fwd:nav:target"));
    if (navTarget !== null) {
        navTarget.appendChild(nav(getMetaProperty("fwd:nav:highlight-path")));
    }

    const headerTarget = $(getMetaProperty("fwd:header:target"));
    if (headerTarget !== null) {
        headerTarget.appendChild(header({
            title: getMetaProperty("fwd:header:title"),
            description: getMetaProperty("fwd:header:description"),
        }));
    }

    const footerTarget = $(getMetaProperty("fwd:footer:target"));
    if (footerTarget !== null) {
        footerTarget.appendChild(footer({
            author: getMetaProperty("fwd:footer:author"),
            authorURL: getMetaProperty("fwd:footer:author-url"),
            license: getMetaProperty("fwd:footer:license"),
            licenseURL: getMetaProperty("fwd:footer:license-url"),
            vcs: getMetaProperty("fwd:footer:vcs"),
            vcsURL: getMetaProperty("fwd:footer:vcs-url"),
            version: getMetaProperty("fwd:footer:version"),
            privacyPolicyURL: getMetaProperty("fwd:footer:privacy-policy-url"),
        }));
    }
});


// Export to namespace
fwdekker = {stringToHtml, $, doAfterLoad, nav, header, footer};