// The documentation view (US09-01): the manuals in `docs/`, rendered in the app.
//
// The same markdown files are the repository's documentation and the deployment's
// documentation. An operator who was handed a URL and an access secret has no
// checkout in front of them, and the network the application runs on is not assumed
// to reach a CDN — so the renderer is vendored and everything here is same-origin.
//
// Nothing on this page is authenticated. It is served from the static mount beside
// the application shell, exactly like `index.html`: the troubleshooting page is
// needed most by whoever cannot get past the access secret, and no documentation
// file contains anything a session would protect.
//
// `marked` and `mermaid` are loaded lazily, on the first documentation page and the
// first diagram. They are large, and every other view does without them.
import { el, errorBanner, setActiveNav } from "./dom.js";

const DOCS_BASE = "/docs";
const VENDOR = "/app/js/vendor";
const INDEX_PAGE = "index";
// A page name comes from the hash, so it is caller input: no scheme, no traversal,
// no absolute path. The server would refuse those too; this refuses them earlier and
// without a request that looks like an attempt.
const PAGE_PATTERN = /^[\w-]+(\/[\w-]+)*$/;

let markedPromise;
let mermaidPromise;

function loadMarked() {
  markedPromise ??= import(`${VENDOR}/marked.esm.js`).then((module) => module.marked);
  return markedPromise;
}

// mermaid ships one large UMD bundle rather than a self-contained ES module, so it
// arrives through a script element. `script-src 'self'` allows it because it is ours
// and same-origin; nothing here relaxes that.
function loadMermaid() {
  mermaidPromise ??= new Promise((resolve, reject) => {
    const script = document.createElement("script");
    script.src = `${VENDOR}/mermaid.min.js`;
    script.onload = () => resolve(window.mermaid);
    script.onerror = () => reject(new Error("the diagram renderer could not be loaded"));
    document.head.appendChild(script);
  });
  return mermaidPromise;
}

// ── the pure parts, unit-tested in js/tests/unit.js ──────────────────────────

/** A stable, readable heading anchor. Letters and digits of any script survive. */
export function slug(text) {
  const cleaned = String(text)
    .toLowerCase()
    .trim()
    .replace(/[^\p{L}\p{N}\s-]/gu, "")
    .replace(/[\s-]+/g, "-")
    .replace(/^-|-$/g, "");
  return cleaned || "section";
}

/** Assign every heading an id, disambiguating repeats the way a reader would
 *  expect: the first `Notes` keeps `#notes`, the second becomes `#notes-2`. */
export function assignHeadingIds(headings) {
  const used = new Map();
  for (const heading of headings) {
    const base = slug(heading.textContent);
    const seen = (used.get(base) || 0) + 1;
    used.set(base, seen);
    heading.id = seen === 1 ? base : `${base}-${seen}`;
  }
  return headings;
}

/**
 * Where a link inside a documentation page should go.
 *
 * Markdown links between documents are relative file paths, which a browser would
 * treat as downloads that leave the application. Returns the in-app target for a
 * link to another document or to a heading, and `null` for anything else — external
 * links stay exactly as the author wrote them.
 */
export function resolveDocLink(currentPage, href) {
  if (!href) return null;
  if (/^[a-z][a-z\d+.-]*:/i.test(href) || href.startsWith("//")) return null;
  if (href.startsWith("#")) return { page: currentPage, anchor: href.slice(1) };
  const [target, anchor = ""] = href.split("#");
  if (!/\.md$/i.test(target)) return null;
  // Resolved against the current document, so `../guides/x.md` means what it means
  // in the repository. The origin is a placeholder; only the path is used.
  const resolved = new URL(target, `https://docs.invalid/${currentPage}.md`);
  const page = decodeURIComponent(resolved.pathname).replace(/^\//, "").replace(/\.md$/i, "");
  return PAGE_PATTERN.test(page) ? { page, anchor } : null;
}

/**
 * Point every in-app link at its route, and leave every other link alone.
 *
 * The resolved page is kept on the element, so the reading order can be read back
 * from the rendered index rather than parsed out of the markdown a second time.
 */
export function rewriteLinks(article, page) {
  for (const link of article.querySelectorAll("a[href]")) {
    const href = link.getAttribute("href");
    const target = resolveDocLink(page, href);
    if (target) {
      link.setAttribute("href", docHash(target.page, target.anchor));
      link.dataset.docPage = target.page;
    } else if (/^https?:/i.test(href)) {
      link.setAttribute("rel", "noreferrer noopener");
      link.setAttribute("target", "_blank");
    }
  }
  return article;
}

/** The reading order, taken from the index document itself — one list, in one
 *  place, that is equally the index on Gitea and the navigation here. */
export function documentIndex(indexElement) {
  const pages = [];
  for (const link of indexElement.querySelectorAll("a[data-doc-page]")) {
    const page = link.dataset.docPage;
    if (page !== INDEX_PAGE && !pages.some((entry) => entry.page === page)) {
      pages.push({ page, title: link.textContent.trim() });
    }
  }
  return pages;
}

// ── the view ─────────────────────────────────────────────────────────────────

async function fetchPage(page) {
  const response = await fetch(`${DOCS_BASE}/${page}.md`, { headers: { accept: "text/markdown" } });
  if (!response.ok) throw new Error(`documentation page unavailable: ${response.status}`);
  return response.text();
}

async function toArticle(markdown, page) {
  const marked = await loadMarked();
  const article = el("article", { class: "doc", "data-testid": "doc", "data-page": page });
  // The only innerHTML in the application, and deliberate: rendering markdown *is*
  // producing HTML. The input is a file from this repository, and the page's CSP
  // (`script-src 'self'`, no `'unsafe-inline'`) means injected script and inline
  // handlers do not run even if one ever were not.
  article.innerHTML = marked.parse(markdown, { async: false });
  assignHeadingIds(article.querySelectorAll("h1, h2, h3, h4, h5, h6"));
  rewriteLinks(article, page);
  return article;
}

function docHash(page, anchor = "") {
  const query = new URLSearchParams(anchor ? { page, anchor } : { page });
  return `#/docs?${query}`;
}

/** Diagrams are authored as ```mermaid blocks so the source is diffable and Gitea
 *  renders them natively. A failure here replaces the diagram, never the page. */
async function renderDiagrams(article) {
  const blocks = [...article.querySelectorAll("pre > code.language-mermaid")];
  if (!blocks.length) return;
  let mermaid;
  try {
    mermaid = await loadMermaid();
    mermaid.initialize({ startOnLoad: false, securityLevel: "strict", theme: "dark" });
  } catch (error) {
    blocks.forEach((block) => block.closest("pre").replaceWith(errorBanner(error.message)));
    return;
  }
  for (const [index, block] of blocks.entries()) {
    const figure = el("figure", { class: "diagram", "data-testid": "diagram" });
    block.closest("pre").replaceWith(figure);
    try {
      const { svg } = await mermaid.render(`diagram-${index}-${Date.now()}`, block.textContent);
      figure.innerHTML = svg;
    } catch (error) {
      figure.replaceWith(errorBanner(`Diagram could not be drawn: ${error.message}`));
    }
  }
}

function notFound() {
  return el(
    "div",
    { class: "alert", role: "alert", "data-testid": "doc-not-found" },
    "That documentation page does not exist. ",
    el("a", { class: "link", href: docHash(INDEX_PAGE) }, "Back to the documentation index")
  );
}

function sidebar(pages, current) {
  return el(
    "nav",
    { class: "card", "aria-label": "Documentation" },
    el("h2", {}, "Documentation"),
    el(
      "ul",
      { class: "album-list", "data-testid": "doc-pages" },
      el(
        "li",
        {},
        el(
          "a",
          {
            href: docHash(INDEX_PAGE),
            "aria-current": current === INDEX_PAGE ? "true" : false,
          },
          "Index"
        )
      ),
      ...pages.map(({ page, title }) =>
        el(
          "li",
          {},
          el(
            "a",
            {
              href: docHash(page),
              "data-page": page,
              "aria-current": page === current ? "true" : false,
            },
            title
          )
        )
      )
    )
  );
}

export async function renderDocs(root, params = {}) {
  setActiveNav("docs");
  const requested = params.page || INDEX_PAGE;
  const page = PAGE_PATTERN.test(requested) ? requested : "";

  let indexArticle;
  try {
    indexArticle = await toArticle(await fetchPage(INDEX_PAGE), INDEX_PAGE);
  } catch (error) {
    root.replaceChildren(errorBanner(`Documentation is unavailable: ${error.message}`));
    return;
  }
  const pages = documentIndex(indexArticle);

  let article;
  if (!page) article = notFound();
  else if (page === INDEX_PAGE) article = indexArticle;
  else {
    try {
      article = await toArticle(await fetchPage(page), page);
    } catch {
      article = notFound();
    }
  }

  root.replaceChildren(el("div", { class: "two-pane" }, sidebar(pages, page), article));
  await renderDiagrams(article);
  scrollToAnchor(params.anchor);
}

// A documentation anchor cannot live in the hash — the hash is the route — so it
// travels as a parameter and is applied after the page renders.
function scrollToAnchor(anchor) {
  if (!anchor) return;
  const target = document.getElementById(anchor);
  if (target) target.scrollIntoView({ block: "start" });
}