/**
 * Loading pdf.js, once and only when a PDF is actually opened.
 *
 * The library is larger than the rest of the application put together, so it is
 * behind a dynamic import: a session that never opens a PDF never downloads it.
 * Vite gives it its own chunk automatically.
 *
 * The worker is bundled with the app rather than fetched from a CDN. pdf.js
 * documents the CDN route, but this instance is meant to run without reaching
 * out to anyone — and a viewer that stops working when a third party has an
 * outage is not one you can rely on offline.
 */

import type { PDFDocumentProxy, PDFPageProxy } from "pdfjs-dist";

export type { PDFDocumentProxy, PDFPageProxy };

/* The legacy build, not the default one.
 *
 * pdf.js 6 calls Uint8Array.prototype.toHex, which only very recent browsers
 * have; on anything older the document fails to parse with a message that says
 * nothing about the cause. The legacy build compiles those away. It costs a
 * little size, and it is the difference between working everywhere and working
 * on this year's browsers. */
type PdfjsModule = typeof import("pdfjs-dist/legacy/build/pdf.mjs");

let loading: Promise<PdfjsModule> | null = null;

/** Resolves to the pdf.js module, fetching it on the first call. */
export function loadPdfjs(): Promise<PdfjsModule> {
  if (!loading) {
    loading = (async () => {
      const pdfjs = await import("pdfjs-dist/legacy/build/pdf.mjs");
      // `?url` keeps the worker a separate file instead of inlining it, which is
      // what a worker needs to be.
      const workerUrl = (await import("pdfjs-dist/legacy/build/pdf.worker.min.mjs?url")).default;
      pdfjs.GlobalWorkerOptions.workerSrc = workerUrl;
      return pdfjs;
    })().catch((err) => {
      // A failed load must not be remembered as a resolved promise, or the
      // viewer would stay broken for the rest of the session.
      loading = null;
      throw err;
    });
  }
  return loading;
}

export interface PdfPageSize {
  /** Page width in PDF points at scale 1 — the space strokes are stored in. */
  width: number;
  height: number;
}

export interface LoadedPdf {
  doc: PDFDocumentProxy;
  pageCount: number;
  /** Size of every page, so the layout can be built before anything is rendered. */
  sizes: PdfPageSize[];
}

/**
 * Opens a PDF from bytes.
 *
 * Takes the bytes rather than a URL because they come from the encrypted
 * attachment route or from the offline cache — pdf.js fetching the URL itself
 * would send no Authorization header and get a 401.
 */
export async function openPdf(bytes: Uint8Array): Promise<LoadedPdf> {
  const pdfjs = await loadPdfjs();
  // pdf.js takes ownership of the buffer it is given and detaches it; a copy
  // keeps the caller's cached bytes usable afterwards.
  const doc = await pdfjs.getDocument({ data: new Uint8Array(bytes) }).promise;

  const sizes: PdfPageSize[] = [];
  for (let n = 1; n <= doc.numPages; n++) {
    const page = await doc.getPage(n);
    const viewport = page.getViewport({ scale: 1 });
    sizes.push({ width: viewport.width, height: viewport.height });
  }
  return { doc, pageCount: doc.numPages, sizes };
}

/**
 * Lays the selectable text over a rendered page.
 *
 * pdf.js draws pages as pictures, so selecting means positioning transparent
 * spans exactly where the glyphs were painted. The scale must match the canvas
 * or the two drift apart and a selection lands on the wrong words.
 *
 * Returns a cancel function — a page can be re-rendered at a new zoom before the
 * previous text layer has finished building.
 */
export async function renderTextLayer(
  page: PDFPageProxy,
  container: HTMLElement,
  cssWidth: number
): Promise<void> {
  const pdfjs = await loadPdfjs();
  const base = page.getViewport({ scale: 1 });
  const scale = cssWidth / base.width;
  const viewport = page.getViewport({ scale });

  container.replaceChildren();
  // Every span's transform is expressed against this, so the layer only lines up
  // with the canvas when it carries the same scale.
  container.style.setProperty("--scale-factor", String(scale));

  const layer = new pdfjs.TextLayer({
    textContentSource: await page.getTextContent(),
    container,
    viewport,
  });
  await layer.render();
}

/**
 * The words on a page, in reading order.
 *
 * pdf.js hands back positioned runs, not sentences: a line can arrive in several
 * pieces and `hasEOL` is the only marker of where one ends. Joining runs without
 * it glues the last word of a line to the first of the next, which then matches
 * nothing when searched for.
 */
export async function extractPageText(page: PDFPageProxy): Promise<string> {
  const content = await page.getTextContent();
  let out = "";
  for (const item of content.items) {
    const run = item as { str?: string; hasEOL?: boolean };
    if (typeof run.str !== "string") continue;
    out += run.str;
    if (run.hasEOL) out += "\n";
    else if (run.str && !run.str.endsWith(" ")) out += " ";
  }
  return out.replace(/[ \t]+/g, " ").trim();
}

/**
 * Renders one page onto a canvas at the given CSS width.
 *
 * The canvas is sized in device pixels and scaled down by CSS, so the page is
 * sharp on a high-density screen instead of being upscaled from a low-res
 * bitmap. Returns a cancel function: scrolling quickly can start more renders
 * than finish, and pdf.js complains loudly if two run on one canvas.
 */
export function renderPage(
  page: PDFPageProxy,
  canvas: HTMLCanvasElement,
  cssWidth: number
): { done: Promise<void>; cancel: () => void } {
  const base = page.getViewport({ scale: 1 });
  const dpr = Math.min(window.devicePixelRatio || 1, 2); // beyond 2 costs memory for no visible gain
  const viewport = page.getViewport({ scale: (cssWidth / base.width) * dpr });

  canvas.width = Math.round(viewport.width);
  canvas.height = Math.round(viewport.height);
  canvas.style.width = `${cssWidth}px`;
  canvas.style.height = `${Math.round(viewport.height / dpr)}px`;

  const ctx = canvas.getContext("2d");
  if (!ctx) return { done: Promise.resolve(), cancel: () => {} };

  const task = page.render({ canvasContext: ctx, viewport, canvas });
  return {
    done: task.promise.catch((err: unknown) => {
      // A cancelled render is the normal outcome of scrolling, not a fault.
      if ((err as { name?: string })?.name !== "RenderingCancelledException") throw err;
    }),
    cancel: () => task.cancel(),
  };
}