// Loads PDF.js (vendored first, CDN fallback) and renders one page at a time,
// scaled to fit its container and sharp on high-DPI screens.

let readyPromise = null;

function loadScript(src) {
  return new Promise((resolve, reject) => {
    const s = document.createElement('script');
    s.src = src;
    s.onload = resolve;
    s.onerror = () => reject(new Error('Failed to load ' + src));
    document.head.appendChild(s);
  });
}

export function ensurePdfjs() {
  if (window.pdfjsLib) return Promise.resolve();
  if (readyPromise) return readyPromise;

  const CDN = 'https://cdnjs.cloudflare.com/ajax/libs/pdf.js/3.11.174';
  readyPromise = loadScript('/vendor/pdfjs/pdf.min.js')
    .then(() => {
      if (!window.pdfjsLib) throw new Error('vendor missing');
      window.__pdfWorker = '/vendor/pdfjs/pdf.worker.min.js';
    })
    .catch(() => loadScript(`${CDN}/pdf.min.js`).then(() => {
      window.__pdfWorker = `${CDN}/pdf.worker.min.js`;
    }))
    .then(() => {
      window.pdfjsLib.GlobalWorkerOptions.workerSrc = window.__pdfWorker;
    });
  return readyPromise;
}

export class PdfViewer {
  constructor(container) {
    this.container = container;
    this.canvas = document.createElement('canvas');
    this.canvas.className = 'pdf-canvas';
    this.ctx = this.canvas.getContext('2d');
    container.appendChild(this.canvas);
    this.pdf = null;
    this.numPages = 0;
    this.page = 1;
    this._busy = false;
    this._queued = null;
    this._onResize = () => this.refit();
    window.addEventListener('resize', this._onResize);
  }

  async load(url) {
    await ensurePdfjs();
    this.pdf = await window.pdfjsLib.getDocument(url).promise;
    this.numPages = this.pdf.numPages;
    return this.numPages;
  }

  async render(n) {
    if (!this.pdf) return;
    n = Math.max(1, Math.min(this.numPages, Math.trunc(n) || 1));
    this.page = n;
    if (this._busy) { this._queued = n; return; }
    this._busy = true;
    try {
      const page = await this.pdf.getPage(n);
      const base = page.getViewport({ scale: 1 });
      const cw = this.container.clientWidth || 800;
      const ch = this.container.clientHeight || 600;
      const dpr = Math.min(window.devicePixelRatio || 1, 2);
      const fit = Math.min(cw / base.width, ch / base.height);
      const vp = page.getViewport({ scale: fit * dpr });
      this.canvas.width = Math.floor(vp.width);
      this.canvas.height = Math.floor(vp.height);
      this.canvas.style.width = Math.floor(vp.width / dpr) + 'px';
      this.canvas.style.height = Math.floor(vp.height / dpr) + 'px';
      await page.render({ canvasContext: this.ctx, viewport: vp }).promise;
    } finally {
      this._busy = false;
      if (this._queued !== null) {
        const q = this._queued;
        this._queued = null;
        if (q !== n) this.render(q);
      }
    }
  }

  refit() { if (this.pdf) this.render(this.page); }

  destroy() { window.removeEventListener('resize', this._onResize); }
}