import { getDocument } from "pdfjs-dist/legacy/build/pdf.mjs";

/** `@napi-rs/canvas` is an optional dependency; null when its native binary is not installed. */
export async function loadCanvas(): Promise<typeof import("@napi-rs/canvas") | null> {
  try {
    return await import("@napi-rs/canvas");
  } catch {
    return null;
  }
}

/** Number of non-white pixels in the box (x0,y0,x1,y1 in pt from the top-left) of page 1 drawn at `scale`. */
export async function inkPixels(
  canvasLib: typeof import("@napi-rs/canvas"),
  bytes: Uint8Array,
  box: { x0: number; y0: number; x1: number; y1: number },
  scale = 2,
): Promise<number> {
  const task = getDocument({ data: new Uint8Array(bytes), useSystemFonts: false, disableFontFace: true });
  const doc = await task.promise;
  try {
    const page = await doc.getPage(1);
    const viewport = page.getViewport({ scale });
    const canvas = canvasLib.createCanvas(Math.ceil(viewport.width), Math.ceil(viewport.height));
    const ctx = canvas.getContext("2d");
    ctx.fillStyle = "#ffffff";
    ctx.fillRect(0, 0, canvas.width, canvas.height);
    await page.render({ canvasContext: ctx as never, canvas: canvas as never, viewport }).promise;
    const x = Math.floor(box.x0 * scale);
    const y = Math.floor(box.y0 * scale);
    const w = Math.ceil((box.x1 - box.x0) * scale);
    const h = Math.ceil((box.y1 - box.y0) * scale);
    const data = ctx.getImageData(x, y, w, h).data;
    let ink = 0;
    for (let i = 0; i < data.length; i += 4) if (data[i] < 250 || data[i + 1] < 250 || data[i + 2] < 250) ink++;
    return ink;
  } finally {
    await task.destroy();
  }
}

/** PNG of one page (1-based) at `scale`, white background; for visual review only. */
export async function pagePng(canvasLib: typeof import("@napi-rs/canvas"), bytes: Uint8Array, pageNo: number, scale = 1.5): Promise<Buffer> {
  const task = getDocument({ data: new Uint8Array(bytes), useSystemFonts: false, disableFontFace: true });
  const doc = await task.promise;
  try {
    const page = await doc.getPage(pageNo);
    const viewport = page.getViewport({ scale });
    const canvas = canvasLib.createCanvas(Math.ceil(viewport.width), Math.ceil(viewport.height));
    const ctx = canvas.getContext("2d");
    ctx.fillStyle = "#ffffff";
    ctx.fillRect(0, 0, canvas.width, canvas.height);
    await page.render({ canvasContext: ctx as never, canvas: canvas as never, viewport }).promise;
    return canvas.toBuffer("image/png");
  } finally {
    await task.destroy();
  }
}