import type { CanvasDocument, StickerAsset } from '../types';
import { serializeDocument } from './svgExport';

const PNG_TYPE = 'image/png';

/**
 * Render the full visible canvas document to a PNG Blob. WebKit returns the
 * Blob directly from toBlob; the standard-track callback form and the
 * data-URL fallback keep other browsers working.
 */
export async function createDesignPreviewBlob(
  document: CanvasDocument,
  stickers: Map<string, StickerAsset>,
): Promise<Blob> {
  const width = Math.max(1, Math.floor(document.width || 0));
  const height = Math.max(1, Math.floor(document.height || 0));
  const svgMarkup = await serializeDocument(document, stickers, { includeBackground: true });
  const svgBlob = new Blob([svgMarkup], { type: 'image/svg+xml;charset=utf-8' });
  const objectUrl = URL.createObjectURL(svgBlob);
  try {
    const image: HTMLImageElement =
      typeof Image === 'function' ? new Image() : globalThis.document.createElement('img');
    image.src = objectUrl;
    await image.decode();

    const canvas = globalThis.document.createElement('canvas');
    canvas.width = width;
    canvas.height = height;
    const ctx = canvas.getContext('2d');
    if (!ctx) throw new Error('设计预览画布上下文不可用');
    ctx.drawImage(image, 0, 0, canvas.width, canvas.height);

    const pngBlob = await canvasToPngBlob(canvas);
    if (pngBlob.type !== PNG_TYPE || pngBlob.size === 0) {
      throw new Error('设计预览未生成有效的 PNG 输出');
    }
    return pngBlob;
  } finally {
    URL.revokeObjectURL(objectUrl);
  }
}

async function canvasToPngBlob(canvas: HTMLCanvasElement): Promise<Blob> {
  if (typeof canvas.toBlob === 'function') {
    try {
      const direct = (canvas.toBlob as unknown as (type: string) => Blob | undefined)('image/png');
      if (direct) return direct;
    } catch {
      // Non-WebKit browsers wait for the callback form below.
    }
    return new Promise<Blob>((resolve, reject) => {
      canvas.toBlob(blob => {
        if (blob) resolve(blob);
        else reject(new Error('设计预览画布未生成 PNG Blob'));
      }, 'image/png');
    });
  }
  if (typeof canvas.toDataURL === 'function') {
    const res = await fetch(canvas.toDataURL('image/png'));
    if (!res.ok) throw new Error('设计预览 PNG 输出读取失败');
    return res.blob();
  }
  throw new Error('当前浏览器不支持设计预览 PNG 输出');
}