/**
 * Downscale an image File to a max dimension and re-encode as JPEG.
 * Phone photos routinely exceed the backend's upload limit; this keeps
 * uploads well under it while preserving enough detail for captioning.
 */
export async function downscaleImage(
  file: File,
  maxDim = 1536,
  quality = 0.85,
): Promise<File> {
  const bitmap = await loadImageBitmap(file);
  const { width, height } = scaleDimensions(bitmap.width, bitmap.height, maxDim);

  const canvas = document.createElement('canvas');
  canvas.width = width;
  canvas.height = height;
  const ctx = canvas.getContext('2d');
  if (!ctx) throw new Error('Canvas 2D context unavailable');
  ctx.drawImage(bitmap, 0, 0, width, height);

  const blob = await new Promise<Blob>((resolve, reject) => {
    canvas.toBlob(
      (b) => (b ? resolve(b) : reject(new Error('canvas.toBlob returned null'))),
      'image/jpeg',
      quality,
    );
  });

  const name = file.name.replace(/\.[^.]+$/, '') + '.jpg';
  return new File([blob], name, { type: 'image/jpeg' });
}

function scaleDimensions(w: number, h: number, maxDim: number): { width: number; height: number } {
  if (w <= maxDim && h <= maxDim) return { width: w, height: h };
  const ratio = w > h ? maxDim / w : maxDim / h;
  return { width: Math.round(w * ratio), height: Math.round(h * ratio) };
}

async function loadImageBitmap(file: File): Promise<ImageBitmap | HTMLImageElement> {
  if ('createImageBitmap' in window) {
    return createImageBitmap(file);
  }
  return new Promise((resolve, reject) => {
    const img = new Image();
    img.onload = () => resolve(img);
    img.onerror = () => reject(new Error('Failed to load image'));
    img.src = URL.createObjectURL(file);
  });
}