/**
 * Rotation d'un Blob image via Canvas.
 * @param {Blob} blob — image source (JPEG)
 * @param {number} degrees — angle de rotation en degrés (0, 90, 180, 270)
 * @returns {Promise<Blob>} Blob JPEG pivoté, ou le blob original si degrees === 0
 */
export async function rotateBlob(blob, degrees) {
  const norm = ((degrees % 360) + 360) % 360;
  if (norm === 0) return blob;

  const url = URL.createObjectURL(blob);

  return new Promise((resolve, reject) => {
    const img = new Image();

    img.onload = () => {
      // Pour 90° ou 270°, largeur et hauteur sont inversées
      const swap = norm === 90 || norm === 270;
      const w = swap ? img.naturalHeight : img.naturalWidth;
      const h = swap ? img.naturalWidth  : img.naturalHeight;

      const canvas = document.createElement('canvas');
      canvas.width  = w;
      canvas.height = h;
      const ctx = canvas.getContext('2d');

      ctx.translate(w / 2, h / 2);
      ctx.rotate((norm * Math.PI) / 180);
      ctx.drawImage(img, -img.naturalWidth / 2, -img.naturalHeight / 2);

      URL.revokeObjectURL(url);
      canvas.toBlob(
        b => b ? resolve(b) : reject(new Error('rotateBlob : toBlob a retourné null')),
        'image/jpeg',
        0.92
      );
    };

    img.onerror = () => {
      URL.revokeObjectURL(url);
      reject(new Error('rotateBlob : impossible de charger le blob'));
    };

    img.src = url;
  });
}