/**
 * Client-side photo downscaling for pickup uploads.
 *
 * Why this exists: the driver app used to PUT the raw camera file
 * straight at PocketBase. PocketBase's request logs show what that
 * cost us — on 2026-10-12 every single attempt came back `400
 * unexpected EOF` (86 of 86, median 7.6s, max 15.6s). That error is
 * Go's multipart parser hitting the end of the request body before
 * the declared Content-Length arrived: a modern Android camera
 * produces a 3–8 MB file, and on mobile data the transfer is getting
 * cut off partway through. Not a PocketBase mime or schema problem —
 * `order_item_attachements` has `mimeTypes=null`, so nothing was
 * rejecting the format.
 *
 * The office side never hit this because `attachment-compressor.ts`
 * in WFDASH re-encodes to 1200px / quality 70 before storing. These
 * constants deliberately mirror `JPG_MAX_WIDTH` and `JPG_QUALITY`
 * there so both sides produce comparable images. A 1200px q70 JPEG
 * lands around 150–300 KB — a sub-second transfer that finishes.
 */

/** Longest edge, in pixels, of the uploaded image. Mirrors WFDASH's JPG_MAX_WIDTH. */
export const MAX_PHOTO_EDGE = 1200;

/** JPEG encoder quality, 0–1. Mirrors WFDASH's JPG_QUALITY (70). */
export const PHOTO_QUALITY = 0.7;

/**
 * Below this, the file is already small enough that re-encoding costs
 * the driver time (and a visible spinner) for no meaningful saving.
 * 150 KB is comfortably inside a reliable mobile upload.
 */
export const SKIP_BELOW_BYTES = 150 * 1024;

/**
 * Formats a canvas can decode and re-encode for us. HEIC/HEIF is
 * excluded deliberately: iPhone-origin files land here via `accept`
 * when a driver picks from the library rather than the camera, and
 * Chrome cannot decode them into a canvas. They fall through to
 * `compressPhoto` returning the original untouched.
 */
const REENCODABLE_TYPES = new Set(['image/jpeg', 'image/png', 'image/webp']);

function loadImage(url: string): Promise<HTMLImageElement> {
  return new Promise((resolve, reject) => {
    const img = new Image();
    img.onload = () => resolve(img);
    img.onerror = () => reject(new Error('decode failed'));
    img.src = url;
  });
}

/**
 * Downscale + re-encode `file` for upload.
 *
 * Never throws and never returns null: on any failure it hands back the
 * original file. Compression here is an optimisation, not a
 * correctness requirement — turning a photo that might have uploaded
 * fine into an error would be strictly worse than trying and failing
 * the way we do today. Callers can treat the return value as
 * "the best file we can produce", so the happy path stays simple.
 */
export async function compressPhoto(file: File): Promise<File> {
  if (file.size <= SKIP_BELOW_BYTES) return file;
  if (!REENCODABLE_TYPES.has(file.type)) return file;

  // Not available during SSR. Only ever called from a click handler in
  // the browser, but a guard is cheaper than a confusing crash.
  if (typeof document === 'undefined') return file;

  let objectUrl: string | null = null;
  try {
    objectUrl = URL.createObjectURL(file);
    const img = await loadImage(objectUrl);

    // Guard against absurd intrinsic sizes before allocating a canvas;
    // some devices report bogus dimensions for corrupt files and a
    // canvas at 99999px would take the tab down with it.
    if (!img.naturalWidth || !img.naturalHeight) return file;
    if (img.naturalWidth > 20000 || img.naturalHeight > 20000) return file;

    const longest = Math.max(img.naturalWidth, img.naturalHeight);
    // Only shrink. Upscaling a small photo wastes bytes and detail.
    if (longest <= MAX_PHOTO_EDGE) return file;

    const scale = MAX_PHOTO_EDGE / longest;
    const width = Math.max(1, Math.round(img.naturalWidth * scale));
    const height = Math.max(1, Math.round(img.naturalHeight * scale));

    const canvas = document.createElement('canvas');
    canvas.width = width;
    canvas.height = height;
    const ctx = canvas.getContext('2d');
    if (!ctx) return file;

    // JPEG has no alpha channel; without this a transparent PNG comes
    // out with black backgrounds where the alpha was.
    ctx.fillStyle = '#ffffff';
    ctx.fillRect(0, 0, width, height);
    ctx.drawImage(img, 0, 0, width, height);

    const blob = await new Promise<Blob | null>((resolve) =>
      canvas.toBlob(resolve, 'image/jpeg', PHOTO_QUALITY),
    );
    if (!blob) return file;

    // If re-encoding somehow produced something bigger, keep the
    // original rather than making the upload worse.
    if (blob.size >= file.size) return file;

    const stem = file.name.replace(/\.[^.]+$/, '') || 'pickup';
    return new File([blob], `${stem}.jpg`, {
      type: 'image/jpeg',
      lastModified: Date.now(),
    });
  } catch {
    // Decode failure, canvas unavailable, out of memory — all handled
    // the same way: try anyway with what we were given.
    return file;
  } finally {
    if (objectUrl) URL.revokeObjectURL(objectUrl);
  }
}