export type CompressionResult = {
  blob: Blob;
  /** MIME type of the produced blob (currently always image/jpeg). */
  type: string;
  originalSize: number;
  compressedSize: number;
  width: number;
  height: number;
};

// Raster images the browser can render to canvas can be compressed to JPEG.
// SVG is vector and must stay as-is. [Gitea #331] GIF and APNG are usually
// animated, and a canvas keeps only the first frame, so they aren't offered.
// Empty type string (undetected MIME) is excluded.
const NOT_COMPRESSIBLE = new Set(['image/svg+xml', 'image/gif', 'image/apng']);
const isCompressibleType = (type: string): boolean =>
  type.startsWith('image/') && !NOT_COMPRESSIBLE.has(type);

/**
 * Pure: whether a WebP file is animated. An animated WebP is a VP8X file
 * whose header has the animation flag (byte 20, bit 0x02) and an ANIM chunk.
 */
export function isAnimatedWebp(bytes: Uint8Array): boolean {
  const ascii = (at: number, len: number) => String.fromCharCode(...bytes.subarray(at, at + len));
  if (bytes.length < 21 || ascii(0, 4) !== 'RIFF' || ascii(8, 4) !== 'WEBP') return false;
  // Bit 0x02 of the VP8X flags byte (no-bitwise: test it arithmetically).
  return ascii(12, 4) === 'VP8X' && Math.floor(bytes[20] / 2) % 2 === 1;
}

/** Pure: whether RGBA pixel data has any pixel that isn't fully opaque. */
export function hasTransparency(rgba: Uint8ClampedArray): boolean {
  for (let i = 3; i < rgba.length; i += 4) {
    if (rgba[i] !== 255) return true;
  }
  return false;
}

/** Returns true if this file can be compressed via canvas (any raster image type). */
export function isCompressible(file: File | Blob): boolean {
  return isCompressibleType(file.type);
}

const JPEG_OUTPUT_TYPE = 'image/jpeg';

/**
 * Compress an image file via canvas.toBlob → JPEG at the given quality.
 * Returns null if the browser cannot render the image (e.g. unsupported codec)
 * or if the source is left untouched to avoid data loss (see below).
 *
 * PNG is skipped entirely: it may carry an alpha channel, and re-encoding to
 * JPEG composites transparency onto an opaque (black) background, corrupting the
 * image. Any other image with transparent pixels, and animated WebP, are left
 * alone for the same reason (#331). Returning null makes callers fall back to uploading the lossless
 * original. The image is decoded with `imageOrientation: 'from-image'` so any
 * EXIF orientation is baked into the pixels instead of being silently dropped.
 */
export async function compressImage(
  file: File | Blob,
  quality = 0.82,
): Promise<CompressionResult | null> {
  if (!isCompressibleType(file.type)) return null;
  // Skip PNG (potential alpha) — re-encoding to JPEG would flatten transparency.
  if (file.type === 'image/png') return null;
  // [Gitea #331] An animated WebP would become a single still frame.
  if (file.type === 'image/webp') {
    try {
      const head = new Uint8Array(await file.slice(0, 32).arrayBuffer());
      if (isAnimatedWebp(head)) return null;
    } catch {
      return null;
    }
  }

  let bitmap: ImageBitmap;
  try {
    bitmap = await createImageBitmap(file, { imageOrientation: 'from-image' });
  } catch {
    // Corrupt/unsupported source: fall back to uploading the lossless original
    // (the caller uses the original file on a null result) rather than rejecting,
    // which would drop the file entirely from the Promise.allSettled upload.
    return null;
  }
  const { width, height } = bitmap;
  const canvas = document.createElement('canvas');
  canvas.width = width;
  canvas.height = height;
  const ctx = canvas.getContext('2d');
  if (!ctx) {
    bitmap.close();
    return null;
  }
  ctx.drawImage(bitmap, 0, 0);
  bitmap.close();
  // [Gitea #331] JPEG has no alpha: a transparent WebP (sticker, logo) came out
  // with a black background. Keep any image with transparency as the original.
  try {
    if (hasTransparency(ctx.getImageData(0, 0, width, height).data)) return null;
  } catch {
    return null;
  }

  return new Promise((resolve) => {
    canvas.toBlob(
      (blob) => {
        if (!blob) {
          resolve(null);
          return;
        }
        resolve({
          blob,
          type: JPEG_OUTPUT_TYPE,
          originalSize: file.size,
          compressedSize: blob.size,
          width,
          height,
        });
      },
      JPEG_OUTPUT_TYPE,
      quality,
    );
  });
}

export function formatFileSize(bytes: number): string {
  if (bytes < 1024) return `${bytes} B`;
  if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`;
  return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
}