import sharp from 'sharp';

export const MAX_DERIVED_IMAGES = 100;
export const MAX_DERIVED_IMAGE_BYTES = 2 * 1024 * 1024;
const DERIVED_IMAGE_ATTEMPTS = [
  { dimension: 2048, quality: 82 },
  { dimension: 1600, quality: 70 },
  { dimension: 1280, quality: 60 },
  { dimension: 960, quality: 50 },
] as const;

export interface PreparedDerivedImage {
  buffer: Buffer;
  mime: 'image/webp';
  width?: number;
  height?: number;
}

export function limitDerivedImages<T>(assets: readonly T[]): { selected: T[]; skipped: number } {
  return {
    selected: assets.slice(0, MAX_DERIVED_IMAGES),
    skipped: Math.max(0, assets.length - MAX_DERIVED_IMAGES),
  };
}

/** Downsample and progressively compress one extracted image to the shared 2MB cap. */
export async function prepareDerivedImage(buffer: Buffer): Promise<PreparedDerivedImage | null> {
  for (const attempt of DERIVED_IMAGE_ATTEMPTS) {
    const output = await sharp(buffer, { failOn: 'none' })
      .rotate()
      .resize({
        width: attempt.dimension,
        height: attempt.dimension,
        fit: 'inside',
        withoutEnlargement: true,
      })
      .webp({ quality: attempt.quality, effort: 4 })
      .toBuffer({ resolveWithObject: true });
    if (output.data.byteLength <= MAX_DERIVED_IMAGE_BYTES) {
      return {
        buffer: output.data,
        mime: 'image/webp',
        ...(output.info.width ? { width: output.info.width } : {}),
        ...(output.info.height ? { height: output.info.height } : {}),
      };
    }
  }
  return null;
}