import sharp from 'sharp';

export type ImageFacts = {
  width: number | null;
  height: number | null;
  hasAlpha: boolean | null;
  opaque: boolean | null;
};

const UNKNOWN: ImageFacts = { width: null, height: null, hasAlpha: null, opaque: null };

/**
 * Measure the file that was actually written.
 *
 * Read from disk rather than echoed from the request, because the request is what was asked for and
 * this is what arrived — and those are precisely the two things that come apart. The caller is an
 * agent that cannot look at the image, so anything it is not told, it cannot know.
 *
 * `hasAlpha` and `opaque` answer different questions and the gap between them is the point:
 *
 *   hasAlpha: false                  no alpha channel at all
 *   hasAlpha: true,  opaque: false   transparency present and used
 *   hasAlpha: true,  opaque: true    an alpha channel the model never used
 *
 * The third is the failure worth catching. `background: "transparent"` is accepted, the job succeeds,
 * and every pixel comes back fully opaque — which for an app icon is the difference between usable and
 * a black square, and is invisible from both ends without this field.
 *
 * Never throws. A generation that produced real bytes must not be recorded as failed because the
 * measurement of it did; nulls say "not known" and the image is still there.
 */
export async function inspectImage(path: string): Promise<ImageFacts> {
  try {
    const image = sharp(path);
    const metadata = await image.metadata();

    // stats() decodes the pixels — the only way to answer "is anything actually transparent", since
    // the header can only say whether a channel exists. Skipped when there is no alpha channel at
    // all, where the answer is already known and the decode would be waste.
    const opaque = metadata.hasAlpha ? (await image.stats()).isOpaque : true;

    return {
      width: metadata.width ?? null,
      height: metadata.height ?? null,
      hasAlpha: metadata.hasAlpha ?? null,
      opaque,
    };
  } catch (error) {
    console.error(`[inspect] could not read ${path}:`, error instanceof Error ? error.message : error);
    return UNKNOWN;
  }
}