/**
 * How big a picture of `w`×`h` units is drawn in a preview of `room` and where its top left lands.
 * `pad` is the room a worn shader asks for around it, as a share of each side (see `quadPadding`).
 * From one device pixel per unit on the zoom is whole device pixels, and the corner sits on one, so
 * pixel art stays sharp; below that the picture only shrinks to fit. `ratio` is device pixels per
 * panel pixel.
 */
export function previewFit(
  w: number,
  h: number,
  room: { w: number; h: number },
  options: { margin: number; maxZoom: number; ratio: number; pad?: { u: number; v: number } | null },
): { zoom: number; x: number; y: number } {
  const { margin, maxZoom, ratio, pad } = options;
  const fits = Math.min(
    (room.w - 2 * margin) / (w * (1 + 2 * (pad?.u ?? 0))),
    (room.h - 2 * margin) / (h * (1 + 2 * (pad?.v ?? 0))),
    maxZoom,
  );
  const device = fits * ratio;
  const zoom = Math.max(0, device >= 1 ? Math.floor(device) / ratio : fits);
  const corner = (space: number, size: number) => Math.round(((space - size * zoom) / 2) * ratio) / ratio;
  return { zoom, x: corner(room.w, w), y: corner(room.h, h) };
}