export type Viewport = { zoom: number; pan: { x: number; y: number } };
export type ViewBounds = { x: number; y: number; width: number; height: number };

/** 境界を余白付きで中央へ収める。座標・倍率だけを扱う純粋関数。 */
export function fitViewport(
  bounds: ViewBounds,
  width: number,
  height: number,
  padding = 40,
  minZoom = 8,
  maxZoom = 160,
): Viewport {
  const zoom = Math.max(
    minZoom,
    Math.min(
      maxZoom,
      Math.min(
        (width - 2 * padding) / Math.max(bounds.width, 1),
        (height - 2 * padding) / Math.max(bounds.height, 1),
      ),
    ),
  );
  return {
    zoom,
    pan: {
      x: (width - (2 * bounds.x + bounds.width) * zoom) / 2,
      y: (height - (2 * bounds.y + bounds.height) * zoom) / 2,
    },
  };
}

/** 指定した画面座標の下にある点を保ったまま倍率を変える。 */
export function zoomAround(
  point: { x: number; y: number },
  viewport: Viewport,
  factor: number,
  minZoom = 5,
  maxZoom = 200,
): Viewport {
  const zoom = Math.max(minZoom, Math.min(maxZoom, viewport.zoom * factor));
  const world = {
    x: (point.x - viewport.pan.x) / viewport.zoom,
    y: (point.y - viewport.pan.y) / viewport.zoom,
  };
  return { zoom, pan: { x: point.x - world.x * zoom, y: point.y - world.y * zoom } };
}