interface Slabbable<T> {
  readonly byteSize: number;
  readonly byteAlignment: number;
  new (buffer: SharedArrayBuffer, byteOffset: number): T;
}

type SlabInstances<T extends Slabbable<any>[]> = {
  [K in keyof T]: T[K] extends Slabbable<infer I> ? I : never;
};

function align(offset: number, alignment: number): number {
  const remainder = offset % alignment;
  return remainder === 0 ? offset : offset + (alignment - remainder);
}

export function slab<T extends Slabbable<any>[]>(...classes: T): SlabInstances<T> {
  const offsets: number[] = [];
  let cursor = 0;
  for (const cls of classes) {
    cursor = align(cursor, cls.byteAlignment);
    offsets.push(cursor);
    cursor += cls.byteSize;
  }

  const buffer = new SharedArrayBuffer(cursor);
  const instances = classes.map((cls, i) => new cls(buffer, offsets[i]));
  return instances as SlabInstances<T>;
}