export const DEFAULT_RELATION_FIELDS = ['id', 'code', 'name'] as const;
export const USER_RELATION_FIELDS = ['id', 'username'] as const;
export const CODE_RELATION_FIELDS = ['id', 'code'] as const;

export type DefaultRelation = {
  readonly id: string;
  readonly code: string;
  readonly name: string;
};

export type UserRelation = {
  readonly id: string;
  readonly username: string;
};

export type CodeRelation = {
  readonly id: string;
  readonly code: string;
};

export function pickRelation<T, K extends keyof NonNullable<T>>(
  source: T | null | undefined,
  fields: readonly K[],
): Pick<NonNullable<T>, K> | null {
  if (source == null) {
    return null;
  }
  const result = {} as Pick<NonNullable<T>, K>;
  for (const field of fields) {
    result[field] = source[field];
  }
  return result;
}

export function pickDefaultRelation(
  source: DefaultRelation | null | undefined,
): DefaultRelation | null {
  return pickRelation(source, DEFAULT_RELATION_FIELDS);
}

export function pickUserRelation(source: UserRelation): UserRelation {
  return (
    pickRelation(source, USER_RELATION_FIELDS) ?? {
      id: source.id,
      username: source.username,
    }
  );
}

export function pickCodeRelation(
  source: CodeRelation | null | undefined,
): CodeRelation | null {
  return pickRelation(source, CODE_RELATION_FIELDS);
}

export function fallbackUserRelation(
  source: UserRelation | null | undefined,
  fallbackId: string,
): UserRelation {
  return pickUserRelation(source ?? { id: fallbackId, username: '' });
}