export interface HasCustomFields {
  /**
   * Points at the schema in `erpCustomFieldsSchemas` that should validate `customFields`.
   * Omit (undefined) to avoid unintentionally overwriting existing server-side values.
   */
  customFieldsSchemaRef?: number | null;

  /**
   * Schema-bound custom fields stored on the ERP entity.
   * Omit (undefined) to avoid unintentionally overwriting existing server-side values.
   */
  customFields?: Record<string, unknown> | null;
}

/**
 * Apply `customFields` and `customFieldsSchemaRef` to an outgoing API payload.
 *
 * IMPORTANT: We only set fields when they are not `undefined` (null is allowed) to avoid
 * unintended overwrites when callers don't specify custom fields.
 */
export function applyCustomFields<T extends Record<string, unknown>>(
  obj: T,
  customFields: Pick<HasCustomFields, "customFieldsSchemaRef" | "customFields">
): T {
  const out = obj as Record<string, unknown>;
  if (customFields.customFieldsSchemaRef !== undefined) {
    out["customFieldsSchemaRef"] = customFields.customFieldsSchemaRef;
  }
  if (customFields.customFields !== undefined) {
    out["customFields"] = customFields.customFields;
  }
  return obj;
}

/**
 * Convenience helper to construct a "full" API payload from a REST-style payload.
 */
export function toFullApiObjectFromRest(
  restObj: Record<string, unknown>,
  source: HasCustomFields
): Record<string, unknown> {
  return applyCustomFields(restObj, source);
}

/**
 * Merge non-empty `customFields` into a REST payload. Returns a shallow copy so callers can
 * keep a strict `Record<string, string>` base type before widening with optional `customFields`.
 */
export function applyNonEmptyCustomFieldsToRest<T extends Record<string, unknown>>(
  obj: T,
  customFields: Record<string, unknown> | null | undefined
): T & { customFields?: Record<string, unknown> } {
  if (customFields != null && Object.keys(customFields).length > 0) {
    return { ...obj, customFields };
  }
  return obj as T & { customFields?: Record<string, unknown> };
}