What is the test for a motion for a continuance in a criminal case?

"A continuance in a criminal case may be granted only for good cause. ( 1050, subd. (e).) Whether good cause exists is a question for the trial court's discretion. [Citation.] . . . While a showing of good cause requires that both counsel and the defendant demonstrate they have prepared for trial with due diligence [citation], the trial court may not exercise its discretion 'so as to deprive the defendant or his [or her] attorney of a reasonable opportunity to prepare.' [Citation.]" (People v. Doolin (2009) 45 Cal.4th 390, 450.) "In reviewing the decision to deny a continuance, '[o]ne factor to consider is whether a continuance would be useful.' [Citation.]" (People v. Mungia (2008) 44 Cal.4th 1101, 1118.) The trial court has discretion to determine whether to grant or deny a motion for continuance. (People v. Sakarias (2000) 22 Cal. 4th 596, 646.) " '[A]n order of denial is seldom successfully attacked.' [Citation.]" (People v. Peeler (1995) 9 Cal. 4th 953, 1003.)