What is the test for finding an ex post facto law unconstitutional as applied to an offender who committed his crime before its enactment?

Weaver v. Graham, 450 U.S. 24, 101 S.Ct. 960, 67 L.Ed.2d 17 (1981), held a Florida statute unconstitutional as applied to a petitioner who committed his crime before its enactment. The statute diminished the number of "gain-time" credits that prisoners could earn for good behavior. Id. at 25-26, 101 S.Ct. 960. Such credits reduced the portion of a prisoner's sentence that he would be required to serve. Id. In finding an ex post facto violation, Weaver identified two critical elements of an ex post facto law: "it must be retrospective, that is, it must apply to events occurring before its enactment, and it must disadvantage the offender affected by it." Id. at 29, 101 S.Ct. 960. The court reasoned that the statute substantially altered the consequences attached to the petitioner's already-completed crime and that it disadvantaged him by lengthening the time he would have to stay in prison. Id. at 33, 101 S.Ct. 960.