The garbage collector presented in this paper makes use of two well known compaction garbage collection algorithms with very different performance characteristics: Cheney's two-space copying collector and Jonker's sliding compaction collector. We propose a scheme which allows either collector to be used. The run-time memory requirements of the program being executed are used to determine the most appropriate collector. This enables us to achieve a fast collector for heap requirements less than half of the heap memory but allows the heap utilization to increase beyond this threshold. Using these ideas we develop a particularly attractive extension to Appel's generational collector. We also describe a particularly fast implementation of the garbage collector which avoids interpreting the structure and current state of closures by attaching specific code to heap objects. This code knows the structure and current state of the object and performs the appropriate actions without having to te...