Repairing high school timetables with polymorphic ejection chains

This paper introduces polymorphic ejection chains, and applies them to the problem of repairing time assignments in high school timetables while preserving regularity. An ejection chain is a sequence of repairs, each of which removes a defect introduced by the previous repair. Just as the elements of a polymorphic list may have different types, so in a polymorphic ejection chain the individual repairs may have different types. Methods for the efficien realization of these ideas, implemented in the author’s KHE framework, are given, and some initial experiments are presented.