For convenience, attached is a quick and dirty Tkinter GUI that lets you step through the Crochemore/Perrin Algorithm on your choice of inputs, just for play/discovery. A good illustration of the memory for periodic needles can be found by testing: haystack="456,123,123,456,123,123,456" needle="123,123,123," The GUI program does not implement the Boyer-Moore/Horspool/Sunday-style shift-table. In the current PR 22904, this table is used in exactly those situations where the GUI says "Matched 0, so jump ahead by 1".