The Extreme methods extend the ideas of the Hard methods. The advanced colouring strategies we
investigate here are X-Cycles, XY-Chains and 3D Medusa. For these colouring
strategies we introduce new ways of chaining candidates together, with regards to colours. Jellyfish is the most rare
method of all, and we have found only 8 occurrences in a set of 100, 000 boards. Jellyfish is just an extension of Swordfish,
but it is quite difficult to spot while playing.
In this category we introduce a new set of methods, with the simplest example being a BUG which stands for Bivalue Universal Grave. This set of methods abuses the fact that the person/programme, who designed the puzzle, made sure it has only one solution. The general idea is that the person who solves the sudoku, should avoid specific configurations of the candidates. If such a configuration is achieved, this means that the board has more than one solution and it is invalid. In order to avoid such a configuration, we can place a value in a cell or eliminate a candidate from a cell. We deliberately do not include any other such methods in our solver, as we believe, that given a general sudoku board, the user should not assume that it is correct. There have been cases where wrong sudoku boards have been published.