A sudoku solver I wrote while watching my sister and my mother solving
sudokus at the beach on the northern shore of the island of Porquerolles
(there are much better beaches than on the southern shore).

The one specificity of the algorithm they use is that they never make
any assumption about what number could be in some cell, and thus never
have to backtrack.  Only deduction is used; and that is actually enough
to solve a vast number of sudokus; even some I could find labelled as
“extremely hard” (and deemed so by human players!).  I wanted to know if
it was enough for *any* grid: it’s not, as examplified by
‘grids/maman.sdk’ and ‘grids/guardian/2094.sdk’ (I don’t have other
examples as the moment).

As it turns out, all the deductive logic can be summed up in one single
method, called ‘locate’ in the solver class; together with the
‘propagate’ method that propagate constraints, it builds the
sudoku-solving algorithm that solves almost everything.  It is thus
quite possible that with some additional research one could solve all
sudokus, but I haven’t investigated this yet.

	Usage: ./sudoku.rb [-g] <grid.sdk>

The -g switch applies the “guessing” method; that is, the aforementioned
method of making hypotheses where we may have to backtrack.  It is not
the default because that I was more interested in exploring the
possibilities of the deductive method; that’s how I found the two
examples above.

See directory grids for example of grids (all solved by the program).