Bird begins with the specification (without regard to efficiency):
solve = filter valid . expand . choices
And ends up with this third version of solve
solve = search . choices
search m
| not (safe m) = []
| complete m' = [map (map head) m']
| otherwise = concat (map search (expand1 m'))
| where m' = prune m
Note: some functions are missing
The interesting idea is how he uses equational reasoning to reach this solve.
On Sun, Jan 1, 2012 at 7:27 PM, Peter Hall
I set myself a learning task of writing a sudoku solver. (https://github.com/peterjoel/sudoku/blob/master/src/Sudoku.hs) It works pretty well, but struggles with some of the harder grids. e.g. data/hard4.txt and data/hard5.txt take around 18 seconds to solve. Obviously there's a limit, but I feel like I should be able to make this faster.
I think the main issue is reading/writing to the cells in the grid, since (!!) is O(n). Even though it never has to go beyond index 8, it will add up over the millions of times it has to do it. I imagine it could be a lot faster if I use a static, non-list data structure, but I was hoping to keep it a bit more flexible.
Also, I'm struggling to get started with performance profiling. Can someone point me to some good resources?
Peter
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-- -- Regards, KC