[Haskell-beginners] suggestions for optimizing sudoku solver

KC kc1956 at gmail.com
Wed Jan 4 04:44:43 CET 2012

I'm amazed at the invariants he comes up with for all the problems in the book.

At the end of Bird's solution:

"Final Remarks
We tested the solver on 36 puzzles recorded at the website
http://haskell.org/haskellwiki/Sudoku. It solved them in 8.8s (on a
1GHz Pentium 3 PC). We also tested them on six minimal puzzles (each
with 17 non-blank entries) chosen randomly from the 32 000 given at
the site. It solved them in 111.4s. There are about a dozen different
Haskell Sudoku solvers at the site. All of these, including a very
nice solver by Lennart Augustsson, deploy coordinate calculations.
Many use arrays and most use monads. Ours is about twice as slow as
Augustsson’s on the nefarious puzzle (a particularly hard puzzle with
the minimum 17 non-blank entries), but about 30 times faster than Yitz
Gale’s solver on easy puzzles. We also know of solvers that reduce the
problem to Boolean satisfiability, constraint satisfaction, model
checking and so on. I would argue that the one presented above is
certainly one of the simplest and shortest. And at least it was
derived, in part, by equational reasoning."

On Tue, Jan 3, 2012 at 7:25 PM, Lyndon Maydwell <maydwell at gmail.com> wrote:
> I was about to recommend Bird's solution as well.
> An incredibly eye-opening pearl. I believe that he sticks with lists
> as the data-structure the entire way through, so that should give an
> indication that acceptable performance can be achieved without
> requiring mutable data-structures, etc.
> The only issue with this approach is that unless you're aware of other
> invariants available you'll just end up copying Bird's solution...
> I certainly wasn't aware of some of the invariants used in the paper,
> and can't think of any others.
> On Wed, Jan 4, 2012 at 11:01 AM, KC <kc1956 at gmail.com> wrote:
>> 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 <peter.hall at memorphic.com> wrote:
>>> 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
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://www.haskell.org/mailman/listinfo/beginners
>> --
>> --
>> Regards,
>> KC
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