Write sudoku solver in Clojure

…yeah, because the world just needs another Sudoku solver. Well, I’m not trying to solve world hunger with it, but just an attempt to practice clojure, I took (read: stole) Peter Norvig’s sudoku solver algorithm (written in Python) and adapted it into Clojure. I put it up on Github under sudoku-clj. The algorithm itself isn’t that hard to understand. The porting to a lisp-y syntax made the code a little longer than its Python counterpart. I’m sure seasoned Lisp/Clojure users can point out dozens of places where more idiomatic/succinct syntax can be used (If you happen to be one, do tell, by the way).

Here’s a few things I noticed:

  • Mutable states in clojure are captured using refs. The object itself (in this case, the grid, which is a hash map) doesn’t mutate, but the reference is changed to point to different grid objects that represent a configuration at a given step.

  • Clojure sequences are Lazy. A few times I tried to print out the current state (remaining digits) of the square, but if you simply do (println seq), you will get a Java-ish toString() output of the sequence object. You need to force the lazy sequence to be evaluated by (println (apply str seq)). Needless to say, you lose the advantage of lazy sequences, so use it sparingly.

  • Python’s list comprehension syntax is fabulous. Clojure’s counterpart for comprehension doesn’t feel as elegent, nor is map a function onto a sequence to achieve that (the way I used it)

  • Cake is yummy!

  • The performance isn’t great…I must have done something wrong, but the easy sudoku grid took about 2 seconds (with the JVM already booted), while the Python algorithm solves it in a fraction of a second.

  • Because assign/eliminate are mutually recursive, my current implementation uses the naive way of doing recursion, i.e., let the stack grow. Clojure has a function trampoline, which adds a level of indirection that applies to mutually recursive functions. It uses recur at tail end position (basically translates the recursive calls into loops) which doesn’t fill your process’s stack. It might not be obvious (to me anyways) how one can do that with a few levels of function calls in between assign/eliminate, but I’m sure there’s a way

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