pyke-who-owns-zebra

A solution to the "Who owns the zebra?" puzzle in Python's Pyke module.

Who owns the zebra?

"Who owns the zebra?" is a well-known logic puzzle. Basically, "There are 5 houses, one nationality lives in each, each has a different pet, etc. In the red house, the Swede drinks coffee. So who owns the zebra?"

Requirements

• Python 2.7.5 is confirmed to work
• python-pyke is the knowledge engine
• python-texttable is used for easy debugging visualization

apt-get install python python-pyke python-texttable

Contains

• clues.kfb ~ Contains the baseline facts provided by the puzzle.
• relations.krb ~ Contains forward-chaining rules to derive additional facts.
• driver.py ~ Runs the test.

Sample Output

$ python driver.py 

== Who owns the zebra? German ==

#   Color    Nationality    Pet    Drink       Smoke    
=======================================================
1   yellow   Norwegian     cats    water    Dunhill     
2   blue     Dane          horse   tea      Blend       
3   red      English       birds   milk     Pall Mall   
4   green    German        zebra   coffee   Prince      
5   white    Swede         dog     beer     Blue Master 

Calculated in 1.21 seconds.

General Logic

The logic here is to create the constraints driven by two core concepts:

• exclusive relationships ~ "if nationality X has pet Y, then no other nationality has pet Y"
• spatial relationships ~ "there are 5 houses next to each other"

I've seen this done with Python's "constraint" module (http://stackoverflow.com/a/320981/128977) but I wanted to see what it takes to recreate the same logic in Pyke.

For instance, where the "constraint" module has AllDifferentConstraint, this solution creates two custom rules if_one_related_then_others_unrelated and if_four_unrelated_then_remaining_is_related.

All done and said, 14 rules are used for this solution:

Type	Rules	Examples
definitional	categories	asserts the primitive categories, eg "is_category(DRINK, coffee)"
inverse	inverse_relationship_positive inverse_relationship_negative inverse_relationship_beside	"if the Swede has a horse, then the horse is owned by the Swede"
transitive	transitive_positive transitive_negative	eg "if the Swede has a horse and the horse is in the red house, then the Swede is in the red house"
exclusive	if_one_related_then_others_unrelated if_four_unrelated_then_other_is_related	eg "if the Swede has a horse, then the Dane does not"
neighbors (basic)	expanded_relationship_beside_left unrelated_to_beside	basic neighbor logic, eg "if the Swede lives next to the cats, then the Swede does not have cats"
neighbors (spatial)	check_next_to_either_edge check_too_close_to_edge check_next_to_with_other_side_impossible left_of_and_only_two_slots_remaining	complex neighbor logic, eg "if the Swede is not in house #2 and cats are not in house #2, and the Swede lives next to cats, then neither Swede nor cats can be in house #1"