A solution to the "Who owns the zebra?" puzzle in Python's Pyke.
Python
Latest commit 5f0ece5 Jun 9, 2014 Fixed typo in comment.
 Failed to load latest commit information. README.md Jun 9, 2014 clues.kfb Jun 9, 2014 driver.py Jun 9, 2014 relations.krb Jun 9, 2014

# 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"