Add the Zebra Puzzle as a practice exercise. (#336)

* Add the Zebra Puzzle as a practice exercise.

* Call error in stub functions instead of returning dummy value

* Add missing parens

config.json

@@ -558,6 +558,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 1
+      },
+      {
+        "slug": "zebra-puzzle",
+        "name": "Zebra Puzzle",
+        "uuid": "cf7f3ccd-babc-417c-ba74-937c7e674065",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 6
       }
     ]
   },

exercises/practice/zebra-puzzle/.docs/instructions.md

@@ -0,0 +1,24 @@
+# Instructions
+
+Solve the zebra puzzle.
+
+1. There are five houses.
+2. The Englishman lives in the red house.
+3. The Spaniard owns the dog.
+4. Coffee is drunk in the green house.
+5. The Ukrainian drinks tea.
+6. The green house is immediately to the right of the ivory house.
+7. The Old Gold smoker owns snails.
+8. Kools are smoked in the yellow house.
+9. Milk is drunk in the middle house.
+10. The Norwegian lives in the first house.
+11. The man who smokes Chesterfields lives in the house next to the man with the fox.
+12. Kools are smoked in the house next to the house where the horse is kept.
+13. The Lucky Strike smoker drinks orange juice.
+14. The Japanese smokes Parliaments.
+15. The Norwegian lives next to the blue house.
+
+Each of the five houses is painted a different color, and their inhabitants are of different national extractions, own different pets, drink different beverages and smoke different brands of cigarettes.
+
+Which of the residents drinks water?
+Who owns the zebra?

exercises/practice/zebra-puzzle/.meta/config.json

@@ -0,0 +1,17 @@
+{
+  "authors": ["blakelewis"],
+  "files": {
+    "solution": [
+      "zebra-puzzle.rkt"
+    ],
+    "test": [
+      "zebra-puzzle-test.rkt"
+    ],
+    "example": [
+      ".meta/example.rkt"
+    ]
+  },
+  "blurb": "Solve the zebra puzzle.",
+  "source": "Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Zebra_Puzzle"
+}

exercises/practice/zebra-puzzle/.meta/example.rkt

@@ -0,0 +1,119 @@
+#lang racket
+
+(provide owns-zebra drinks-water)
+
+(struct house (number color owner pet drink smokes) #:transparent)
+
+(define all-houses
+  (for*/list ([n '(1 2 3 4 5)]
+              [c '(red green ivory yellow blue)]
+              [o '(Englishman Spaniard Ukrainian
+		   Norwegian Japanese)]
+              [p '(dog snails fox horse zebra)]
+              [d '(coffee tea milk orange-juice water)]
+              [s '(Old-Gold Kools Chesterfields
+		    Lucky-Strike Parliaments)])
+      (house n c o p d s)))
+
+(define house-constraints
+  (list 
+    (λ (h) (eq? (eq? (house-owner h) 'Englishman)
+	        (eq? (house-color h) 'red)))
+    (λ (h) (eq? (eq? (house-owner h) 'Spaniard)
+	        (eq? (house-pet h) 'dog)))
+    (λ (h) (eq? (eq? (house-drink h) 'coffee)
+	        (eq? (house-color h) 'green)))
+    (λ (h) (eq? (eq? (house-owner h) 'Ukrainian)
+	        (eq? (house-drink h) 'tea)))
+    (λ (h) (eq? (eq? (house-smokes h) 'Old-Gold)
+	        (eq? (house-pet h) 'snails)))
+    (λ (h) (eq? (eq? (house-smokes h) 'Kools)
+	        (eq? (house-color h) 'yellow)))
+    (λ (h) (eq? (eq? (house-drink h) 'milk)
+	        (= (house-number h) 3)))
+    (λ (h) (eq? (eq? (house-owner h) 'Norwegian)
+	        (= (house-number h) 1)))
+    (λ (h) (eq? (eq? (house-smokes h) 'Lucky-Strike)
+	        (eq? (house-drink h) 'orange-juice)))
+    (λ (h) (eq? (eq? (house-owner h) 'Japanese)
+	        (eq? (house-smokes h) 'Parliaments)))
+    (λ (h) (eq? (= (house-number h) 2)
+	        (eq? (house-color h) 'blue)))
+    (λ (h) (implies (eq? (house-color h) 'green)
+	            (not (= (house-number h) 1))))
+    (λ (h) (implies (eq? (house-color h) 'ivory)
+		    (not (= (house-number h) 5))))))
+
+(define (apply-constraints constraints items)
+  (for/fold ([it items])
+            ([constraint constraints])
+     (filter constraint it)))
+
+(define feasible-houses
+  (apply-constraints house-constraints all-houses))
+
+(define (next-door? h1 h2)
+  (= (abs (- (house-number h1) (house-number h2))) 1))
+
+(define two-house-constraints
+  (list
+     (λ (h1 h2) (implies (and (eq? (house-color h1) 'green)
+			       (eq? (house-color h2) 'ivory))
+		         (= (house-number h1)
+			    (add1 (house-number h2)))))
+     (λ (h1 h2) (implies (and (eq? (house-smokes h1) 'Chesterfields)
+			      (eq? (house-pet h2) 'fox))
+			 (next-door? h1 h2)))
+     (λ (h1 h2) (implies (and (eq? (house-smokes h1) 'Kools)
+			      (eq? (house-pet h2) 'horse))
+			 (next-door? h1 h2)))))
+
+(define (check-street constraint street)
+  (for*/and ([h1 street]
+	     [h2 street])
+    (constraint h1 h2)))
+
+(define street-constraints
+  (map (λ (c) (λ (s) (check-street c s))) two-house-constraints))
+
+(define houses-by-number
+  (for/list ([n '(1 2 3 4 5)])
+    (filter (λ (h) (= n (house-number h))) feasible-houses)))
+
+(define (disjoint? h1 h2)
+  (not (or (eq? (house-color h1) (house-color h2))
+	   (eq? (house-owner h1) (house-owner h2))
+	   (eq? (house-pet h1) (house-pet h2))
+	   (eq? (house-drink h1) (house-drink h2))
+	   (eq? (house-smokes h1) (house-smokes h2)))))
+
+(define (extends? street h)
+  (andmap (λ (h1) (disjoint? h h1)) street))
+
+(define (build-streets houses street)
+  (if (null? houses)
+    (list street)
+    (for/fold
+      ([streets '()])
+      ([h (car houses)]
+       #:when (extends? street h))
+      (append streets (build-streets (cdr houses) (cons h street))))))
+
+(define all-streets
+  (build-streets houses-by-number '()))
+
+(define feasible-streets
+  (apply-constraints street-constraints all-streets))
+
+(define (find-owner condition)
+  (if (= 1 (length feasible-streets))
+    (for/first ([h (car feasible-streets)]
+		#:when (condition h))
+      (house-owner h))
+    (null)))
+
+(define (owns-zebra)
+  (find-owner (λ (h) (eq? (house-pet h) 'zebra))))
+
+(define (drinks-water)
+  (find-owner (λ (h) (eq? (house-drink h) 'water))))

exercises/practice/zebra-puzzle/.meta/tests.toml

@@ -0,0 +1,16 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[16efb4e4-8ad7-4d5e-ba96-e5537b66fd42]
+description = "resident who drinks water"
+
+[084d5b8b-24e2-40e6-b008-c800da8cd257]
+description = "resident who owns zebra"

exercises/practice/zebra-puzzle/zebra-puzzle-test.rkt

@@ -0,0 +1,18 @@
+#lang racket/base
+
+(require "zebra-puzzle.rkt")
+
+(module+ test
+  (require rackunit rackunit/text-ui))
+
+(module+ test
+  (define suite
+    (test-suite
+      "zebra puzzle tests"
+
+      (test-equal? "who owns the zebra?"
+		   (owns-zebra) 'Japanese)
+
+      (test-equal? "who drinks water?"
+		   (drinks-water) 'Norwegian)))
+  (run-tests suite))

exercises/practice/zebra-puzzle/zebra-puzzle.rkt

@@ -0,0 +1,9 @@
+#lang racket
+
+(provide owns-zebra drinks-water)
+
+(define (owns-zebra)
+  (error "Not implemented yet"))
+
+(define (drinks-water)
+  (error "Not implemented yet"))