Test based on question on Racket Users

Author: AlexKnauth

typed-racket-test/succeed/for-last.rkt

@@ -1,6 +1,6 @@
 #lang typed/racket
 
-(require typed/rackunit)
+(require math/array typed/rackunit)
 
 (check-equal? (for/last : (U String #f) ([i '(1 2 3 4 5)]
                                          #:when (even? i))
@@ -23,3 +23,25 @@
                 (list i j))
               (list 3 2))
 
+
+;; based on a question by 曹朝 on the Racket Users mailing list:
+;; https://groups.google.com/d/msg/racket-users/0tOGWZ9O57c/jRXJYkUdAQAJ
+(: shortest-edit-distance (-> String String (U #f Integer)))
+(define (shortest-edit-distance str0 str1)
+(let* ([l0 : Integer (string-length str0)]
+       [l1 : Integer (string-length str1)]
+       [table : (Mutable-Array Integer) (array->mutable-array (make-array (vector l0 l1) 0))])
+  (for*/last : (U #f Integer) ([i0 : Integer (in-range l0)]
+                               [i1 : Integer (in-range l1)])
+    (let* ([c0 : Char (string-ref str0 i0)]
+           [c1 : Char (string-ref str1 i1)]
+           [base : Integer (cond
+                             [(and (= i0 0) (= i1 0)) 0]
+                             [(= i0 0) (array-ref table (vector i0 (sub1 i1)))]
+                             [(= i1 0) (array-ref table (vector (sub1 i0) i1))]
+                             [else (min (array-ref table (vector i0 (sub1 i1)))
+                                        (array-ref table (vector (sub1 i0) i1))
+                                        (array-ref table (vector (sub1 i0) (sub1 i1))))])]
+           [answer : Integer (if (char=? c0 c1) base (add1 base))])
+      (array-set! table (vector i0 i1) answer)
+      answer))))