Add hamming exercise

config.json

@@ -4,6 +4,7 @@
   "repository": "https://github.com/exercism/xracket",
   "active": false,
   "problems": [
-    "bob"
+    "bob",
+    "hamming"
   ]
 }

hamming/example.rkt

@@ -0,0 +1,11 @@
+#lang racket
+
+(define (hamming-distance dna1 dna2)
+  (if (= (string-length dna1) (string-length dna2))
+    (length (for/list ([c1 (in-string dna1)]
+                       [c2 (in-string dna2)]
+                      #:unless (eq? c1 c2))
+                      c1))
+    #f))
+
+(provide hamming-distance)

hamming/hamming-test.rkt

@@ -0,0 +1,33 @@
+#lang racket
+
+(require rackunit
+         "example.rkt")
+
+(check-equal? 0 (hamming-distance "A" "A")
+              "identical strands")
+(check-equal? 0 (hamming-distance "GGACTGA" "GGACTGA")
+              "long identical strands")
+(check-equal? 1 (hamming-distance "A" "G")
+              "complete distance in single nucleotide strands")
+(check-equal? 2 (hamming-distance "AG" "CT")
+              "complete distance in small strands")
+(check-equal? 1 (hamming-distance "AT" "CT")
+              "small distance in small strands")
+(check-equal? 1 (hamming-distance "GGACG" "GGTCG")
+              "small distance")
+(check-equal? 2 (hamming-distance "ACCAGGG" "ACTATGG")
+              "small distance in long strands")
+(check-equal? 1 (hamming-distance "AGA" "AGG")
+              "non-unique character in first strand")
+(check-equal? 1 (hamming-distance "AGG" "AGA")
+              "non-unique character in second strand")
+(check-equal? 4 (hamming-distance "GATACA" "GCATAA")
+              "large distance")
+(check-equal? 9 (hamming-distance "GGACGGATTCTG" "AGGACGGATTCT")
+              "large distance in off-by-one strand")
+(check-equal? 0 (hamming-distance "" "")
+              "empty strands")
+(check-equal? #f (hamming-distance "AATG" "AAA")
+              "disallow first strand longer")
+(check-equal? #f (hamming-distance "ATA" "AGTG")
+              "disallow second string longer")