with iterate

src/org/acooke/lab/perms.clj

@@ -31,3 +31,15 @@
   (let [slow (cons "" (lazy-seq delayed))]
     (def delayed (helper slow))
     delayed))
+
+; http://stackoverflow.com/questions/11567625/tying-the-knot-in-clojure-circular-references-without-explicit-ugly-mutation
+
+(defn stream4 [seed]
+  (defn helper [slow]
+    (apply concat (map (fn [c] (map #(str c %) seed)) slow)))
+  (apply concat (iterate helper seed)))
+
+(defn stream5 [seed]
+  (defn helper [slow]
+    (mapcat (fn [c] (map #(str c %) seed)) slow))
+  (apply concat (iterate helper seed)))

test/org/acooke/lab/perms-test.clj

@@ -50,4 +50,36 @@
   (let [s (take 25 (stream3 ["a" "b" "c"]))]
     (is (= s ["a" "b" "c" "aa" "ab" "ac" "ba" "bb" "bc" "ca" "cb" "cc" "aaa" "aab" "aac" "aba" "abb" "abc" "aca" "acb" "acc" "baa" "bab" "bac" "bba"]))))
 
+(deftest test-perms4-2
+  (let [s (take 2 (stream4 ["a" "b" "c"]))]
+    (is (= s ["a" "b"]))))
+
+(deftest test-perms4-3
+  (let [s (take 3 (stream4 ["a" "b" "c"]))]
+    (is (= s ["a" "b" "c"]))))
+
+(deftest test-perms4-4
+  (let [s (take 4 (stream4 ["a" "b" "c"]))]
+    (is (= s ["a" "b" "c" "aa"]))))
+
+(deftest test-perms4-25
+  (let [s (take 25 (stream4 ["a" "b" "c"]))]
+    (is (= s ["a" "b" "c" "aa" "ab" "ac" "ba" "bb" "bc" "ca" "cb" "cc" "aaa" "aab" "aac" "aba" "abb" "abc" "aca" "acb" "acc" "baa" "bab" "bac" "bba"]))))
+
+(deftest test-perms5-2
+  (let [s (take 2 (stream5 ["a" "b" "c"]))]
+    (is (= s ["a" "b"]))))
+
+(deftest test-perms5-3
+  (let [s (take 3 (stream5 ["a" "b" "c"]))]
+    (is (= s ["a" "b" "c"]))))
+
+(deftest test-perms5-4
+  (let [s (take 4 (stream5 ["a" "b" "c"]))]
+    (is (= s ["a" "b" "c" "aa"]))))
+
+(deftest test-perms5-25
+  (let [s (take 25 (stream5 ["a" "b" "c"]))]
+    (is (= s ["a" "b" "c" "aa" "ab" "ac" "ba" "bb" "bc" "ca" "cb" "cc" "aaa" "aab" "aac" "aba" "abb" "abc" "aca" "acb" "acc" "baa" "bab" "bac" "bba"]))))
+
 (run-tests)