Improve laziness and fix tests

Author: NthPortal

src/main/scala/collection/immutable/LazyListExtensions.scala

@@ -0,0 +1,22 @@
+package scala.collection.immutable
+
+object LazyListExtensions {
+
+  implicit class LazyListCycle[T](ll: LazyList[T]) {
+    /**
+     * When called on a finite `LazyList`, returns a circular structure
+     * that endlessly repeats the elements in the input.
+     * The result is a true cycle occupying only constant memory.
+     */
+    def cycle: LazyList[T] =
+      if (ll.knownSize == 0) LazyList.empty
+      else LazyList.empty #::: {
+        if (ll.isEmpty) LazyList.empty
+        else {
+          lazy val result: LazyList[T] = ll #::: result
+          result
+        }
+      }
+  }
+
+}

src/main/scala/scala/collection/immutable/LazyListExtensions.scala

@@ -63,18 +63,16 @@ class TestLazyListExtensions {
     val xs = LazyList(1, 2, 3)
     val cyc = xs.cycle
     assertTrue(cyc.tail eq cyc.tail.tail.tail.tail)
-    assertTrue(cyc.tail eq cyc.drop(4))   // TODO: fails
-    assertTrue(cyc eq cyc.tail.tail.tail) // TODO: fails
-    assertTrue(cyc eq cyc.drop(3))        // TODO: fails
-    assertTrue(cyc eq cyc.drop(3))        // TODO: fails
+    assertTrue(cyc.tail.tail eq cyc.drop(4).tail)
+    assertTrue(cyc.tail eq cyc.drop(3).tail)
   }
   @Test
   def testConstantMemory2(): Unit = {
     var counter = 0
     def count(): Int = { counter += 1; counter }
     val xs = count() #:: count() #:: count() #:: LazyList.empty
     val cyc = xs.cycle
-    assertEquals(1, counter)
+    assertEquals(0, counter)
     assertEquals(10, cyc.take(10).size)
     assertEquals(3, counter)
   }