impl ++ test. and fix bug

Author: matsu-chara

src/main/scala/finger_tree/FingerTree.scala

@@ -89,12 +89,19 @@ sealed trait FingerTree[+A] {
       es.foldLeft[FingerTree[B]](pr) { case (acc, e) => acc.snoc(e) }
     case (pr, Single(f)) =>
       es.foldLeft[FingerTree[B]](pr) { case (acc, e) => acc.snoc(e) }.snoc(f)
+    case (Empty, sf) =>
+      es.foldRight[FingerTree[B]](sf) { case (e, acc) => acc.cons(e) }
+    case (Single(r), sf) =>
+      es.foldRight[FingerTree[B]](sf) { case (e, acc) => acc.cons(e) }.cons(r)
     case (Deep(pr, m1, sf1), Deep(pr1, m2, sf)) =>
-      val esNode = (sf1.toList ++ es ++ pr1.toList).grouped(3).toList.map {
-        case a :: b :: c :: Nil => Node3(a, b, c)
-        case a :: b :: Nil      => Node2(a, b)
-        case _ :: Nil | Nil => throw new IllegalArgumentException
+      def makeNode(xs: List[B]): List[Node[B]] = xs match {
+        case a :: b :: Nil      => Node2(a, b) :: Nil
+        case a :: b :: c :: Nil => Node3(a, b, c) :: Nil
+        case a :: b :: res      => Node2(a, b) :: makeNode(res)
+        case _ :: Nil | Nil     => throw new IllegalArgumentException
       }
+
+      val esNode = makeNode(sf1.toList ++ es ++ pr1.toList)
       val m = m1.append(esNode, m2)
       Deep(pr, m, sf)
   }

src/test/scala/finger_tree/FingerTreeTest.scala

@@ -66,4 +66,27 @@ class FingerTreeTest extends FunSuite with DiagrammedAssertions {
     }
   }
 
+  test("++") {
+    def fromCons(range: Range): FingerTree[Int] = range.foldLeft[FingerTree[Int]](empty) {
+      case (acc, i) => acc.cons(i)
+    }
+    def fromSnoc(range: Range): FingerTree[Int] = range.foldLeft[FingerTree[Int]](empty) {
+      case (acc, i) => acc.snoc(i)
+    }
+
+    val ft1 = fromSnoc(1 to 5)
+    val ft2 = fromCons(6 to 10)
+    // format: off
+    val expect1 = Deep(One(1),Deep(One(Node2(2,3)),Empty,Three(Node2(4,5),Node2(10,9),Node2(8,7))),One(6))
+    // format: on
+    assert(ft1 ++ ft2 === expect1)
+
+    val ft10 = fromSnoc(11 to 20)
+    val ft20 = fromCons(21 to 30)
+    // format: off
+    val expect2 = Deep(One(11),Deep(One(Node3(12,13,14)),Deep(One(Node2(Node3(15,16,17),Node2(18,19))),Empty,One(Node3(Node2(20,30),Node2(29,28),Node3(27,26,25)))),One(Node3(24,23,22))),One(21))
+    // format: on
+    assert(ft10 ++ ft20 == expect2)
+  }
+
 }