From ab8903875c25562f98a78c385aba1b087cedb7a9 Mon Sep 17 00:00:00 2001
From: Alexey Filippov <afilippov@acm.org>
Date: Sun, 5 May 2019 17:29:26 +0100
Subject: [PATCH] #4 - Add finger tree concatenation

---
 src/main/scala/finger/FingerTreeDeque.scala   |  6 +++--
 src/main/scala/finger/Fingers.scala           | 22 +++++++++++++++++++
 .../scala/finger/FingerTreeDequeSpec.scala    |  4 ++--
 3 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/src/main/scala/finger/FingerTreeDeque.scala b/src/main/scala/finger/FingerTreeDeque.scala
index d0dc4c2..495a0e8 100644
--- a/src/main/scala/finger/FingerTreeDeque.scala
+++ b/src/main/scala/finger/FingerTreeDeque.scala
@@ -1,9 +1,9 @@
 package finger
 
 import finger.Fingers.{Empty, FingerTree}
-import okasaki.Deque
+import okasaki.CatenableDeque
 
-class FingerTreeDeque[E] extends Deque[E, FingerTree[E]] {
+class FingerTreeDeque[E] extends CatenableDeque[E, FingerTree[E]] {
   override def init(q: FingerTree[E]): FingerTree[E] = Fingers.init(q)
 
   override def last(q: FingerTree[E]): E = Fingers.last(q)
@@ -19,4 +19,6 @@ class FingerTreeDeque[E] extends Deque[E, FingerTree[E]] {
   override def head(q: FingerTree[E]): E = Fingers.head(q)
 
   override def tail(q: FingerTree[E]): FingerTree[E] = Fingers.tail(q)
+
+  override def ++(a: FingerTree[E], b: FingerTree[E]): FingerTree[E] = Fingers.concat(a, b)
 }
diff --git a/src/main/scala/finger/Fingers.scala b/src/main/scala/finger/Fingers.scala
index c8cbdf3..13138a5 100644
--- a/src/main/scala/finger/Fingers.scala
+++ b/src/main/scala/finger/Fingers.scala
@@ -222,4 +222,26 @@ object Fingers {
       case NilR => throw new IllegalStateException("init() of an empty tree")
       case SnocR(t, _) => t
     }
+
+  // ==========================================================================================
+  def app3[A](fta: FingerTree[A], ts: List[A], ftb: FingerTree[A]): FingerTree[A] =
+    (fta, ftb) match {
+      case (Empty, xs) => cons1(ts, xs)
+      case (xs, Empty) => snoc1(xs, ts)
+      case (Single(x), xs) => cons(x, cons1(ts, xs))
+      case (xs, Single(x)) => snoc(snoc1(xs, ts), x)
+      case (Deep(pr1, m1, sf1), Deep(pr2, m2, sf2)) =>
+        Deep(pr1, app3(m1, nodes(toList(sf1) ++ ts ++ toList(pr2)), m2), sf2)
+    }
+
+  def nodes[A](xs: List[A]): List[Node[A]] = xs match {
+    case List(a, b) => List(Node2(a, b))
+    case List(a, b, c) => List(Node3(a, b, c))
+    case List(a, b, c, d) => List(Node2(a, b), Node2(c, d))
+    case a :: b :: c :: rest => Node3(a, b, c) :: nodes(rest)
+  }
+
+  def concat[A](xs: FingerTree[A], ys: FingerTree[A]): FingerTree[A] =
+    app3(xs, Nil, ys)
+
 }
diff --git a/src/test/scala/finger/FingerTreeDequeSpec.scala b/src/test/scala/finger/FingerTreeDequeSpec.scala
index 27e458e..7c5bacb 100644
--- a/src/test/scala/finger/FingerTreeDequeSpec.scala
+++ b/src/test/scala/finger/FingerTreeDequeSpec.scala
@@ -1,6 +1,6 @@
 package finger
 
-import okasaki.{DequeSpec, IntElements}
+import okasaki.{CatenableDequeSpec, IntElements}
 
-class FingerTreeDequeSpec extends DequeSpec(new FingerTreeDeque[Int])
+class FingerTreeDequeSpec extends CatenableDequeSpec(new FingerTreeDeque[Int])
   with IntElements
\ No newline at end of file