June 2018 Update

Task/Doubly-linked-list-Element-insertion/Perl-6/doubly-linked-list-element-insertion.pl6

@@ -1,3 +1,50 @@
+role DLElem[::T] {
+    has DLElem[T] $.prev is rw;
+    has DLElem[T] $.next is rw;
+    has T $.payload = T;
+
+    method pre-insert(T $payload) {
+        die "Can't insert before beginning" unless $!prev;
+        my $elem = ::?CLASS.new(:$payload);
+        $!prev.next = $elem;
+        $elem.prev = $!prev;
+        $elem.next = self;
+        $!prev = $elem;
+        $elem;
+    }
+
+    method post-insert(T $payload) {
+        die "Can't insert after end" unless $!next;
+        my $elem = ::?CLASS.new(:$payload);
+        $!next.prev = $elem;
+        $elem.next = $!next;
+        $elem.prev = self;
+        $!next = $elem;
+        $elem;
+    }
+
+    method delete {
+        die "Can't delete a sentinel" unless $!prev and $!next;
+        $!next.prev = $!prev;
+        $!prev.next = $!next;   # conveniently returns next element
+    }
+}
+
+role DLList[::DLE] {
+    has DLE $.first;
+    has DLE $.last;
+
+    submethod BUILD {
+	$!first = DLE.new;
+	$!last = DLE.new;
+	$!first.next = $!last;
+	$!last.prev = $!first;
+    }
+
+    method list { ($!first.next, *.next ...^ !*.next).map: *.payload }
+    method reverse { ($!last.prev, *.prev ...^ !*.prev).map: *.payload }
+}
+
 class DLElem_Str does DLElem[Str] {}
 class DLList_Str does DLList[DLElem_Str] {}