Skip list insertion is complete but there are more specs to write jus…

…t to be thorough

lib/treetop/runtime/interval_skip_list.rb

@@ -1,9 +1,8 @@
 class IntervalSkipList
-  attr_reader :nodes, :head
+  attr_reader :head
 
   def initialize
     @node_heights = [1, 2, 1, 3, 1, 2, 2]
-    @nodes = []
     @head = Node.new(nil, max_height)
   end
 
@@ -16,16 +15,46 @@ def empty?
   end
 
   def insert(value)
-    node = Node.new(value, next_node_height)
-    nodes << node
-    0.upto(node.height - 1) do |i|
-      head.next[i] = node
+    path = Array.new(max_height, nil)
+    found_node = find(value, path)
+    if found_node && found_node.value == value
+      return found_node
+    else
+      return make_node(value, path)
     end
-    node
+  end
+
+  def nodes
+    nodes = []
+    cur_node = head.next[0]
+    until cur_node.nil?
+      nodes << cur_node
+      cur_node = cur_node.next[0]
+    end
+    nodes
   end
 
   protected
   attr_reader :node_heights
+  def find(value, path)
+    cur_node = head
+    (max_height - 1).downto(0) do |cur_height|
+      while (next_node = cur_node.next[cur_height]) && next_node.value < value
+        cur_node = next_node
+      end
+      path[cur_height] = cur_node
+    end
+    cur_node.next[0]
+  end
+
+  def make_node(value, path)
+    new_node = Node.new(value, next_node_height)
+    0.upto(new_node.height - 1) do |i|
+      new_node.next[i] = path[i].next[i]
+      path[i].next[i] = new_node
+    end
+    return new_node
+  end
 
   def next_node_height
     height = node_heights.shift

spec/runtime/interval_skip_list_spec.rb

@@ -11,7 +11,7 @@
 
   before do
     @list = IntervalSkipList.new
-    @expected_node_heights = [01, 2, 1, 3, 1, 2, 2]
+    @expected_node_heights = [1, 2, 1, 3, 1, 2, 2]
   end
 
   describe "when nothing has been inserted" do
@@ -34,7 +34,7 @@
         head.height.should == list.max_height
       end
 
-      it "has null next pointers" do
+      it "has nil next pointers" do
         0.upto(list.max_height - 1) do |i|
           head.next[i].should be_nil
         end
@@ -84,7 +84,7 @@
         inserted_node.value.should == 1
       end
 
-      it "has null next pointers" do
+      it "has nil next pointers" do
         inserted_node.next.each do |next_pointer|
           next_pointer.should be_nil
         end
@@ -121,6 +121,10 @@
       it "has a height of the first expected node height" do
         inserted_node.height.should == expected_node_heights[0]
       end
+
+      it "has its single next pointer pointing at the second inserted node" do
+        inserted_node.next[0].should == inserted_nodes[1]
+      end
     end
 
     describe "the second inserted node" do
@@ -137,6 +141,12 @@
       it "has a height of the second expected node height" do
         inserted_node.height.should == expected_node_heights[1]
       end
+
+      it "has nil next pointers" do
+        inserted_node.next.each do |next_pointer|
+          next_pointer.should be_nil
+        end
+      end
     end
   end