From ad1bc62a7c89166b70d0b14e06c1b1f6eae8e8e2 Mon Sep 17 00:00:00 2001
From: Mark Beck <>
Date: Tue, 18 Nov 2025 09:29:05 +0100
Subject: [PATCH] feat: change to array of next nodes

---
 .../java/de/hs_mannheim/pr2/Skiplist.java     | 153 +++++++-----------
 .../java/de/hs_mannheim/pr2/SkiplistTest.java |  26 ++-
 2 files changed, 82 insertions(+), 97 deletions(-)

diff --git a/src/main/java/de/hs_mannheim/pr2/Skiplist.java b/src/main/java/de/hs_mannheim/pr2/Skiplist.java
index 4ea28ef..f10c153 100644
--- a/src/main/java/de/hs_mannheim/pr2/Skiplist.java
+++ b/src/main/java/de/hs_mannheim/pr2/Skiplist.java
@@ -1,12 +1,12 @@
 package de.hs_mannheim.pr2;
 
+import java.lang.reflect.Array;
 import java.util.Comparator;
 import java.util.Random;
 
-/**
- * Hello world!
- */
 public class Skiplist<T> {
+    private static final double PROBABILITY = 0.5;
+    private static final int MAX_LEVEL = 32;
 
     Node head = null;
     int size = 0;
@@ -36,121 +36,89 @@ public class Skiplist<T> {
         return size == 0;
     }
 
-    private void insertAtLvl(int lvl, Node oldNode) {
-        Node newNode = new Node(oldNode.value);
-        newNode.down = oldNode;
-
-        if (this.lvl < lvl) {
-            throw new RuntimeException();
-        } else if (this.lvl == lvl) {
-            Node newHead = new Node(null, true);
-            newHead.down = this.head;
-            this.head = newHead;
-            this.lvl += 1;
-        }
-
-        if (rng.nextDouble() < 0.5) {
-            insertAtLvl(lvl + 1, newNode);
-        }
-
-        Node currentNode = this.head;
-        for (int i = 0; i < this.lvl - lvl - 1; i++) {
-            currentNode = currentNode.down;
-        }
-
-        while (true) {
-            if (currentNode.next == null || cpr(currentNode.next.value, newNode.value) > 0) {
-                newNode.next = currentNode.next;
-                currentNode.next = newNode;
-                return;
-            }
-            currentNode = currentNode.next;
+    private int randomLevel() {
+        int level = 1;
+        while (rng.nextDouble() < PROBABILITY && level < MAX_LEVEL) {
+            level++;
         }
+        return level;
     }
 
     public void add(T data) {
-        Node node = new Node(data);
+        int level = randomLevel();
+        Node node = new Node(data, level);
         Node currentNode = head;
         int counter = 0;
-        while (true) {
-            if (currentNode.next == null || cpr(currentNode.next.value, data) > 0) {
-                if (currentNode.down == null) {
-                    node.next = currentNode.next;
-                    currentNode.next = node;
-                    break;
-                } else {
-                    currentNode = currentNode.down;
-                }
-            } else {
-                currentNode = currentNode.next;
+
+        for (; level > 0; level--) {
+            while (currentNode.next[level-1] != null && cpr(currentNode.next[level-1].value, data) < 0) {
+                currentNode = currentNode.next[level-1];
+                counter++;
             }
-            counter++;
+            node.next[level-1] = currentNode.next[level-1];
+            currentNode.next[level-1] = node;
         }
         size += 1;
-        if (rng.nextDouble() < 0.5) {
-            insertAtLvl(1, node);
-        }
         System.out.println("adding took " + counter + " steps");
     }
 
-    private void removeUpper(int lvl, Node node) {
-        Node currentNode = this.head;
-        for (int i = 0; i < this.lvl - lvl - 1; i++) {
-            currentNode = currentNode.down;
-        }
-        while (currentNode.next != null) {
-            if (currentNode.next.down == node) {
-                removeUpper(lvl + 1, currentNode.next);
-                currentNode.next = currentNode.next.next;
-                return;
-            }
-            currentNode = currentNode.next;
-        }
+    private void remove(Node node) {
+        int level = node.next.length;
+        Node currentNode = head;
+        int counter = 0;
 
+        for (; level > 0; level--) {
+            while (currentNode.next[level-1] != null) {
+                if (currentNode.next[level-1] == node) {
+                    currentNode.next[level-1] = node.next[level-1];
+                    break;
+                }
+                currentNode = currentNode.next[level-1];
+                counter++;
+            }
+        }
+        size -= 1;
+        System.out.println("removing took " + counter + " steps");
     }
 
     public T pop() {
         if (size == 0) {
             return null;
         } 
-        Node head = this.head;
-        while (head.down != null) {
-            head = head.down;
-        }
-        removeUpper(1, head.next);
-        T result = head.next.value;
-        head.next = head.next.next;
-        size -= 1;
-        return result;
+
+        Node node = this.head.next[0];
+        remove(node);
+        return node.value;
     }
 
-    public T get(int location) {
-        if (location >= size) {
-            return null;
-        }
+    public boolean contains(T data) {
         Node currentNode = head;
-        while (currentNode.down != null) {
-            currentNode = currentNode.down;
-        }
-        currentNode = currentNode.next;
+        int counter = 0;
 
-        for (int i = 0; i < location; i++) {
-            currentNode = currentNode.next;
+        for (int level = MAX_LEVEL; level > 0; level--) {
+            while (currentNode.next[level-1] != null && cpr(currentNode.next[level-1].value, data) <= 0) {
+                if (currentNode.next[level-1].value.equals(data)) {
+                    System.out.println("looking for took " + counter + " steps");
+                    return true;
+                }
+                currentNode = currentNode.next[level-1];
+                counter++;
+            }
         }
-        return currentNode.value;
+        System.out.println("looking for took " + counter + " steps");
+        return false;
     }
 
     @Override
     public String toString() {
-        Node currentHead = head;
         StringBuilder result = new StringBuilder();
-        while (currentHead != null) {
-            Node currentNode = currentHead;
-            while (currentNode != null) {
-                result.append(currentNode);
-                currentNode = currentNode.next;
+        Node node = this.head;
+        for (int i = 0; i <= size; i++) {
+            result.append(String.format("%5s |", node.toString()));
+            for (int j = 0; j < node.next.length; j++) {
+                result.append("#");
             }
-            currentHead = currentHead.down;
+            node = node.next[0];
             result.append("\n");
         }
         return result.toString();
@@ -162,17 +130,20 @@ public class Skiplist<T> {
     // }
 
     private class Node {
-        Node next = null;
-        Node down = null;
+        Node[] next;
         T value;
         boolean dummy = false;
 
-        Node(T value) {
+        @SuppressWarnings("unchecked")
+        Node(T value, int level) {
             this.value = value;
+            this.next = (Node[]) Array.newInstance(Node.class, level);
         }
 
+        @SuppressWarnings("unchecked")
         Node(T value, Boolean dummy) {
             this.dummy = true;
+            this.next = (Node[]) Array.newInstance(Node.class, MAX_LEVEL);
         }
 
         @Override
diff --git a/src/test/java/de/hs_mannheim/pr2/SkiplistTest.java b/src/test/java/de/hs_mannheim/pr2/SkiplistTest.java
index 5978e57..7b720ad 100644
--- a/src/test/java/de/hs_mannheim/pr2/SkiplistTest.java
+++ b/src/test/java/de/hs_mannheim/pr2/SkiplistTest.java
@@ -1,6 +1,7 @@
 package de.hs_mannheim.pr2;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import org.junit.jupiter.api.Test;
@@ -63,15 +64,28 @@ public class SkiplistTest {
     }
 
     @Test
-    public void canGet() {
+    public void canContains() {
         Skiplist<Integer> list = new Skiplist<Integer>();
         list.add(1);
-        list.add(2);
         list.add(3);
-        assertEquals(1, list.get(0).intValue());
-        assertEquals(2, list.get(1).intValue());
-        assertEquals(3, list.get(2).intValue());
-        assertEquals(null, list.get(3));
+        list.add(2);
+        list.add(4);
+        list.add(10);
+        list.add(7);
+        list.add(8);
+        list.add(6);
+        list.add(3);
+        assertTrue(list.contains(1));
+        assertTrue(list.contains(2));
+        assertTrue(list.contains(3));
+        assertTrue(list.contains(4));
+        assertTrue(list.contains(6));
+        assertTrue(list.contains(7));
+        assertTrue(list.contains(8));
+        assertTrue(list.contains(10));
+        assertFalse(list.contains(5));
+        assertFalse(list.contains(9));
+        assertFalse(list.contains(11));
     }