Сделать бесконечную историю со скоростью доступа О(1).

Author: AlekseyGur

src/ru/alexgur/kanban/service/HistoryManager.java

@@ -10,6 +10,8 @@ public interface HistoryManager {
 
     void clear();
 
+    void remove(int id);
+
     List<Task> getHistory();
 
 }

src/ru/alexgur/kanban/service/InMemoryHistoryManager.java

@@ -1,47 +1,104 @@
 package ru.alexgur.kanban.service;
 
 import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
 import java.util.List;
 
 import ru.alexgur.kanban.model.Task;
 
 public class InMemoryHistoryManager implements HistoryManager {
 
-    public static final int HISTORY_MAX_SIZE = 10;
-
-    private ArrayList<Task> history = new ArrayList<>();
+    private Node head;
+    private Node tail;
+    private HashMap<Integer, Node> history = new HashMap<>();
 
     @Override
     public void clear() {
         clearImpl();
     }
 
+    @Override
+    public void remove(int id) {
+        removeImpl(id);
+    }
+
     @Override
     public void add(Task task) {
         addImpl(task);
     }
 
     @Override
     public List<Task> getHistory() {
-        return getHistoryImpl();
+        List<Task> listTasks = new ArrayList<>();
+        for (Node node : getTasks()) {
+            listTasks.add(node.getTask());
+        }
+        return listTasks;
+    }
+
+    private Node linkLast(Task task) {
+        final Node oldtail = tail;
+        final Node newNode = new Node(oldtail, task, null);
+        tail = newNode;
+        if (oldtail == null) {
+            head = newNode;
+        } else {
+            oldtail.setNext(newNode);
+        }
+        return newNode;
+    }
+
+    private ArrayList<Node> getTasks() {
+        ArrayList<Node> nodes = new ArrayList<>();
+        Node currentNode = head;
+        while (currentNode != null) {
+            nodes.add(currentNode);
+            currentNode = currentNode.getNext();
+        }
+        Collections.reverse(nodes);
+        return nodes;
     }
 
     private void addImpl(Task task) {
         if (task != null) {
-            history.add(task);
-            if (history.size() > HISTORY_MAX_SIZE) {
-                history.remove(0);
+            if (history.containsKey(task.getId())) {
+                Node node = history.remove(task.getId());
+                removeNode(node);
             }
+            Node node = linkLast(task);
+            history.put(task.getId(), node);
+        }
+    }
+
+    private void removeNode(Node node) {
+        Node prev = node.getPrev();
+        Node next = node.getNext();
+
+        if (prev != null && next != null) {
+            // Удаляетяся элемент в середине списка
+            next.setPrev(prev);
+            prev.setNext(next);
+        } else if (prev == null && next != null) {
+            // Удаляетяся первый элемент
+            head = next;
+            next.setPrev(null);
+        } else if (prev != null && next == null) {
+            // Удаляетяся последний элемент
+            tail = prev;
+            prev.setNext(null);
         }
+
+        history.remove(node.getTask().getId());
     }
 
-    private List<Task> getHistoryImpl() {
-        List<Task> res = history;
-        return List.copyOf(res);
+    private void removeImpl(int id) {
+        history.remove(id);
     }
 
     private void clearImpl() {
+        head = null;
+        tail = null;
         history.clear();
     }
-
 }

src/ru/alexgur/kanban/service/Node.java

@@ -0,0 +1,50 @@
+package ru.alexgur.kanban.service;
+
+import ru.alexgur.kanban.model.Task;
+
+public class Node {
+    private Task task;
+    private Node next;
+    private Node prev;
+
+    public Node(Node prev, Task task, Node next) {
+        this.task = task;
+        this.next = next;
+        this.prev = prev;
+    } 
+
+    @Override
+    public int hashCode() {
+        return task.hashCode();
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        Node other = (Node) obj;
+        return task.equals(other.task);
+    }
+
+    public Task getTask() {
+        return task;
+    }
+
+    public void setTask(Task task) {
+        this.task = task;
+    }
+
+    public Node getNext() {
+        return next;
+    }
+
+    public void setNext(Node next) {
+        this.next = next;
+    }
+
+    public Node getPrev() {
+        return prev;
+    }
+
+    public void setPrev(Node prev) {
+        this.prev = prev;
+    }
+}