An undo-redo data structure based on the concepts of DoublyLinkedList and behaves exactly the same way as undo-redo features do in Photoshop and Microsoft Word. Regret an Android library also uses this data structure.

The list adds nodes in sequential order as a LinkedList would do when the pointer is at the end of the list.

If the pointer is on the head node or between existing nodes as shown below when adding a new entry, all nodes to the right of the pointer inclusive the node pointed at, will be replaced with the new node.

Each node contains of an instance of Action with field members: String key and Object value used for holding the key-value of an entry.

To add to the UndoRedoList you call undoRedo.add(KEY_TEXT_COLOR, Color.BLACK, Color.RED); where KEY_TEXT_COLOR is used to identify the type of data and Color.BLACK and Color.RED is the old and new value.

UndoRedoList is a linear data structure and has similar performance as Java's LinkedList. The following is the time-complexity for the important methods in UndoRedoList

• add() is always O(1) regardless of the pointer position
• undo() or redo() is O(1) because we can only traverse to the next or previous node

Copyright 2019 Muddi Walid

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