Welcome! Today, we will delve into managing a document's editing history using stacks. Imagine building a text editor; you would need to handle actions like adding text, undoing it, and redoing those changes. We will see how these features can be efficiently implemented using stacks. By the end of this lesson, you will possess an in-depth understanding of applying stacks in practical scenarios.
Before starting the coding portion, let's dissect the methods we will implement. These methods will manage a document's edit history, allowing us to apply changes, undo them, and redo them effectively.
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applyChange(change: String): This method applies a change to the document. The change, represented as a string, is stored in a way that allows us to remember the order of applied changes. Any previously undone changes are discarded. -
undo(): String?: This method undoes the most recent change and allows us to store it for a possible redo. It returns the change that was undone. If there are no changes available to undo, it returnsnull. -
redo(): String?: This method redoes the most recent undone change, making it active again. It returns the change that was redone. If there are no changes available to redo, it returnsnull. -
getChanges(): List<String>: This method returns a list of all applied changes in the order they were applied.
To implement these methods efficiently, we'll use two stacks. We use stacks to keep track of applied changes and undone changes because of their LIFO (Last In, First Out) property. The most recent change is always at the top, making it easy to undo and redo.
Let's break down each method's implementation and study how they interact with the stacks.
Let's implement the solution step-by-step. Firstly, we define our class and set up the initial state.
Kotlin1class DocumentHistory { 2 // Stacks to track current changes and undone changes that can be redone 3 private val changesStack = mutableListOf<String>() 4 private val redoStack = mutableListOf<String>() 5}
Here, DocumentHistory is the class, and we initialize changesStack as a MutableList to act as our stack of applied changes, and redoStack as another MutableList for undone changes.
Next, we put into effect the method to apply changes.
Kotlin1class DocumentHistory { 2 /* Other methods and properties omitted for brevity */ 3 4 // Adds a new change and clears the redo history 5 fun applyChange(change: String) { 6 changesStack.add(change) 7 redoStack.clear() 8 } 9}
With applyChange, we take a string change and append it to the changesStack list. Additionally, we clear the redoStack to ensure that once a new change is applied, any previously undone changes cannot be redone.
Now, we will implement the method to undo the most recent change.
Kotlin1class DocumentHistory { 2 /* Other methods and properties omitted for brevity */ 3 4 // Removes and returns the most recent change, moving it to the redo stack 5 // Returns null if there are no changes to undo 6 fun undo(): String? { 7 return if (changesStack.isNotEmpty()) { 8 val change = changesStack.removeAt(changesStack.lastIndex) 9 redoStack.add(change) 10 change 11 } else { 12 null 13 } 14 } 15}
Here, undo checks if changesStack is not empty. If it's not, it removes the last item from the list, simulating a stack pop operation, adds it to redoStack, and returns the undone change. Otherwise, it returns null.
Now we'll create the method to redo the most recent undone change.
Kotlin1class DocumentHistory { 2 /* Other methods and properties omitted for brevity */ 3 4 // Restores the most recently undone change back to the document history 5 // Returns null if there are no changes to redo 6 fun redo(): String? { 7 return if (redoStack.isNotEmpty()) { 8 val change = redoStack.removeAt(redoStack.lastIndex) 9 changesStack.add(change) 10 change 11 } else { 12 null 13 } 14 } 15}
The redo method checks if redoStack is not empty. If it's not, it removes the last item, simulating a LIFO operation, adds it back to changesStack, and returns the redone change. Otherwise, it returns null.
Lastly, we implement the method to retrieve all applied changes.
Kotlin1class DocumentHistory { 2 /* Other methods and properties omitted for brevity */ 3 4 // Returns a copy of the current document history in chronological order 5 // Changes are returned in the order they were applied 6 fun getChanges(): List<String> { 7 return changesStack.toList() 8 } 9}
The getChanges method simply returns a copy of the changesStack list, which includes all changes applied to the document in the order they were applied.
Here is the final implementation of our DocumentHistory class, combining all the methods we've discussed:
Kotlin1class DocumentHistory { 2 // Stacks to track current changes and undone changes that can be redone 3 private val changesStack = mutableListOf<String>() 4 private val redoStack = mutableListOf<String>() 5 6 // Adds a new change to the history and clears any undone changes that could be redone 7 fun applyChange(change: String) { 8 changesStack.add(change) 9 redoStack.clear() 10 } 11 12 // Removes and returns the most recent change, moving it to the redo stack 13 // Returns null if there are no changes to undo 14 fun undo(): String? { 15 return if (changesStack.isNotEmpty()) { 16 val change = changesStack.removeAt(changesStack.lastIndex) 17 redoStack.add(change) 18 change 19 } else { 20 null 21 } 22 } 23 24 // Restores the most recently undone change back to the document history 25 // Returns null if there are no changes to redo 26 fun redo(): String? { 27 return if (redoStack.isNotEmpty()) { 28 val change = redoStack.removeAt(redoStack.lastIndex) 29 changesStack.add(change) 30 change 31 } else { 32 null 33 } 34 } 35 36 // Returns a copy of the current document history in chronological order 37 fun getChanges(): List<String> { 38 return changesStack.toList() 39 } 40}
Let's test this with an example:
Kotlin1fun main() { 2 val docHist = DocumentHistory() 3 4 // Apply changes 5 docHist.applyChange("Added header") 6 docHist.applyChange("Added footer") 7 println(docHist.getChanges()) // Output: [Added header, Added footer] 8 9 // Undo last change 10 println(docHist.undo()) // Output: Added footer 11 println(docHist.getChanges()) // Output: [Added header] 12 13 // Redo last undone change 14 println(docHist.redo()) // Output: Added footer 15 println(docHist.getChanges()) // Output: [Added header, Added footer] 16 17 // Undo all changes 18 println(docHist.undo()) // Output: Added footer 19 println(docHist.undo()) // Output: Added header 20 println(docHist.getChanges()) // Output: [] 21 22 // Try undoing when no changes are left 23 println(docHist.undo()) // Output: null 24 25 // Redo changes 26 println(docHist.redo()) // Output: Added header 27 println(docHist.redo()) // Output: Added footer 28 println(docHist.getChanges()) // Output: [Added header, Added footer] 29 30 // Try redoing when no changes are left to redo 31 println(docHist.redo()) // Output: null 32}
In today's lesson, we learned how to manage a document's editing history using stacks. We implemented methods to apply changes, undo the last change, redo the most recent undone change, and retrieve all applied changes. This exercise provided us with practical experience in using stacks to efficiently track and revert operations in a real-life scenario. Keep practicing similar challenges to deepen your understanding of data structures in various applications. Fantastic job today, and keep up the good work!
