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, 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.

• applyChange(change): 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(): This method undoes the most recent change and allows us to store it for possible redo. It returns the change that was undone. If there are no changes available to undo, it returns null.
• redo(): 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 returns null.
• getChanges(): This method returns an array 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. First, we define our class and set up the initial state.

JavaScript
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1class DocumentHistory {
2    constructor() {
3        this.changesStack = [];
4        this.redoStack = [];
5    }
6}

Here, DocumentHistory is the class, and we initialize an empty array named changesStack to act as our stack of applied changes and another empty array named redoStack to act as our stack for undone changes.

Next, we put into effect the method to apply changes.

JavaScript
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1class DocumentHistory {
2    constructor() {
3        this.changesStack = [];
4        this.redoStack = [];
5    }
6
7    applyChange(change) {
8        this.changesStack.push(change);
9        this.redoStack = [];
10    }
11}

With applyChange, we take the string change and push it to the changesStack array. 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.

JavaScript
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1class DocumentHistory {
2    constructor() {
3        this.changesStack = [];
4        this.redoStack = [];
5    }
6
7    applyChange(change) {
8        this.changesStack.push(change);
9        this.redoStack = [];
10    }
11
12    undo() {
13        if (this.changesStack.length === 0) {
14            return null;
15        }
16        const change = this.changesStack.pop();
17        this.redoStack.push(change);
18        return change;
19    }
20}

Here, undo checks if the changesStack array is empty. If it is, it returns null. Otherwise, it pops the last item from the array, simulating a stack pop operation, pushes it to the redoStack, and returns the undone change.

Now we'll create the method to redo the most recent undone change.

JavaScript
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1class DocumentHistory {
2    constructor() {
3        this.changesStack = [];
4        this.redoStack = [];
5    }
6
7    applyChange(change) {
8        this.changesStack.push(change);
9        this.redoStack = [];
10    }
11
12    undo() {
13        if (this.changesStack.length === 0)
14            return null;
15        const change = this.changesStack.pop();
16        this.redoStack.push(change);
17        return change;
18    }
19
20    redo() {
21        if (this.redoStack.length === 0){
22            return null;
23        }
24        const change = this.redoStack.pop();  // Redo the last undone change
25        this.changesStack.push(change);
26        return change;
27    }
28}

The redo method checks if the redoStack is empty. If it is, it returns null. Otherwise, it pops the last item from the stack, simulating a LIFO operation, pushes it back to the changesStack, and returns the redone change.

Lastly, we implement the method to retrieve all applied changes.

JavaScript
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1class DocumentHistory {
2    constructor() {
3        this.changesStack = [];
4        this.redoStack = [];
5    }
6
7    applyChange(change) {
8        this.changesStack.push(change);
9        this.redoStack = [];
10    }
11
12    undo() {
13        if (this.changesStack.length === 0)
14            return null;
15        const change = this.changesStack.pop();
16        this.redoStack.push(change);
17        return change;
18    }
19
20    redo() {
21        if (this.redoStack.length === 0){
22            return null;
23        }
24        const change = this.redoStack.pop();  // Redo the last undone change
25        this.changesStack.push(change);
26        return change;
27    }
28
29    getChanges() {
30        // Copying the array to avoid external modifications
31        // Returning a reference to the actual array would allow external code to modify our stack
32        return [...this.changesStack]; 
33    }
34}

The getChanges method simply returns a copy of the changesStack array, which includes all changes applied to the document in the order they were applied. Copying the array avoids external modifications, ensuring that the original changesStack remains intact.

With all the necessary methods implemented, let's test this with an example:

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1const docHist = new DocumentHistory();
2
3// Apply changes
4docHist.applyChange("Added header");
5docHist.applyChange("Added footer");
6console.log(docHist.getChanges());  // Output: ['Added header', 'Added footer']
7
8// Undo last change
9console.log(docHist.undo());  // Output: "Added footer"
10console.log(docHist.getChanges());  // Output: ['Added header']
11
12// Redo last undone change
13console.log(docHist.redo());  // Output: "Added footer" 
14console.log(docHist.getChanges());  // Output: ['Added header', 'Added footer']
15
16// Undo all changes
17console.log(docHist.undo());  // Output: "Added footer"
18console.log(docHist.undo());  // Output: "Added header"
19console.log(docHist.getChanges());  // Output: []
20
21// Try undoing when no changes are left
22console.log(docHist.undo());  // Output: null
23
24// Redo changes
25console.log(docHist.redo());  // Output: "Added header"
26console.log(docHist.redo());  // Output: "Added footer"
27console.log(docHist.getChanges());  // Output: ['Added header', 'Added footer']
28
29// Try redoing when no changes are left to redo
30console.log(docHist.redo());  // Output: null

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!