Implement Undo Redo in C++ Gap Buffer Text Editor

Implementing undo and redo in a C++ text editor using a gap buffer starts with tracking changes via snapshots of the buffer’s pointers or lightweight deltas for insertions and deletions. Beginners, grab a char array, set up five key pointers—head, gap start, gap end, buffer end, and cursor—and then layer on two stacks: one for undo states and one for redo. This keeps things efficient without copying the whole buffer every keystroke, as seen in solid implementations like lazyhacker’s gapbuffer.

Contents

• Understanding Gap Buffers in C++
• Building a Basic Gap Buffer
• Undo/Redo Fundamentals for Text Editors
• Data Structures for Undo/Redo
• Step-by-Step Undo Implementation
• Adding Redo Functionality
• Best Practices for Beginners
• Performance Tips and Common Pitfalls
• Sources
• Conclusion

Understanding Gap Buffers in C++

Why choose a gap buffer for your C++ text editor? It’s killer for cursor-based edits—insertions and deletions fly because the “gap” (unused space) slides right to your cursor position, minimizing data shifts. Unlike a plain string that copies everything on resize, this setup assumes most edits cluster around where you’re typing. Wikipedia nails it: track gap endpoints with pointers, ignore the gap’s guts, and amortize rare full copies over cheap pointer tweaks Gap buffer - Wikipedia.

Picture this: a char array holds your text, split by a gap. Five pointers rule the show:

• Buffer head (start of real text).
• Gap start.
• Gap end (first non-gap char).
• Buffer end.
• Cursor position (always at gap start, never inside the gap).

New text? Move the gap to cursor, write in. Delete? Widen the gap. Simple. But for undo/redo? That’s where it gets fun—you can’t just revert chars; you need to rewind the whole state.

Building a Basic Gap Buffer

Let’s code a starter gap buffer. Beginners, don’t overthink—begin with a fixed-size array, say 1024 chars, and grow later.

class GapBuffer {
private:
 std::vector<char> buf;
 size_t head = 0; // Start of text
 size_t gap_start = 0; // Cursor/gap beginning
 size_t gap_end = 0; // End of gap
 size_t buf_end = 0; // End of all data

public:
 GapBuffer(size_t size = 1024) : buf(size, '\0') {}

 void insert(char c) {
 move_gap_to(gap_start); // Ensure gap at cursor
 if (gap_end - gap_start > 0) {
 buf[gap_start] = c;
 gap_start++;
 buf_end++;
 } else {
 // Grow buffer or shift—keep simple for now
 }
 }

 void move_gap_to(size_t pos) {
 // Shift text to move gap; memmove for speed
 if (pos > gap_start) {
 // Rightward shift
 std::memmove(&buf[gap_start], &buf[pos], buf_end - pos);
 } else {
 // Leftward
 std::memmove(&buf[pos + (gap_end - gap_start)], &buf[gap_start], gap_start - pos);
 }
 gap_end = pos + (gap_end - gap_start);
 gap_start = pos;
 }

 // Get char at logical pos (skip gap)
 char at(size_t pos) const {
 if (pos < gap_start) return buf[head + pos];
 return buf[gap_end + (pos - gap_start)];
 }
};

Test it: insert ‘H’, ‘i’, move cursor, insert ’ there’. Boom, “Hi there”. Draw from GeeksforGeeks Gap Buffer for the memmove tricks—they’re battle-tested.

Undo/Redo Fundamentals for Text Editors

Undo isn’t magic; it’s replaying inverse ops. Press Ctrl+Z? Reverse the last insert or delete. Redo? Forward from there. Stack Overflow threads break it down: use a vector of states, pop/push as you go Undo/Redo implementation - Stack Overflow.

For gap buffers, full snapshots work but eat memory on big files. Better: deltas. Track what changed—position, inserted string, or deleted chunk. GeeksforGeeks shows stacks of ops like append/undo Implement Undo and Redo features.

Two stacks: undo (past states), redo (future). New edit after undo? Nuke redo. Cursor moves? Ignore 'em—users hate undoing arrow keys.

Data Structures for Undo/Redo

C++ structs shine here. From cdacamar’s editor notes, union for insert/delete data Text Editor Data Structures:

struct Delta {
 enum class Type { Insert, Delete };
 Type type;
 size_t pos;
 std::string text; // Inserted or deleted chunk

 Delta(Type t, size_t p, std::string s) : type(t), pos(p), text(std::move(s)) {}
};

class UndoManager {
private:
 std::vector<Delta> undo_stack;
 std::vector<Delta> redo_stack;
 static constexpr size_t MAX_UNDOS = 100; // Beginner cap

public:
 void push_undo(Delta d) {
 undo_stack.push_back(d);
 redo_stack.clear(); // Fresh edit kills redo
 if (undo_stack.size() > MAX_UNDOS) undo_stack.erase(undo_stack.begin());
 }

 Delta pop_undo() { 
 if (undo_stack.empty()) return {};
 Delta d = undo_stack.back(); undo_stack.pop_back();
 redo_stack.push_back(inverse(d)); // Flip for redo
 return d;
 }
};

Inverse? Insert becomes delete at same pos, vice versa. Ties perfectly to gap buffer’s move_gap_to(pos).

Step-by-Step Undo Implementation

Hook your GapBuffer to UndoManager. Before any edit:

1. Capture delta: for insert, pos = cursor, text = new char(s).
2. push_undo(delta).
3. Apply edit.

Undo:

1. Delta d = pop_undo();
2. if d.type == Insert: delete from d.pos, length d.text.size().
3. Gap buffer delete: move_gap_to(d.pos), widen gap by len.

Code snippet:

void GapBuffer::do_undo(UndoManager& mgr) {
 Delta d = mgr.pop_undo();
 if (d.type == Delta::Insert) {
 move_gap_to(d.pos);
 gap_end += d.text.size(); // "Delete" by gap growth
 buf_end -= d.text.size();
 } else { // Delete: re-insert
 move_gap_to(d.pos);
 std::memcpy(&buf[gap_start], d.text.data(), d.text.size());
 gap_start += d.text.size();
 buf_end += d.text.size();
 gap_end = gap_start;
 }
}

Rishabh’s GitHub lib inspires the stack swap undo-redo-for-text-editors.

Adding Redo Functionality

Symmetric to undo. pop_redo(), apply forward, push to undo (inverse).

Delta UndoManager::pop_redo() {
 if (redo_stack.empty()) return {};
 Delta d = redo_stack.back(); redo_stack.pop_back();
 undo_stack.push_back(inverse(d));
 return d;
}

// In GapBuffer::do_redo similar to do_undo but flip logic

Clear redo on new pushes. Emacs-style branching? Advanced—stick linear for starters Rethinking Undo.

Best Practices for Beginners

Start small: fixed buffer, no resize. Nail insert/delete/move first, sans undo. Then add stacks.

• Cap stacks at 50-100 ops. Memory hog otherwise.
• Batch tiny edits (multiple chars) into one delta.
• Ignore cursor-only moves Stack Overflow cursor tip.
• Test: “ABC”, undo insert C → “AB”, redo → “ABC”, new ‘D’ → redo gone.
• Use std::vector over arrays for dynamic growth.

Dacap’s undo lib for inspo later dacap/undo.

Performance Tips and Common Pitfalls

Shifting the gap? O(n) worst-case, but rare if cursor jumps little. Deltas beat snapshots—lazyhacker’s pointers prove it gapbuffer GitHub.

Pitfalls:

• Gap inside gap? Never—enforce cursor at start.
• Large deletes: string copies bloat; consider pointers to buffer slices.
• Threading? Single-threaded editor first.

Profile with big files. Gap buffers crush ropes for local edits.

Sources

1. Text Editor Data Structures - invoke::thought()
2. GitHub - lazyhacker/gapbuffer
3. Undo/Redo implementation - Stack Overflow
4. GitHub - rishabhsaini1001/undo-redo-for-text-editors
5. Implement Undo and Redo features of a Text Editor - GeeksforGeeks
6. Rethinking Undo - invoke::thought()
7. Gap buffer - Wikipedia
8. Need Help writing an undo/redo function - Stack Overflow
9. Gap Buffer Data Structure - GeeksforGeeks
10. GitHub - dacap/undo

Conclusion

Nail a C++ gap buffer text editor with undo/redo by starting simple—pointers first, deltas next—and you’ll have a responsive editor rivaling pros. Beginners win by limiting scope, testing obsessively, and iterating from basics like the sources here. Scale up: add resize, multi-line, then polish. Your first Ctrl+Z working? That’s the hook—keeps you coding.