Add UndoSystem implementation and refactor UndoNode for simplicity.

UndoTree.cc

@@ -1,47 +1,3 @@
+// Placeholder translation unit for UndoTree struct definition.
+// Undo logic is implemented in UndoSystem.
 #include "UndoTree.h"
-
-#include <cassert>
-
-
-void
-UndoTree::Begin(const UndoKind kind, const size_t row, const size_t col)
-{
-	if (this->pending != nullptr) {
-		if (this->pending->Kind() == kind) {
-			return;
-		}
-		this->Commit();
-	}
-
-	assert(this->pending == nullptr);
-	this->pending = new UndoNode(kind, row, col);
-	assert(this->pending != nullptr);
-}
-
-
-void
-UndoTree::Commit()
-{
-	if (this->pending == nullptr) {
-		return;
-	}
-
-	if (this->root == nullptr) {
-		assert(this->current == nullptr);
-
-		this->root    = this->pending;
-		this->current = this->pending;
-		this->pending = nullptr;
-
-		return;
-	}
-
-	assert(this->current != nullptr);
-	if (this->current->Next() != nullptr) {
-		this->current->DeleteNext();
-	}
-
-	this->current->Next(this->pending);
-	this->current = this->pending;
-	this->pending = nullptr;
-}