6 files changed, 57 insertions, 12 deletions
diff --git a/src/CellBuffer.cxx b/src/CellBuffer.cxx
index b3111914e..691a811dd 100644
--- a/src/CellBuffer.cxx
+++ b/src/CellBuffer.cxx
@@ -1160,8 +1160,8 @@ int CellBuffer::UndoSavePoint() const noexcept {
 	return uh->SavePoint();
 }
 
-void CellBuffer::SetUndoCurrent(int action) noexcept {
-	uh->SetCurrent(action);
+void CellBuffer::SetUndoCurrent(int action) {
+	uh->SetCurrent(action, Length());
 }
 
 int CellBuffer::UndoCurrent() const noexcept {
diff --git a/src/CellBuffer.h b/src/CellBuffer.h
index 5422e92aa..916937845 100644
--- a/src/CellBuffer.h
+++ b/src/CellBuffer.h
@@ -183,7 +183,7 @@ public:
 	int UndoActions() const noexcept;
 	void SetUndoSavePoint(int action) noexcept;
 	int UndoSavePoint() const noexcept;
-	void SetUndoCurrent(int action) noexcept;
+	void SetUndoCurrent(int action);
 	int UndoCurrent() const noexcept;
 	void SetUndoTentative(int action) noexcept;
 	int UndoTentative() const noexcept;
diff --git a/src/Document.cxx b/src/Document.cxx
index 52081e1cd..8ab837daa 100644
--- a/src/Document.cxx
+++ b/src/Document.cxx
@@ -363,7 +363,7 @@ int Document::UndoSavePoint() const noexcept {
 	return cb.UndoSavePoint();
 }
 
-void Document::SetUndoCurrent(int action) noexcept {
+void Document::SetUndoCurrent(int action) {
 	cb.SetUndoCurrent(action);
 }
 
diff --git a/src/Document.h b/src/Document.h
index 185066a6c..81cb299c0 100644
--- a/src/Document.h
+++ b/src/Document.h
@@ -409,7 +409,7 @@ public:
 	int UndoActions() const noexcept;
 	void SetUndoSavePoint(int action) noexcept;
 	int UndoSavePoint() const noexcept;
-	void SetUndoCurrent(int action) noexcept;
+	void SetUndoCurrent(int action);
 	int UndoCurrent() const noexcept;
 	void SetUndoTentative(int action) noexcept;
 	int UndoTentative() const noexcept;
diff --git a/src/UndoHistory.cxx b/src/UndoHistory.cxx
index 1a956b998..62f871a60 100644
--- a/src/UndoHistory.cxx
+++ b/src/UndoHistory.cxx
@@ -173,6 +173,14 @@ bool UndoActions::AtStart(size_t index) const noexcept {
 	return !types[index-1].mayCoalesce;
 }
 
+size_t UndoActions::LengthTo(size_t index) const noexcept {
+	size_t sum = 0;
+	for (size_t act = 0; act < index; act++) {
+		sum += lengths.ValueAt(act);
+	}
+	return sum;
+}
+
 void ScrapStack::Clear() noexcept {
 	stack.clear();
 	current = 0;
@@ -395,15 +403,49 @@ bool UndoHistory::AfterOrAtDetachPoint() const noexcept {
 	return detach && (*detach <= currentAction);
 }
 
-void UndoHistory::SetCurrent(int action) noexcept {
-	// Find position in scraps for action
-	memory = {};
-	size_t position = 0;
+intptr_t UndoHistory::Delta(int action) noexcept {
+	intptr_t sizeChange = 0;
 	for (int act = 0; act < action; act++) {
-		position += actions.lengths.ValueAt(act);
+		const intptr_t lengthChange = actions.lengths.SignedValueAt(act);
+		sizeChange += (actions.types[act].at == ActionType::insert) ? lengthChange : -lengthChange;
+	}
+	return sizeChange;
+}
+
+bool UndoHistory::Validate(intptr_t lengthDocument) noexcept {
+	// Check history for validity
+	const intptr_t sizeChange = Delta(currentAction);
+	if (sizeChange > lengthDocument) {
+		// Current document size too small for changes made in undo history.
+		return false;
 	}
-	scraps->SetCurrent(position);
+	const intptr_t lengthOriginal = lengthDocument - sizeChange;
+	intptr_t lengthCurrent = lengthOriginal;
+	for (int act = 0; act < actions.SSize(); act++) {
+		const intptr_t lengthChange = actions.lengths.SignedValueAt(act);
+		if (actions.positions.SignedValueAt(act) > lengthCurrent) {
+			// Change outside document.
+			return false;
+		}
+		lengthCurrent += (actions.types[act].at == ActionType::insert) ? lengthChange : -lengthChange;
+		if (lengthCurrent < 0) {
+			return false;
+		}
+	}
+	return true;
+}
+
+void UndoHistory::SetCurrent(int action, intptr_t lengthDocument) {
+	// Find position in scraps for action
+	memory = {};
+	const size_t lengthSum = actions.LengthTo(action);
+	scraps->SetCurrent(lengthSum);
 	currentAction = action;
+	if (!Validate(lengthDocument)) {
+		currentAction = 0;
+		DeleteUndoHistory();
+		throw std::runtime_error("UndoHistory::SetCurrent: invalid undo history.");
+	}
 }
 
 int UndoHistory::Current() const noexcept {
diff --git a/src/UndoHistory.h b/src/UndoHistory.h
index 1186a1e0e..13aaca98c 100644
--- a/src/UndoHistory.h
+++ b/src/UndoHistory.h
@@ -53,6 +53,7 @@ struct UndoActions {
 	[[nodiscard]] intptr_t SSize() const noexcept;
 	void Create(size_t index, ActionType at_, Sci::Position position_, Sci::Position lenData_, bool mayCoalesce_);
 	[[nodiscard]] bool AtStart(size_t index) const noexcept;
+	[[nodiscard]] size_t LengthTo(size_t index) const noexcept;
 };
 
 class ScrapStack {
@@ -112,7 +113,9 @@ public:
 	bool AfterDetachPoint() const noexcept;
 	bool AfterOrAtDetachPoint() const noexcept;
 
-	void SetCurrent(int action) noexcept;
+	[[nodiscard]] intptr_t Delta(int action) noexcept;
+	[[nodiscard]] bool Validate(intptr_t lengthDocument) noexcept;
+	void SetCurrent(int action, intptr_t lengthDocument);
 	[[nodiscard]] int Current() const noexcept;
 	[[nodiscard]] int Type(int action) const noexcept;
 	[[nodiscard]] Sci::Position Position(int action) const noexcept;