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diff --git a/src/ChangeHistory.h b/src/ChangeHistory.h
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index 000000000..8a6e745dc
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+++ b/src/ChangeHistory.h
@@ -0,0 +1,112 @@
+// Scintilla source code edit control
+/** @file ChangeHistory.h
+ ** Manages a history of changes in a document.
+ **/
+// Copyright 2022 by Neil Hodgson <neilh@scintilla.org>
+// The License.txt file describes the conditions under which this software may be distributed.
+
+#ifndef CHANGEHISTORY_H
+#define CHANGEHISTORY_H
+
+namespace Scintilla::Internal {
+
+constexpr int changeOriginal = 0;
+constexpr int changeRevertedOriginal = 1;
+constexpr int changeSaved = 2;
+constexpr int changeModified = 3;
+constexpr int changeRevertedToChange = 4;
+
+// As bit flags
+constexpr unsigned int bitRevertedToOriginal = 1;
+constexpr unsigned int bitSaved = 2;
+constexpr unsigned int bitModified = 4;
+constexpr unsigned int bitRevertedToModified = 8;
+
+struct InsertionSpan {
+	Sci::Position start;
+	Sci::Position length;
+	int edition;
+	enum class Direction { insertion, deletion } direction;
+};
+
+using EditionSet = std::vector<int>;
+using EditionSetOwned = std::unique_ptr<EditionSet>;
+
+class ChangeStack {
+	std::vector<size_t> steps;
+	std::vector<InsertionSpan> insertions;
+public:
+	void Clear() noexcept;
+	void AddStep();
+	void PushDeletion(Sci::Position positionDeletion, int edition);
+	void PushInsertion(Sci::Position positionInsertion, Sci::Position length, int edition);
+	[[nodiscard]] size_t PopStep() noexcept;
+	[[nodiscard]] InsertionSpan PopSpan() noexcept;
+	void SetSavePoint() noexcept;
+};
+
+struct ChangeLog {
+	ChangeStack changeStack;
+	RunStyles<Sci::Position, int> insertEdition;
+	SparseVector<EditionSetOwned> deleteEdition;
+
+	void Clear(Sci::Position length);
+	void InsertSpace(Sci::Position position, Sci::Position insertLength);
+	void DeleteRange(Sci::Position position, Sci::Position deleteLength);
+	void Insert(Sci::Position start, Sci::Position length, int edition);
+	void CollapseRange(Sci::Position position, Sci::Position deleteLength);
+	void PushDeletionAt(Sci::Position position, int edition);
+	void InsertFrontDeletionAt(Sci::Position position, int edition);
+	void SaveRange(Sci::Position position, Sci::Position length);
+	void PopDeletion(Sci::Position position, Sci::Position deleteLength);
+	void SaveHistoryForDelete(Sci::Position position, Sci::Position deleteLength);
+	void DeleteRangeSavingHistory(Sci::Position position, Sci::Position deleteLength);
+	void SetSavePoint();
+
+	Sci::Position Length() const noexcept;
+	[[nodiscard]] size_t DeletionCount(Sci::Position start, Sci::Position length) const noexcept;
+	void Check() const noexcept;
+};
+
+enum class ReversionState { clear, reverting, detached };
+
+class ChangeHistory {
+	ChangeLog changeLog;
+	std::unique_ptr<ChangeLog> changeLogReversions;
+	int historicEpoch = -1;
+
+public:
+	ChangeHistory(Sci::Position length=0);
+
+	void Insert(Sci::Position position, Sci::Position insertLength, bool collectingUndo, bool beforeSave);
+	void DeleteRange(Sci::Position position, Sci::Position deleteLength, bool reverting);
+	void DeleteRangeSavingHistory(Sci::Position position, Sci::Position deleteLength, bool beforeSave, bool isDetached);
+
+	void StartReversion();
+	void EndReversion() noexcept;
+
+	void SetSavePoint();
+
+	void UndoDeleteStep(Sci::Position position, Sci::Position deleteLength, bool isDetached);
+
+	[[nodiscard]] Sci::Position Length() const noexcept;
+
+	// Setting up history before this session
+	void SetEpoch(int epoch) noexcept;
+	void EditionCreateHistory(Sci::Position start, Sci::Position length);
+
+	// Queries for drawing
+	[[nodiscard]] int EditionAt(Sci::Position pos) const noexcept;
+	[[nodiscard]] Sci::Position EditionEndRun(Sci::Position pos) const noexcept;
+	[[nodiscard]] unsigned int EditionDeletesAt(Sci::Position pos) const noexcept;
+	[[nodiscard]] Sci::Position EditionNextDelete(Sci::Position pos) const noexcept;
+
+	// Testing - not used by Scintilla
+	[[nodiscard]] size_t DeletionCount(Sci::Position start, Sci::Position length) const noexcept;
+	EditionSet DeletionsAt(Sci::Position pos) const;
+	void Check() noexcept;
+};
+
+}
+
+#endif