diff --git a/src/gamelog_internal.h b/src/gamelog_internal.h
--- a/src/gamelog_internal.h
+++ b/src/gamelog_internal.h
@@ -11,60 +11,139 @@
 #define GAMELOG_INTERNAL_H
 
 #include "gamelog.h"
+#include <iterator>
 
-static const uint GAMELOG_REVISION_LENGTH = 15;
+/**
+ * Information about the presence of a Grf at a certain point during gamelog history
+ * Note about missing Grfs:
+ * Changes to missing Grfs are not logged including manual removal of the Grf.
+ * So if the gamelog tells a Grf is missing we do not know whether it was readded or completely removed
+ * at some later point.
+ */
+struct GRFPresence{
+	const GRFConfig *gc;  ///< GRFConfig, if known
+	bool was_missing;     ///< Grf was missing during some gameload in the past
 
-/** Contains information about one logged change */
+	GRFPresence(const GRFConfig *gc) : gc(gc), was_missing(false) {}
+	GRFPresence() = default;
+};
+typedef SmallMap<uint32_t, GRFPresence> GrfIDMapping;
+
 struct LoggedChange {
-	GamelogChangeType ct; ///< Type of change logged in this struct
-	union {
-		struct {
-			byte mode;       ///< new game mode - Editor x Game
-			byte landscape;  ///< landscape (temperate, arctic, ...)
-		} mode;
-		struct {
-			char text[GAMELOG_REVISION_LENGTH]; ///< revision string, _openttd_revision
-			uint32 newgrf;   ///< _openttd_newgrf_version
-			uint16 slver;    ///< _sl_version
-			byte modified;   ///< _openttd_revision_modified
-		} revision;
-		struct {
-			uint32 type;     ///< type of savegame, @see SavegameType
-			uint32 version;  ///< major and minor version OR ttdp version
-		} oldver;
-		GRFIdentifier grfadd;    ///< ID and md5sum of added GRF
-		struct {
-			uint32 grfid;    ///< ID of removed GRF
-		} grfrem;
-		GRFIdentifier grfcompat; ///< ID and new md5sum of changed GRF
-		struct {
-			uint32 grfid;    ///< ID of GRF with changed parameters
-		} grfparam;
-		struct {
-			uint32 grfid;    ///< ID of moved GRF
-			int32 offset;    ///< offset, positive = move down
-		} grfmove;
-		struct {
-			char *name;      ///< name of the setting
-			int32 oldval;    ///< old value
-			int32 newval;    ///< new value
-		} setting;
-		struct {
-			uint64 data;     ///< additional data
-			uint32 grfid;    ///< ID of problematic GRF
-			byte bug;        ///< type of bug, @see enum GRFBugs
-		} grfbug;
-	};
+	LoggedChange(GamelogChangeType type = GLCT_NONE) : ct(type) {}
+	virtual ~LoggedChange() {}
+	virtual void FormatTo(std::back_insert_iterator<std::string> &output_iterator, GrfIDMapping &grf_names, GamelogActionType action_type) = 0;
+
+	GamelogChangeType ct;
+};
+
+struct LoggedChangeMode : LoggedChange {
+	LoggedChangeMode() : LoggedChange(GLCT_MODE) {}
+	LoggedChangeMode(byte mode, byte landscape) :
+		LoggedChange(GLCT_MODE), mode(mode), landscape(landscape) {}
+	void FormatTo(std::back_insert_iterator<std::string> &output_iterator, GrfIDMapping &grf_names, GamelogActionType action_type) override;
+
+	byte mode;      ///< new game mode - Editor x Game
+	byte landscape; ///< landscape (temperate, arctic, ...)
+};
+
+struct LoggedChangeRevision : LoggedChange {
+	LoggedChangeRevision() : LoggedChange(GLCT_REVISION) {}
+	LoggedChangeRevision(const std::string &text, uint32_t newgrf, uint16_t slver, byte modified) :
+		LoggedChange(GLCT_REVISION), text(text), newgrf(newgrf), slver(slver), modified(modified) {}
+	void FormatTo(std::back_insert_iterator<std::string> &output_iterator, GrfIDMapping &grf_names, GamelogActionType action_type) override;
+
+	std::string text; ///< revision string, _openttd_revision
+	uint32_t newgrf;  ///< _openttd_newgrf_version
+	uint16_t slver;   ///< _sl_version
+	byte modified;    //< _openttd_revision_modified
+};
+
+struct LoggedChangeOldVersion : LoggedChange {
+	LoggedChangeOldVersion() : LoggedChange(GLCT_OLDVER) {}
+	LoggedChangeOldVersion(uint32_t type, uint32_t version) :
+		LoggedChange(GLCT_OLDVER), type(type), version(version) {}
+	void FormatTo(std::back_insert_iterator<std::string> &output_iterator, GrfIDMapping &grf_names, GamelogActionType action_type) override;
+
+	uint32_t type;    ///< type of savegame, @see SavegameType
+	uint32_t version; ///< major and minor version OR ttdp version
+};
+
+struct LoggedChangeGRFAdd : LoggedChange, GRFIdentifier {
+	LoggedChangeGRFAdd() : LoggedChange(GLCT_GRFADD) {}
+	LoggedChangeGRFAdd(const GRFIdentifier &ident) :
+		LoggedChange(GLCT_GRFADD), GRFIdentifier(ident) {}
+	void FormatTo(std::back_insert_iterator<std::string> &output_iterator, GrfIDMapping &grf_names, GamelogActionType action_type) override;
+};
+
+struct LoggedChangeGRFRemoved : LoggedChange {
+	LoggedChangeGRFRemoved() : LoggedChange(GLCT_GRFREM) {}
+	LoggedChangeGRFRemoved(uint32_t grfid) :
+		LoggedChange(GLCT_GRFREM), grfid(grfid) {}
+	void FormatTo(std::back_insert_iterator<std::string> &output_iterator, GrfIDMapping &grf_names, GamelogActionType action_type) override;
 
-	~LoggedChange();
+	uint32_t grfid; ///< ID of removed GRF
+};
+
+struct LoggedChangeGRFChanged : LoggedChange, GRFIdentifier {
+	LoggedChangeGRFChanged() : LoggedChange(GLCT_GRFCOMPAT) {}
+	LoggedChangeGRFChanged(const GRFIdentifier &ident) :
+		LoggedChange(GLCT_GRFCOMPAT), GRFIdentifier(ident) {}
+	void FormatTo(std::back_insert_iterator<std::string> &output_iterator, GrfIDMapping &grf_names, GamelogActionType action_type) override;
+};
+
+struct LoggedChangeGRFParameterChanged : LoggedChange {
+	LoggedChangeGRFParameterChanged() : LoggedChange(GLCT_GRFPARAM) {}
+	LoggedChangeGRFParameterChanged(uint32_t grfid) :
+		LoggedChange(GLCT_GRFPARAM), grfid(grfid) {}
+	void FormatTo(std::back_insert_iterator<std::string> &output_iterator, GrfIDMapping &grf_names, GamelogActionType action_type) override;
+
+	uint32_t grfid; ///< ID of GRF with changed parameters
+};
+
+struct LoggedChangeGRFMoved : LoggedChange {
+	LoggedChangeGRFMoved() : LoggedChange(GLCT_GRFMOVE) {}
+	LoggedChangeGRFMoved(uint32_t grfid, int32_t offset) :
+		LoggedChange(GLCT_GRFMOVE), grfid(grfid), offset(offset) {}
+	void FormatTo(std::back_insert_iterator<std::string> &output_iterator, GrfIDMapping &grf_names, GamelogActionType action_type) override;
+
+	uint32_t grfid; ///< ID of moved GRF
+	int32_t offset; ///< offset, positive = move down
+};
+
+struct LoggedChangeSettingChanged : LoggedChange {
+	LoggedChangeSettingChanged() : LoggedChange(GLCT_SETTING) {}
+	LoggedChangeSettingChanged(const std::string &name, int32_t oldval, int32_t newval) :
+		LoggedChange(GLCT_SETTING), name(name), oldval(oldval), newval(newval) {}
+	void FormatTo(std::back_insert_iterator<std::string> &output_iterator, GrfIDMapping &grf_names, GamelogActionType action_type) override;
+
+	std::string name; ///< name of the setting
+	int32_t oldval;   ///< old value
+	int32_t newval;   ///< new value
+};
+
+struct LoggedChangeGRFBug : LoggedChange {
+	LoggedChangeGRFBug() : LoggedChange(GLCT_GRFBUG) {}
+	LoggedChangeGRFBug(uint64_t data, uint32_t grfid, byte bug) :
+		LoggedChange(GLCT_GRFBUG), data(data), grfid(grfid), bug(bug) {}
+	void FormatTo(std::back_insert_iterator<std::string> &output_iterator, GrfIDMapping &grf_names, GamelogActionType action_type) override;
+
+	uint64_t data;  ///< additional data
+	uint32_t grfid; ///< ID of problematic GRF
+	byte bug;       ///< type of bug, @see enum GRFBugs
+};
+
+struct LoggedChangeEmergencySave : LoggedChange {
+	LoggedChangeEmergencySave() : LoggedChange(GLCT_EMERGENCY) {}
+	void FormatTo(std::back_insert_iterator<std::string> &output_iterator, GrfIDMapping &grf_names, GamelogActionType action_type) override;
 };
 
 
 /** Contains information about one logged action that caused at least one logged change */
 struct LoggedAction {
-	std::vector<LoggedChange> change; ///< First logged change in this action
+	std::vector<std::unique_ptr<LoggedChange>> change; ///< Logged changes in this action
 	GamelogActionType at; ///< Type of action
-	uint64 tick;          ///< Tick when it happened
+	uint64_t tick;        ///< Tick when it happened
 };
 
 struct GamelogInternalData {