commit 707b99d48761162f75b078e4c31e5505f7ce1840
Author: Bela Lubkin <filbo_git@armory.com>
Date:   Wed Sep 2 02:26:11 2020 -0700

    added the ability to undo tape erasure.  When the in-memory tape
    is erased with over 20 seconds of play on it, a copy is saved for
    possible undo.  Enter ":undo-tape" or ":ut" from the main menu to
    undo tape erasure.
    
    The guard time preserves long tapes against short abortive actions.
    
    Note: tape undo does not pay attention to which level is loaded.
    Undoing an erased tape from a different level is unlikely to be
    helpful -- unless you are trying to test whether two levels are
    effectively identical.

diff --git a/src/events.c b/src/events.c
index 834b423..690235f 100644
--- a/src/events.c
+++ b/src/events.c
@@ -1825,6 +1825,11 @@ static void HandleKeysSpecial(Key key)
     {
       InsertSolutionTape();
     }
+    else if (is_string_suffix(cheat_input, ":undo-tape") ||
+	is_string_suffix(cheat_input, ":ut"))
+    {
+      RestoreTapeForUndo();
+    }
     else if (is_string_suffix(cheat_input, ":play-solution-tape") ||
 	     is_string_suffix(cheat_input, ":pst"))
     {
diff --git a/src/main.c b/src/main.c
index cb08365..0bb0536 100644
--- a/src/main.c
+++ b/src/main.c
@@ -132,6 +132,7 @@ struct LevelInfo	level, level_template;
 struct PlayerInfo	stored_player[MAX_PLAYERS], *local_player = NULL;
 struct HiScore		highscore[MAX_SCORE_ENTRIES];
 struct TapeInfo		tape;
+struct TapeInfo		tapeUndo;
 struct GameInfo		game;
 struct GlobalInfo	global;
 struct BorderInfo	border;
diff --git a/src/main.h b/src/main.h
index ae52c9f..8f1e53a 100644
--- a/src/main.h
+++ b/src/main.h
@@ -3716,6 +3716,7 @@ extern int			graphics_action_mapping[];
 extern struct LevelInfo		level, level_template;
 extern struct HiScore		highscore[];
 extern struct TapeInfo		tape;
+extern struct TapeInfo		tapeUndo;
 extern struct GlobalInfo	global;
 extern struct BorderInfo	border;
 extern struct ViewportInfo	viewport;
diff --git a/src/tape.c b/src/tape.c
index 0f124e8..3be2ef1 100644
--- a/src/tape.c
+++ b/src/tape.c
@@ -517,10 +517,38 @@ void TapeSetDateFromNow(void)
   TapeSetDateFromEpochSeconds(time(NULL));
 }
 
+void SaveTapeForUndo(void)
+{
+  // Only if a bit of playing has happened.  This leaves the 'precious'
+  // previous run undisturbed if no real amount of playing was done.
+  // (20s could be a setting)
+
+  if (tape.length_seconds >= 20) {
+    tapeUndo = tape;
+  }
+}
+
+void RestoreTapeForUndo(void)
+{
+  struct TapeInfo tmp;
+
+  if (!TAPE_IS_STOPPED(tape))
+    TapeStop();
+
+  // Swap so undo is undoable...
+  tmp = tape;
+  tape = tapeUndo;
+  tapeUndo = tmp;
+
+  DrawCompleteVideoDisplay();
+}
+
 void TapeErase(void)
 {
   int i;
 
+  SaveTapeForUndo();
+
   tape.counter = 0;
   tape.length = 0;
   tape.length_frames = 0;
diff --git a/src/tape.h b/src/tape.h
index a33c859..f60a419 100644
--- a/src/tape.h
+++ b/src/tape.h
@@ -239,6 +239,8 @@ void TapeStopPlaying(void);
 byte *TapePlayAction(void);
 void TapeStop(void);
 void TapeErase(void);
+void SaveTapeForUndo(void);
+void RestoreTapeForUndo(void);
 unsigned int GetTapeLengthFrames(void);
 unsigned int GetTapeLengthSeconds(void);
 void TapeQuickSave(void);