Goals

• Safe: it should not be possible to destroy a game by crashing during a save.
• Effective: Lost progress should be bounded.
• Non-cheatable: Adding kill -9 as a move should not change the game.
• Efficient: subjective, of course :)

Note

I (Pete Mack) independently wrote a partial implementation of this, for replaying movies. It starts out with a checkpoint: a save file, screen size, and complete option dump (including macros and keymaps.) Then it logs keystrokes from there. To replay: open the save file, and watch the movie. For a guaranteed autosave, it would be necessary to include an option to restart playing after reading the log. However, any such logging is easily hacked for save-file abuse. (Just hack the source and allow replay to position n-1.) It does log (and flush) every keystroke prior to executing any commands, so a full replay is guaranteed to die.

Note that this was not designed as an autosave feature, so some modification would be necessary.

Implementation

• Saving is done by writing a 1000new.sorear file, and atomic-renaming it.

  • On systems where the first rename fails, 1000 will be renamed to 1000old, then 1000new to 1000, and 1000old deleted. Possibly less safe?

• Between saves, incoming events are appended to the save file.

• The interesting bits come at the interaction between them

• To always restore at a sane point, only save before prompting for a command

• After certain types of mode shift, humans need 0.7s, if we do it then it'll be unnoticable

  • After level changes, when 8000 characters are in the journal
  • During level changes might be interesting on slow systems
  • Don't store level data, generate before replay

• When we load, replay all stored characters

  • Ignore save&quit commands during replay
  • Will require respawn support - if you die and lose power, replaying the journal will kill the character but we want a birth screen
  • Note that if Angband segfaults, there is a good chance it will do so again on replay - nice for debugging
  • Not so nice for playing, add -flush-journal (which IS cheating misused)